The Branching Bush of Horse Evolution

13 09 2007

Note: Welcome, visitors from The Sandwalk and Pharyngula! I am certainly humbled by the amount of praise and attention this post has received, and although it’s not as scientifically rigorous as I would have liked to be (I still have much to learn), I hope that you find it to be an enjoyable read all the same.

Update: I’ve created something of an appendix to this article about how creationists have presented horse evolution in some of their books. It can be found here.

Eohippus
One of Charles R. Knight’s interpretations of Eohippus

When the name of O.C. Marsh is invoked, it is often to tell of his participation in the great “Bone Wars” of the late 19th century, sparring with fellow osteophile E.D. Cope in the pages of the New York Herald. Twisted tales of deceit and sabotage were promulgated in the sensationalist paper, and while both men helped to bring about an American revolution in vertebrate paleontology, the scars of their bitter squabbling have yet to fully heal. Such scientific in-fighting might seem worthy only of a historical footnote or an introduction to the stereotyped image of “smash-and-grab” paleontology of the time which is almost romantically referred to, but the truth of the matter goes far deeper than the public beard-pulling that is so often remembered.

The tiff between Cope and Marsh is strange in that is seems to exist in the popular literature out of time, removed from the context in which it had originally existed. Charles Darwin had published his earth-shaking work On the Origin of Species by Natural Selection a scant 31 years before the ink almost ran red with rage on the pages of the Herald, the question of evolution being of far more importance in the public consciousness than dinosaurs. The full establishment of the dinosaur as a cultural (and dare I say, mythical) creature in the mind of the American public only seemed to take place after the Bone Wars, the appointment of Henry Fairfield Osborn to the American Museum of Natural History (specifically hired to establish a vertebrate paleontology program) and the popular reports of the dinosaur that carried the namesake of Andrew Carnegie, Diplodocus carnegei, being the more immediate beginnings of the public’s love affair with the extinct creatures. Before Brontosaurus and Tyrannosaurus became household names, the public eye was focused upon horses and birds.

The latter half of the 19th century was a stirring time for biological science, especially involving the new areas of vertebrate paleontology and evolution, the august authorities in England keeping on eye on the up-and-comers starting their own careers in the states. Early on, paleontologist E.D. Cope impressed T.H. Huxley with his 1866 discovery of Laelaps aquilunguis, but in a paleontological clean-sweep Marsh would eventually have his name attached to Cope’s dinosaur and the admiration of not only T.H. Huxley, but Charles Darwin himself. As for the renaming of Laelaps, Marsh found that the name was already taken by a genus of mite, renaming the New Jersey greensand dinosaur Dryptosaurus in 1877 (although Cope, throughout the rest of his career, called the dinosaur Laelaps). It would take more than some taxonomic shuffling to impress the eminent British anatomists and paleontologists, however, and Marsh’s ticket into Huxley’s good graces came in the form of toothed Cretaceous birds like Hesperornis (Marsh, 1872).

While the discovery of ancient bones was exciting to some, evolution was an even more popular topic, and the question that surrounded every fossil was “How does this fit into the grand scheme of evolution?” The 1861 discovery of Archaeopteryx from the lagerstatten of Solnhofen, Germany seemed to arrive right on cue to confirm that evolution had taken place in times previously referred to as “antediluvial”, and Marsh’s subsequent discovery of birds with teeth in the American West further confirmed the notion that aves had evolved from reptilian ancestors (Huxley even being so progressive as to name the dinosaurs as the probable ancestral stock). Charles Darwin himself recognized the importance of Marsh’s discoveries as well, and two years after Marsh visited Darwin at Down House in 1878, Darwin wrote the following letter to Marsh on or about August 31, 1880;

I received some time ago your very kind note of July 28th, & yesterday the magnificent volume. I have looked with renewed admiration at the plates, & will soon read the text. Your work on these old birds & on the many fossil animals of N. America has afforded the best support to the theory of evolution, which has appeared within the last 20 years. The general appearance of the copy which you have sent me is worthy of its contents, and I can say nothing stronger than this.

With cordial thanks, believe me yours very sincerely,

Charles Darwin

Toothed birds were not Marsh’s only claim to evolutionary fame, however; by 1876 his assistants had collected enough fossil horse material to show that the horse was not “a gift from the Old World to the New” (as an European authority had once said during a lecture), but quite the reverse. In fact, the timing of the discovery and study of the horses could not have been better for Marsh, as in 1876 T.H. Huxley visited Yale and was duly impressed with the American Professor and his fossil horses. Huxley was absolutely enthralled by Marsh’s fossil equids, and Huxley’s son Leonard wrote of the visitation upon the New World horses as follows;

At each inquiry, whether he had a specimen to illustrate such and such a point or exemplify a transition from earlier and less specialized forms to later and more specialized ones, Professor Marsh would simply turn to his assistant and bid him fetch box number so and so, until Huxley turned upon him and said ‘I believe you are a magician; whatever I want, you just conjure up.'”

Eohippus
Restoration of Eohippus. From “The Dawn Horse or Eohippus” by Chester Stock (1947).

Huxley even featured Marsh’s discoveries of toothed birds and fossil horses in a set of three lectures he delivered at Chickering Hall in New York, the visit of such a famous evolutionist being front page news (with Marsh sharing in good press since his fossils were discussed by so prominent a figure as Huxley). The only thing that could have made the event sweeter would have been the knowledge of an ancestral horse with five toes (what was regarded as the “primitive” condition for mammals as far as digits go), and Huxley prophesied that such a creature would likely be found in North America. In truth, while it did not precisely fit the bill, a horse bearing a vestigial fifth toe had already been found and was collecting dust in Marsh’s Peabody museum. Writing to Huxley on July 12, 1877, Marsh revealed that little Eohippus (a name that was given up when it was discovered that Richard Owen’s Hyracotherium had priority, only to be later changed back to Eohippus in recent years) had been right under his nose all along;

I had him “corralled” in the basement of our Museum when you were there, but he was so covered with Eocene mud, I did not know him from Orohippus. I promise you his grandfather in time for your next horse lecture if you will give me proper notice.

Although the popular press did not take much note of the re-discovery of Eohippus, Huxley was well pleased, and promised to show Marsh all the “lions” of British science during his aforementioned 1878 visit. Such close ties would be important to Marsh later on, serving to keep Cope out of some respectable circles as well as giving Marsh a good amount of prestige. Oddly enough, however, Cope had his own horse genealogy (although ignoring Marsh’s labels) that went from four toes to one, and it was Cope’s “dawn horse” that provided the basis for some of the first Eohippus reconstructions, not Marsh’s. While Cope missed out on a golden opportunity in 1872 when he was provided a jaw fragment of an early horse, Marsh’s skeleton (as far as I can tell) remained locked away while Cope’s employee J.L. Wortmann uncovered the rest of Cope’s specimen of Eohippus in 1880. Cope named his animal Protorohippus, and it was his reconstruction that ultimately influenced Charles R. Knight and, subsequently, Rudolph Zallinger when he painted his famous Age of Mammals mural. For those who did not get at least a chuckle out of the last line, Zallinger created his mammalian masterpiece for the Peabody museum, the very establishment that O.C. Marsh had created to start his professional career.

As can be said of any scientist, however, Cope and Marsh were both products of their time and (especially in their respective cases) their egos, and while the fact that horses evolved was proved beyond doubt, both men made mistakes when it came to evolution. While Cope, late in his career, bemoaned the fact that Marsh had poisoned the well when it came to making connections with Huxley and other British scientists, it is doubtful that Cope would have lasted long amongst those of the Darwinian school of evolution. In the 1896 book The Primary Factors of Organic Evolution, Cope saw the evolution of the horse being orthogenic, or proceeding in such a way as to imply direction towards a more adapted or perfected form. As this concerns horses, Cope wrote;

Examination of all these genealogical lines reveals a certain definiteness of end and directness of approach. We discover no accessions of characters which are afterwards lost, as would naturally occur as a result of undirected variation. Nor do we discover anything like the appearance of sports along the line, the word sport being used in the sense of a variation widely divergent from its immediate ancestor. On the contrary, the more thorough becomes our knowledge of the series, the more evident does it become that progressive evolution has advanced by minute increments along a definite line, and that variations off this line have not exerted an appreciable influence on the result.

Such notions would have gotten Cope banned from Finch’s Beak gentleman’s association (if one had actually existed), the concept of directed evolution undermining one of the most important points that Darwin had attempted to make about the “transmutation” of life on earth. As we shall later see, however, such notions of orthogenesis may have had some influence on one of Cope’s latter-day pupils, Henry Fairfield Osborn, as well.


An illustration of the horse “Clique,” which had an extra toe on each fore-foot. Marsh examined this horse while still alive, and the horse was donated to Yale after its death in 1891. From Marsh, O.C. 1892. “Recent Polydactyl Horses.”

Marsh, as has already been determined, definitely had the attention of the progenitors of evolution by natural selection, and through the efforts of Matt Wedel, Randy Irmis, and Mike Taylor a number of Marsh’s writings have become available for viewing on the internet (The Marsh Repository). In a 1879 paper published some time after Huxley’s visit, “Polydactyle Horses, Recent and Extinct,” Marsh prefaces the rather short fossil section with several pages about known horses within recent history that had extra digits. The most typical condition for the differing equines was having an extra digit on the inside of the front hooves, one that did not touch the ground. Coupled with a brief appeal to similar observations of extra toes from development, this approach was indeed a wise one; not only do most living horses have vestiges of digits that have been lost, sometimes a multi-toed condition still occurs in living animals, seemingly fitting with the same trends seen in extinct genera.

Marsh's Geneology of the Horse
O.C. Marsh’s concept of “The Geneology of the Horse,” a decidedly straight-line progression. From Marsh, O.C. 1879. “Polydactyly Horses, Recent and Extinct.”

What is notable about Marsh’s interpretation of the history of horse evolution is how straightforward it is. Although missing Cope’s differing evolutionary hypotheses, Marsh makes no qualifications about the fossils he found representing only the “types” of different horses; horses evolved along a straight line, and while a few steps may be missing, it was not indicative of the widely branching pattern recognized by later scientists. The “extraneous” toes seem to become reduced in a gradual fashion, while size and tooth height increased (although the patterns on the teeth, as can be seen in the illustration, vary quite a bit in the “higher” forms). Perhaps Marsh’s adherence to a strict linear progression was at least partly inspired by the diagnosis of Huxley. In an obituary written by Marsh to commemorate Huxley’s life, Marsh made special mention of his horses;

One of Huxley’s lectures in New York was to he on the genealogy of the horse, a subject which he had already written about, based entirely upon European specimens. My own explorations had led me to conclusions quite different from his, and my specimens seemed to me to prove conclusively that the horse originated in the New World and not in the Old, and that its genealogy must be worked out here. With some hesitation, I laid the whole matter frankly before Huxley, and he spent nearly two days going over my specimens with me, and testing each point I made. He then informed me that all this was new to him, and that my facts demonstrated the evolution of the horse beyond question, and for the first time indicated the direct line of descent of an existing animal.

Such interpretations of evolution and the fossil record could only exist within a certain paleontological framework; the more bones that were found from different times and locales the more the old notions would splinter and crack. Vertebrate paleontologists who would succeed Cope and Marsh could not study what they did not have, however, but they still recognized the importance of the horse in showing evolution to be a reality. In 1891 Henry Fairfield Osborn, an independently wealthy Princeton professor and one of E.D. Cope’s friends and supporters during the embroiled Herald fiasco, was appointed the first curator of vertebrate paleontology at the American Museum of Natural History in New York City. The museum was somewhat embarrassed at not possessing any sizable collection of vertebrate fossil material, and even though Cope eventually sold some of his collection to the AMNH for a sum that disappointed the beleaguered Philadelphian, the halls of the great institution were still found wanting of ancient creatures that would bring it notoriety.

Osborn, despite his off-kilter ideas about human evolution that plagued his later years, largely made the AMNH what it is today, having some of the best and brightest collectors and preparators of the 20th century under his employ. While such gems as Barnum Brown’s two Tyrannosaurus rex skeletons, the specimen that remains on display today being Brown’s self-confessed “favorite child,” definitely helped to make the museum famous, some of Osborn’s favorite subjects were the fossil horses. Early on in his career, Osborn attempted to raise $10,000 from museum trustees for a project involving horse evolution, but the appeal was denied. Osborn kept at it and eventually succeeded, however, securing $15,000 from William C. Whitney in 1897, funds used to send collectors and curators like James W. Gidley, Bill Thomson, W.D. Matthew, and Walter Granger out into the field to collect ever more horses from Texas, South Dakota, Colorado, and other locales. Indeed, Osborn soon had many new horse fossils to study and display, creating one of the most notable (and among Biblical fundamentalists, controversial) displays of evolution ever presented to the public.

*(WWII caused the museum to send the first, more incomplete Tyrannosaurus rex skeleton, to the Carnegie Museum out of fear that the museum would be bombed and both would be lost. This may seem like an ill-founded fear, but many fossils like Spinosaurus were lost because German museums were struck with Allied payloads.)

Osborn did much to enhance the AMNH collections during the close of the 19th century, although his rather strange views about mammalian evolution (fueled in part by racism and part by Osborn’s membership in the Presbyterian church) never found wide acceptance. Despite his pet hypotheses, Osborn sent paleontologists far and wide in search of specimens to confirm his ideas, and at least in the case of the Roy Chapman Andrews expeditions during the early 1920’s, unexpected boons abounded. While Marsh held that he had moved horse ancestry out of the Old World and safely into America’s domain, Osborn saw the origin of major placental mammalian groups stemming from Asia (including the origin of humans), the hypothetical five-toed ancestor of the horse remaining elusive in North America because it was “really” buried somewhere in Asia. Osborn described his hypothesis as follows;

In the dispersal center, during the Age of Reptiles and the beginning of the Age of Mammals, there evolved the most remote ancestors of all the higher kinds of mammalian life which exist today, including, for example, the five-toed horses, which have not yet been discovered in either Europe or America. That the very earliest horses known in either Europe or America were four-toed indicates that their ancestors may have lost their fifth toe while still resident in the Asiatic homeland.

Roy Chapman Andrews did not bring Osborn any Asiatic five-toed horses from the expeditions into Mongolia in the early 1920’s, although the mammals Paraceratherium and Andrewsarchus were exciting enough in and of themselves. .

The lack of the most ancestral mammalian fossils did not stop Obsorn from attempting to further his own hypotheses, however, and in order to understand how straight-line evolution may have been presented at the AMNH we need to know how Osborn obfuscated the role of “chance” in evolution (using it almost in the same context as modern creationists do), calling the idea that natural selection works on random variations a “dogma.” Osborn instead preferred an Aristotelian “law,” quoting the philosopher in his 1917 book The Origin and Evolution of Life;

So far as law is concerned, we observe that the evolution of life forms is like that of the stars: their origin and evolution as revealed through palaeontology go to prove that Aristotle was essentially right when he said that “Nature produces those things which, being continually moved by a certain principle contained in themselves, arrive at a certain end.”

Such a notion could be regarded as the “Restless Gene” hypothesis, with what Osborn then referred to as the “hereditary-chromatin” in the animal filling needs as they arose in order to achieve a particular end. Despite his confusion about the role of “law” and “chance” in nature, Osborn did recognize that there were certain ratios in limb structures that were present in animals filling different ecological niches, even closely related ones. In the same book, Osborn writes the following about early horses;

No form of sudden change of character (saltation, mutation of de Vries) or of the chance theory of evolution accounts for such precise steps in mechanical adjustment [as in the limb structure of horses]; because for all the proportional changes, which make up ninety-five percent of mammalian evolution, we must seek a similar cause, namely, the cause of acceleration, balance or persistence, and retardation. This cause may prove to be in the nature of physiochemical interactions regulated by selection. The great importance of selection in the evolution of proportion is demonstrated by the universal law that the limb proportions of mammals are closely adjusted to provide for escape from enemies at each stage in development.

This chain of reasoning, such as it is, nearly works backwards from evolution’s “products” (which it is never done fiddling with), much like the lampoon (which I believe stems from Voltaire, although I have been unable to find the quote) that the nose was placed on the human face to hold up ones glasses.

Equus scotti
Assemblage of bones, illustrated as discovery in situ, of the Pleistocene horse Equus scotti. From Gidley, James Williams. 1900 “A new species of Pleistocene horse from the Staked Plains of Texas“. Bulletin of the AMNH ; v. 13, article 13.

Equus scotti
A mounted skeleton of Equus scotti at the AMNH, constructed out of two skeletons. From Gidley, James Williams. 1901. “Tooth characters and revision of the North American species of the genus Equus.” Bulletin of the AMNH ; v. 14, article 9.

Even though Osborn’s ideas of evolution did not catch on, the idea of horse evolution as a more-or-less straight line was still a popular one, at least in works and representations meant for public consumption. The diversity of fossil horses, thanks to many of the expeditions undertaken by Osborn’s department at the beginning of the 20th century, had considerably expanded, and the idea of an evolutionary “bush” for horses began to take root. Such a representation can be seen in one such generalized and “primitive” bush provided by J.W. Gidley in a 1907 paper on horses from the Miocene and Pliocene of North America. Indeed, the diversity of three-toed forms suggested that ancestry was perhaps more complicated than previously thought, more than one form of horse existing at any one time depending on the available habitats. Osborn noted this in his 1917 book as well, but it seemed to be only a supplementary bit of information behind his ideas of a biogenetic law. One of Osborn’s hires, J.W. Gidley, had a more accurate view of horse evolution, however, and produced the first known branching phylogeny of horses through the Miocene and Pliocene.

Old Horse Evolution Tree
A hypothesis as to the relationships of horse subfamilies by J.W. Gidley in 1907. This is the first known branching diagram for horse evolution. From Gidley, James Williams. 1907. “Revision of the Miocene and Pliocene Equidae of North America.” Bulletin of the AMNH ; v. 23, article 35.

As can be seen from Marsh’s earlier phyletic progression, much of horse evolution seemed to be dictated by features of teeth, the number of toes, and certain aspects of the skull, but as Gidley notes in his paper more material is needed if we are to truly understand the relationships of horses. Indeed, things were not so clean and neat as implied by Marsh’s illustration, even with the inclusion of new taxa. In the summary of the research, Gidley concludes;

As at present understood, the fact seems to be fairly well established that there is a considerable phyletic hiatus between the groups of the Equidae as above subdivided, which are as yet not bridged over by intermediate forms. Such a hiatus seems especially marked between the Anchitheriinae and the Protohippinae, while these groups greatly overlap each other in time. So far as indicated by any known species the Anchitheriinae could not well have stood in direct ancestral line to the latter group or to the Equiinae. There seems also to be almost as decided a gap between the Anchitheriinae and the known species of the older group, the Hyracotheriinie. The Equiinae may well have been derived from some species of the Protohippus division of the Protohippinae.

Outside of engaging in a more detailed study, Gidley also made note that various genera of horses overlapped in time with each other. While this does not rule out anagenesis entirely, it is a problem if there is such a large diversity of horses with similar features living alongside each other rather than a few isolated populations moving in a straight-line progression. The overlap was recognized and illustrated by W.D. Matthew almost 20 years after Gidley’s paper, showing where and when fossil horses existed;

Geological and Geographic Range of Equidae
Visual representation of the geological span and geographical ranges of equids through the Cenozoic. Such a representation could easily be misunderstood as endorsing straight-line evolution of horses. From Matthew, W.D. 1926. “The Evolution of the Horse: A Record and Its InterpretationThe Quarterly Review of Biology, Vol. 1, No. 2., pp. 139-185.

While the illustration, if followed closely, does show a branching pattern of evolution, to an untrained eye the evolution of horses through time seems to go in a relatively straight line, the overlap seemingly giving way to an almost orthogenic trend. I doubt that Matthew’s article was regular Sunday night reading for families of the late 1920’s and so I doubt that it contributed directly to mistaken notions of horse evolution, but another illustration from the same paper could more easily cause confusion;

Horse Evolution Simplified
A simplified, “straight-line” version of horse evolution (Click the image for a larger version). This figure was also reproduced in George McCready Price’s The Predicament of Evolution. From Matthew, W.D. 1926. “The Evolution of the Horse: A Record and Its InterpretationThe Quarterly Review of Biology, Vol. 1, No. 2., pp. 139-185.

This model is similar to Marsh’s (see above) in that horses seem to have followed a very simple ancestor/descendant progression through time. While it is true that living horses did have ancestors with multiple toes and we could trace their line backwards through time to the exclusion of other closely related genera, diagrams like this one seem to have “won out” in the public mind over those that more fully encompassed horse diversity. A 1940 paper by R.A. Stirton would be much clearer when it came to the branching horse lineage;

Stirton Horse Phylogeny
From Stirton, R. A. 1940. Phylogeny of North American Equidae. Bull. Dept. Geol. Sci., Univ. California 25(4): 165-198.

Stirton’s illustration is interesting as it shows a fairly straightforward line of descent through Miohippus is the Upper Oligocene, Miohippus giving rise to some side branches that would eventually go extinct before modern times. The radiation of the ancestors and close relatives of modern horses did not start, according to the phylogeny, until the Upper Miocene and Merychippus, Pliohippus eventually giving rise to Equus in the Upper Pliocene. Further, it is interesting to see how close Stirton’s phylogeny is to the work of later researchers, especially that fossil horse authority Bruce McFadden;

McFadden Horse Phylogeny
From MacFadden, Bruce. 1985. “Patterns of Phylogeny and Rates of Evolution in Fossil Horses: Hipparions from the Miocene and Pliocene of North AmericaPaleobiology, Vol. 11, No. 3. (Summer, 1985), pp. 245-257.

The phylogeny is extremely similar to Stirton’s through Parahippus, but the upper branches are a bit more detailed. Instead of having the genus Equus be a descendant of Pliohippus, Pliohippus is relegated to an offshoot that goes extinct before the Pliocene, the genus Dinohippus giving rise to Equus and the recent horses of the New and Old World in that genera. We will come back to the work of MacFadden later, but it is important to note how close the ideas of researchers in decades past were with modern understanding in this area.

Quinn Horse Phylogeny
From Quinn, J. H. 1955. Miocene Equidae of the Texas Gulf Coastal Plain. Bur. Econ. Geol., Univ. Texas Pub. 5516: 102 pp. (Click for larger image)

J.H. Quinn’s 1955 phylogeny of the horses of the Texas Gulf Coastal Plain was even more wildly branching than Stirton’s, and while Quinn’s focus was a bit more narrow, the tree is much more divergent. Other researchers had the genus Equus arising in the late Pliocene (and even as late as the Pleistocene), Quinn’s version has Equus appearing as early as the middle Miocene, Merychippus, again being nominated as the progenitor of all the subsequent forms in the area. While this version of horse evolution has been extensively reshuffled and revised, it is important to note that the idea that horses evolved in a straight line was well out of fashion by the middle of the 20th century at the very latest. Why, then, did it hang on for so long in the public mind?

Mercyhippus
A mount of Mercyhippus isonesus quintus. From Simpson, George Gaylord. 1932. “Mounted skeletons of Eohippus, Merychippus, and Hesperosiren.” American Museum novitates ; no. 587

Part of the problem with museums is that it takes a lot of time, money, and effort to revise exhibitions, and for some time the American Museum of Natural History horse display (THE display that illustrated horse evolution for many years) followed a progression like that of W.D. Matthew’s simplified diagram (see above). While this was eventually changed when the fossil halls were refurbished, it still seemed to show a straight line of descent, and even the display that stands on the fourth floor of the museum today reflects such a transition. If you read the plaques and take the time to compare the skeletons the branching nature of horse evolution is apparent, but the fossils themselves are arranged from Eohippus to Equus in a two parallel straight lines, showing an overall smaller-to-larger and many-to-one toe progression. Likewise, popular books on evolution and paleontology seemed hard-pressed to let go of straight-line evidence. While it could be said that such books were correct in that we could follow the line of descent from modern horses backwards to the exclusion of other groups, this approach seems to do more harm than good in the long run. Take A.S. Romer’s Man and the Vertebrates: Vol. I, for example. Originally published in 1933, my 1954 Pelican Books paperback edition shows the fossil limbs of Eohippus, Miohippus, Merychippus, and Equus from left to right, once again giving the illusion of a pure line of anagenesis. No hint of a larger diversity is given outside a brief mention of the modern forms of Zebra, Ass, and Prezwalski’s Horse.

Eohippus to Equus
Comparison of Eohippus to Equus. There’s a lot of evolution in that dashed line. From “The Dawn Horse or Eohippus” by Chester Stock (1947).

The 1966 edition of Romer’s Vertebrate Paleontology fairs better overall, but is still found wanting. The same straight-line illustration I just mentioned is found in the section treating perissodactyls as a group, and the skeletons of Eohippus, Mesohippus, and Hippidion are shown left to right across pages 266 and 267. While the text does mention an overall diversity of forms, as well as using certain genera for the “type” from which modern horses evolved, the overall visual impression of simple anagenesis remains. Again, I doubt the casual reader picked up Romer’s book for light nightly reading, but it is strange that the progressive ideas about evolution during that time are so poorly represented.

A similar time-capsule is Edwin Colbert’s Evolution of the Vertebrates, originally printed in 1955. The 1966 edition is the one that I acquired, and it is an interesting contrast to Romer’s book. At first Colbert seems to fall into the same trappings of straight-line evolution, showing a simple progression (in text with arrows) from Hyracotherium (Eohippus) -> Orohippus ->Epihippus -> Mesohippus -> Miohippus, spanning the Lower Eocene to the Upper Oligocene. After this progression, however, Colbert does note that there was a proliferation in forms;

By the end of the Oligocene epoch the horses had through these changes attained the status of advanced browsers, capable of eating leaves and soft plants and able to run fairly rapidly and for sustained periods over hard ground. With the advent of Miocene times there was a branching out of horses along several lines of development, probably as a response to an increase in the variety of environments available to them, and especially because of the spread of early grasses and other flowering plants.

An illustration on page 364 makes something of an attempt to reflect this visually, following the phylogeny of R.A. Stirton (see above) but in a more subdued and compressed manner. Being that only the genus names are mentioned, Colbert’s tree looks especially bare, although it must be conceded that it is a more accurate depiction of horse evolution than Romer’s. The illustration on page 148 of the 1961 paperback edition of G.G. Simpson’s Horses more closely follows Stirton’s phylogeny as well, and the plates likewise show the branching of tooth shapes and other characters rather than grouping forms separated by large expanses of time. The relatively rich fossil record of horses would be important to Simpson in another way as well; in his Neo-Darwinian Synthesis-era work, Tempo and Mode in Evolution (1944), Simpson was able to conclude that horses in general seemed to evolve faster than unrelated groups of animals like ammonites but more slowly than mammals like elephants. Although his hypothesis of a near-constant, albeit accelerated, rate for horse evolution has not held up today, the idea that evolution can occur more quickly or more slowly was a very important idea, an idea that took new form in Eldredge & Gould’s hypothesis of punctuated equilibria decades later.

McFadden Horse Phylogeny
From McFadden, Bruce. 2005. “Fossil Horses – Evidence of Evolution.” Science Vol. 307. no. 5716, pp. 1728 – 1730

So what of our current understanding of horse evolution? As I had mentioned earlier, one of the foremost authorities on the topic is Bruce MacFadden, and in 2005 he authored a straightforward summation of the current state of things in an article entitled “Fossil Horses – Evidence for Evolution.” As MacFadden notes, the overall “look” for the tree, featuring lines that did not leave modern descendants, hasn’t changed much since the time of Stirton and other earlier scientists. There has been much shuffling around and plenty of new discoveries, however, and although the diversity of late horses often gets the most attention it is now being revealed that early members of the horse lineage had a wider diversity as well. It almost seems like there’s an evolutionary bottleneck during the Oligocene, with the beginnings of more diversity in the Miocene, Mercyhippus once again leading the charge on to later forms.

MacFadden also takes a moment to correct a common misconception about horse evolution; there was no unalterable progression from small to large consonant with Cope’s Rule;

Although the 55-My old fossil horse sequence has been used as a classic example of Cope’s rule, this notion is now known to be incorrect. Rather than a linear progression toward larger body size, fossil horse macroevolution is characterized by two distinctly different phases. From 55 to 20 Ma, primitive horses had estimated body sizes between ~10 and 50 kg. In contrast, from 20 Ma until the present, fossil horses were more diverse in their body sizes. Some clades became larger (like those that gave rise to Equus), others remained relatively static in body size, and others became smaller over time.

Still, our current understanding is incomplete, and further fossil finds will continue to rustle the branches of the evolutionary bush. In fact, I would not be surprised if more early forms came to light, and I would be especially interested to see if the “Oligocene Bottleneck” is real or merely a factor of fossil collecting bias. There should be no mistake about the amazing entanglement of branches horses represent, however, and it is somewhat surprising that the public does not often hear about the true form of horse phylogeny. While I did not do an in-depth study of how horse evolution was portrayed in the popular media, from what I have seen it seems that past scientists and authors have often opted for simplicity, getting the public to accept evolution has occurred being more important than giving them an accurate depiction of how evolution works. This is a harsh lesson that we are still learning, as inaccuracies in books, museum displays, and other outlets can leave the door open for creationists to spread distrust of science. It is not enough to merely present someone who is unfamiliar with evolution with our “best” example of anagenesis; if we do, it should be in context with the larger theme of unity and diversity of forms, not a throw-away that is supposed to dazzle in and of itself. The evolution of the horse, in fact, is a perfect example of evolution and can be an extremely powerful tool in education if used properly, but for whatever reason the common theme so far has been for many popular science writers and educators to fall out of the saddle.

Evolution is truly an amazing phenomenon; who would have ever conceived of a small, four-toed animal giving rise to Black Beauty? Our overall conception of “more” being better may even make such a move from four toes to one seem counterintuitive, yet the evidence (from fossils to that of development) is clear in its implications. Horses did not spring up out of the ground from the dust, nor were they “spoken into being” by an omnipotent power that decided that Adam should have a faithful steed. Every bone in their body cries out as to their past, and we are all the more enriched to understand that just like the horse, Homo sapiens is a still-changing product of a long and rich evolutionary history, too.





Mega-Post Preview

11 09 2007

I’ve been working on a brand new historical/evolutionary review post all day, and I’m sad to say it’s not yet completed. I was expecting a finished product by now, so as a substitute, here’s the first few paragraphs of the post that should give you some clues as to what I’ll be covering. I haven’t gone back to fully edit yet so please excuse any errors. I hope to have the whole thing finished tomorrow (it’s currently a little more than halfway done);

Eohippus
One of Charles R. Knight’s interpretations of Eohippus

When the name of O.C. Marsh is invoked, it is often to tell of his participation in the great “Bone Wars” of the late 19th century, sparring with fellow osteophile E.D. Cope in the pages of the New York Herald. Twisted tales of deceit and sabotage were promulgated in the sensationalist paper, and while both men helped to bring about an American revolution in vertebrate paleontology, the scars of their bitter squabbling have yet to fully heal. Such scientific in-fighting might seem worthy only of a historical footnote or an introduction to the stereotyped image of “smash-and-grab” paleontology of the time which is almost romantically referred to, but the truth of the matter goes far deeper than the public beard-pulling that is so often remembered.

The tiff between Cope and Marsh is strange in that is seems to exist in the popular literature out of time, removed from the context in which it had originally existed. Charles Darwin had published his earth-shaking work On the Origin of Species by Natural Selection a scant 31 years before the ink almost ran red with rage on the pages of the Herald, the question of evolution being of far more importance in the public consciousness than dinosaurs. The full establishment of the dinosaur as a cultural (and dare I say, mythical) creature in the mind of the American public only seemed to take place after the Bone Wars, the appointment of Henry Fairfield Osborn to the American Museum of Natural History (specifically hired to establish a vertebrate paleontology program) and the popular reports of the dinosaur that carried the namesake of Andrew Carnegie, Diplodocus carnegei, being the more immediate beginnings of the public’s love affair with the extinct creatures. Before Brontosaurus and Tyrannosaurus became household names, the public eye was focused upon horses and birds.

The latter half of the 19th century was a stirring time for biological science, especially involving the new areas of vertebrate paleontology and evolution, the august authorities in England keeping on eye on the up-and-comers starting their own careers in the states. Early on, paleontologist E.D. Cope impressed T.H. Huxley with his 1866 discovery of Laelaps aquilunguis, but in a paleontological clean-sweep Marsh would eventually have his name attached to Cope’s dinosaur and the admiration of not only T.H. Huxley, but Charles Darwin himself. As for the renaming of Laelaps, Marsh found that the name was already taken by a genus of mite, renaming the New Jersey greensand dinosaur Dryptosaurus in 1877 (although Cope, throughout the rest of his career, called the dinosaur Laelaps). It would take more than some taxonomic shuffling to impress the eminent British anatomists and paleontologists, however, and Marsh’s ticket into Huxley’s good graces came in the form of toothed Cretaceous birds like Hesperornis (Marsh, 1872).





There is a grandeur in this view of life…

7 09 2007

Gorilla
A female gorilla at the Bronx Zoo.

It is often all-too-easy to forget about the wonder that is in nature when one becomes embroiled in the culture war surrounding evolution and creationism. The battles are fought in public classrooms, sundry media outlets, and (perhaps most of all) the internet, but those who do recognize the intricacy and beauty of evolution should not forget to step back every once in a while and look at what so-inspired Darwin in the first place. Nature offers up more treasures and wonders than I could ever fully appreciate during my short tenure on this planet, and without this sense of unity and amazement science can quickly turn into a rather dry and forbidding set of mental exercises.

Aldo Leopold recognized this problem all too well. In his essay “Song of the Gavilan”, collected in the A Sand County Almanac (which should be required reading for any naturalist), Leopold tells of how bright minds are often told to ignore the “music” of nature;

There are men charged with the duty of examining the construction of the plants, animals, and soils which are the instruments of the great orchestra. These men are called professors. Each selects one instrument and spends his life taking it apart and describing its strings and sounding boards. This process of dismemberment is called research. The place for the dismemberment is called a university.

A professor may pluck the strings of this own instrument, but never that of another, and if he listens for music he must never admit it to his fellows or to his students. For all are restrained by an ironbound taboo which decrees that the construction of instruments is the domain of science, while the detection of harmony is the domain of poets.

Professors serve science and science serves progress. It serves progress so well that many of the more intricate instruments are stepped upon and broken in the rush to spread progress to all backward lands. One by one the parts are thus stricken from the songs of songs. If the professor is able to classify each instrument before it is broken, he is well content.

Science contributes moral as well as material blessings to the world. The great moral contribution is objectivity, or the scientific point of view. This means doubting everything except facts; it means hewing to the facts, let the chips fall where they may. One of the facts hewn to by science is that every river needs more people, and all people need more inventions, and hence more science; the good life depends on the indefinite extension of this chain of logic. That the good life on any river may likewise depend on the perception of its music, and the preservation of some music to perceive, is a form of doubt not yet entertained by science.

It would be a mistake to paint all practicing scientists with such a broad brush, but the danger of becoming so objective that the melodies of songbirds and the soft rushing of streams become muted is a very real one. This is strange, especially because it was from wonder that science was born, an attempt to explain what had hitherto been subjugated beneath superstition and religion as it exists, not how we may wish it to be. Still, despite the move away from superstition and natural theology, especially since the writings of Darwin came crashing into the public consciousness, religion attempts to retain a hold on “the birds of the air,” “the beasts of the field,” and “every thing that creepeth upon the earth.” In a recent post on the evolution/creationism debate, the author of the blog Doxoblogy opined;

I’ve got only this to say…looking at creation will inevitably point you to an ‘eternal power and divine nature’ that exists beyond us. Looking in Scripture will introduce this ‘eternal power and divine nature’ to you as the Creator God to whom we owe our love and worship (Romans 1:16-2:11). The God of creation is also the God of Scripture and He has a Son, Jesus.

Clearly there is a sense of awe operating here, and there are seemingly countless flash-animated greeting cards, books, videos, and other resources enforcing the notion that every aggregate of soil, blade of grass, or molecule of water practically screams that life was created by the Judeo-Christian God of the Bible. Such arguments have even become politically fashionable, allowing current presidential candidate John McCain, in an attempt to eat his cake and have it too, to say “I believe in evolution. But I also believe, when I hike the Grand Canyon and see it at sunset, that the hand of God is there also” when asked if he “believed” in evolution. Such a notion clearly points to subjective notions of beauty, as McCain did not say “When I look at a lamprey or a hagfish, I see the hand of God at work.” The overall association of God with the aesthetically pleasing could explain why Thomas Kinkade paintings, which differ so little that I can scarcely tell one from another, as a staple in evangelical Christian households. Perhaps there are some on the fringe that would prefer to think of a tapeworm or liver fluke when contemplating the glory of God, but the vast majority of creeping, crawling, sucking, oozing, and pulsating things on the planet are not generally thought of as being “first in the ways of God.” Even Darwin expressed his doubts about a Creator that was seemingly so cruel. In a famous letter to the American botanist Asa Gray, Darwin confided;

I am bewildered. I had no intention to write atheistically. But I own that I cannot see as plainly as others do, and as I should wish to do, evidence of design and beneficence on all sides of us. There seems to me too much misery in the world. I cannot persuade myself that a beneficent and omnipotent God would have designedly created the Ichneumonidae [parasitic wasps] with the express intention of their [larvae] feeding within the living bodies of Caterpillars, or that a cat should play with mice.

While generally forgotten today, the view that nature is filled with goodness or pleasurable things was, as far as I currently understand, put forth by William Paley in Natural Theology, one of Paley’s core beliefs being that the ability to feel pleasure (and not just pain) was evidence of a beneficent Creator. Still, the understanding that Darwin helped usher humanity, albeit kicking and screaming, into was an understanding of nature that is neither inherently good nor evil. The world is not “for” mankind just as it is not expressly for the benefit of water beetles, Cape Buffalo, or the Northern Flicker. If it were otherwise the world could perhaps divided into creatures that were “good” and others that were “evil,” but no clear distinction exists nor has it ever; attributing such labels to the world around us only speaks to our own ignorance and hubris.

Sometimes I have to wonder if those who propose to have seen God in nature have truly spent any time out in nature or studying its diversity. A poster of some far-flung locale at sunset with one of the Psalm’s printed on the bottom is not understanding nature, a motley organization of life that (try as we might) we are still very much a part of, and even the most repulsive or disgusting of creatures has a worth that does not rely on our “refined” tastes. When an animal dies, insects and bacteria take advantage of the bonanza, putrefying and decomposing the body , beginning the process that will return the creature to the earth. If special circumstances occur, it may well see the light of day again as a fossil, but more likely than not it was be completely broken up, the accumulation of energy and elements in its body being transferred into other organisms and into the ground, allowing different forms of life to flourish. This does not make a maggoty, decomposing carcass any more attractive (or smell any more fragrant), but if we divorce it from our rather superficial requirements of beauty, we can gain an understanding of nature that previously eluded us.

Perhaps my words are only those of a young man, “green” in terms of experience and landlocked in a land of impervious surface and strip malls. Such inexperience may hinder my perceptions, but when I look closely at nature I see neither angels nor demons, God and the Devil being absent from the crashing of the waves along the shore or the lighting strikes of a late-August thunderstorm. This does not mean, however, that I view nature divorced from any sense of awe or deeper emotional feeling, and I would imagine that many of my readers here would tell you the same. When I was covered in wet muck of the Inversand Marl Pit, my heart skipped a beat as a pulled out a chocolate-colored bone fragment that had been kept snug in the greensand for at least 65 million years; my mind reeled at what I could have discovered (and what else might remain buried), and I had to hold back my excitement as I asked my professor if I had really found bone or not. Any book describing the adventures and work of a more professional and seasoned paleontologist or field scientist will reveal much the same thing; objectivity is necessary for the sake of accuracy, but it often comes after a rush of excitement or amazement at a new observation or discovery.

This post is a bit of a throw-away, however, as Charles Darwin long-ago succinctly summed up the thesis of my long argument;

There is grandeur in this view of life, with its several powers, having originally been breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved. – Charles Darwin, On the Origin of Species by Natural Selection

Gorillas
A female gorilla and two babies at the Bronx Zoo





Convergence or Parallel Evolution?

6 09 2007

Many of the world’s great natural history museums devote at least one hall to creatures that no longer exist today. In the old tradition, in order to keep any young upstarts from getting any ideas about evolution, skeletons or parts of skeletons were grouped by the functions they performed, a visitor being likely to find the wing of a bat and the wing of the bird in the same display case even though the two animals extremely distantly related. Newer layouts, conversely, have largely ignored the end-function of one line or another to group animals together by homology and their shared characters, the most well-known example being the remodeled 4th Floor of the American Museum of Natural History in New York City which has attempted to arrange its fossil collections as a walk-through cladistic diagram.

Still, the generally discarded of grouping animals by their adaptations to general habitats or niches is not without it’s charms. Over and over again, evolution has produced forms that seem to converge on certain body plans, varying habitats making some traits advantageous and others a liability, helping to adapt different organisms to their local ecologies. Flight has independently evolved several times (and the ability to glide an even greater number of times), as well as adaptations to marine environments, saber-like canine teeth, immense sails along the spine, and slicing premolar teeth, although each time such familiar features seem to arise it shows that there is more than one way to solve an evolutionary problem from any given point in an organism’s natural history. Not everything can be chalked up to convergence of form in order to carry out particular functions, however. Parallel evolution, although sometimes difficult to determine, also allows relatively closely related forms to take the same evolutionary paths, showing many of the same anatomical characters even though they diverged from a common ancestor at some point in the past and occupy at least two different lines of descent. In fact, it is often these weird and wonderful creatures that are forgotten or overlooked, more people recognizing the term “saber-toothed cat” (or, loathe as I am to say it, “saber-toothed tiger”) or the genus Smilodon than the term “Nimravid” or the genus Dinictis. The following entry, therefore, will be an attempt to navigate through the somewhat “entangled bank” of evolutionary relationships among animals that appear to be shaped in similar ways by the environment but constrained by their species’ history, showing us that there is more than one way to make a saber-toothed cat.

Back into the pool: Of Ichthyosaurs, Sharks, and Cetaceans

Perhaps one of the most well-known (or at least widely cited) examples of evolutionary convergence has been that of the similar body shapes of sharks, ichthyosaurs, and cetaceans. It’s difficult to see these three distinct groups of creatures side by side and not recognize the similarities, but why are they similar in the first place? If they belong to groups that are distantly-related branches of the evolutionary “bush,” why should they have developed similar body forms?

Icthy Shark Porp
One of the most well-known examples of evolutionary convergence; (From Top to Bottom) An ichthyosaur Ophthalmosaurus icenicus, a Porpoise, and a Spiny Dogfish (Squalus acanthias)

Shark Icthy Porp
From the 1925 creationist book The Predicament of Evolution by George McReady Price.

Creationists have been quick to seize upon the idea of convergence as if it were one of evolution’s weak points. In 1926, George McCready Price wrote the following in one of the more well-known early American anti-evolution texts, The Predicament of Evolution;

For instance, we have the shark, the ichthyosaur (an extinct kind of fish-shaped reptile), and the dolphin (a true warmblooded mammal, and not a fish at all), all of which greatly resemble each other in external shape and general appearance. Each has the same long, sharp snout, the same powerful tail, the same general fishlike shape. And yet the first of these is a true fish, the second was just as true a reptile, while the third is a mam-mal, bringing forth its young alive and feeding them by milk, just as does a cow or a horse, though it lives in the sea.

Here the evolutionists have to say that this peculiar shape and general form has been evolved separately and independently in each of these three instances. Indeed, Henry Fairfield Osborn, President of the American Museum of Natural History, New York City, declares that a very similar shape and form has been independently evolved “at least twenty-four times.”—”Encyc. Brit.,” Vol. XX, p. 578…

From this large group of facts we become convinced that these many similar or identical structures, which must have been evolved quite independently (if evolved at all), make too great a draft on our credulity. At least, these hundreds of examples of “parallel evolution” greatly weaken our confidence in homology, or similarity of parts and organs, as a proof of blood relationship.

Such arguments have become traditional amongst creationist apologists, suggesting that if convergent evolution does occur then we must throw homology out the window as similar structures will only mislead us as to the true affinities of the creatures being studied. As we will later see with Cuvier’s Ptero-dactyl, this can be a danger for scientists who are unwary and wish to shoehorn creatures into existing taxonomic categories, but not for those who actually look beyond superficial appearances.

The reason why the shark, the ichthyosaur, and the porpoise should all look vaguely the same is because they live(d) in the same environment; the ocean. An organism that is suspended in a fluid that is much denser than air can be adapted in various ways to such an “alien” environment, but physics does dictate what shapes can be taken based upon life history. It is possible to be a floating filter feeder, exhibiting a round shape, but such a strategy is essentially out of the question for animals that need to move quickly and to hunt for food. What is required is not only a powerful propulsive organ to keep the organism moving forward, but also extra appendages to allow for the control of movement and a streamlined shape to reduce drag (and hence reduce energy costs for moving through the water).

Knight
One of Charles R. Knight’s renditions of an ichthyosaur.

In fact, sharks as a whole provide a good model for various forms of ichthyosaurs. While ichthyosaurs are generally presented as already being streamlined and possessing a large caudal fin with two equally long lobes, we would be loathe to forget that they too are products of evolution and many fossils show us that they were not always an Euryapsid (thank you, johannes) answer to modern-day Lamnid sharks. Early ichtyhosaurs actually had more of a “bump” towards the back of their tail rather than a full-blown caudal fin, their overall body shape and lack of a large propulsive surface keeping them from moving too quickly through the water. A similar tail type/form can be seen in many modern day sharks like the Nurse Shark, which generally live along the bottom feeding on crustaceans and inhabitants that can be sucked out of coral crevices. Being that ichthyosaurs lack gills, it is unlikely that their early representatives were bottom-dwellers, instead preferring shallow areas, which can be especially productive in terms of food.

Modification of the “tail kink” (which was at first thought to be a taphonomic feature, early reconstructions showing “amphibious” ichthyosaurs with straight tails) seen in early forms allowed for the eventual evolution of a crescent-moon shaped tail, as well as adaptations in the skull and of the limbs into fins (the addition of digits and the addition of bones in the digits being quite common in the latest forms). This more-familiar shape would allow ichthyosaurs maximum propulsion with their caudal fin (the spine going downwards instead of upwards, as in sharks) while they would be able to exert control over their motions with their pectoral fins and would be kept from rolling in the water by their dorsal fins. The evolution of large eyes and other features aside, the overall shape and basic skeletal structure of ichthyosaurs seems to be an optimal design for medium-to-large, fast-moving, oceanic predators (although mosasaurs, pliosaurs, and plesiosaurs took different evolutionary routes).

What allowed ichthyosaurs to develop an effective side-to-side motion of the tail would not work for cetaceans, however. Ichthyosaurs developed their mode of propulsion by side-to-side motions of the spine, perhaps swimming in a mode similar to eels or cat sharks at first, a common form of locomotion in modern reptiles. This sort of motion is usually accomplished on land via a sprawling gait, the limbs being held out to the sides and the animal exhibiting a bit of a side-to-side motion as it moves along.

Whether the ancestors of icthyosaurs were sprawlers (to a greater or lesser extent, predisposing them to side-to-side motions of the tail and body) or not, cetaceans evolved much more recently in evolutionary history, and developed from ancestors that carried their legs directly underneath their body. The plasticity of early archaeocetes and their artiodactyl ancestors was greatly diminished, their hip and spine structure adapted to up-and-down undulations rather than the side-to-side motion seen in the video of the salamander. This sort of constraint has not stopped mammals from becoming adapted to the water, however, and clues to the evolution of cetacean movement can be seen in living animals like Giant River Otters;

In the water, undulations of the spine accompanied with some propulsion from the limbs proves to be very effective, and it’s not hard to imagine an archaeocete like Ambulocetus, as my friend Neil so aptly described, as a “sexy otter.” Once undulation of the spine became established as a method of moving through the water, the eventual addition of a tail fluke would do for cetaceans what the crescent-shaped tail of tuna, sharks, and icthyosaurs acheived in terms of speed and power, the body being adapted towards a streamlined appearance with (again) the pectoral fins providing lift/control and the dorsal fin preventing rolling. Larger forms of whales, namely the Mysticetes or Baleen Whales, grew to immense size and gave up some of the features that seem to be convergent with sharks and the smaller ichthyosaurs (some, in fact, did acheive whale-size), but they are derived from more predatory designs and their niche as massive, far-ranging suspension feeders free them from some constrains while imposing some new ones.

Harder Ichthyosaur
A painting of leaping ichthyosaurs by Heinrich Harder (circa 1916)

Human engineering has recognized similar constraints for motion in the water and even in the air; planes and submarines most closely resemble sharks and dolphins in overall shape, the placement and size of the wings on a 747 having much the same function as the large pectoral fins of far-ranging pelagic fish like the Blue Shark. Life in the water adapted all three groups of animals towards the same shape because there does not seem to be any other way to be a fast-moving, medium-to-large sized marine predator; speed and some degree of maneuverability are paramount. Some other lines have diverged from this shape, as noted before, but the sharks, dolphins, and (I don’t think it’s too much of a stretch to say) ichthyosaurs all occupied essentially the same niche and therefore were adapted in a particular fashion.

Do not think, however, that the convergence of three lines towards one body plan gives credence to a kind of “orthogenesis” or progressive force driving evolution. There was no sort of supernatural or external force manipulating the genetic material of these groups with the shape of a dolphin or shark in mind. Rather, the environment and local ecology determined what form would be favored through time, and even though the three groups may look the same and have significant convergences, they also have many traits in common with their ancestors, allowing us to trace their evolutionary history (which is why no one is arguing that dolphins, sharks, and ichthyosaurs are closely related or form a small monophyletic grouping).

A marsupial you wouldn’t want to meet

Living members of the Carnivora (bears, cats, dogs, civets, weasels, etc.) have always caught my attention, but there was an entire group of carnivorous mammals, now extinct, that have left no living representatives. The last known member of this group was named Thylacoleo carnifex by Richard Owen, and it has some of the strangest dentition ever seen in a marsupial. Marsupial mammals are well-known in Australia, creatures like kangaroos, koalas, and wombats coming most immediately to mind out of living extant taxa. There was a much more diverse population of marsupials during the Pleistocene, however, and the “marsupial lion” was likely a formidable predator.

Thylacoleo
A skull of Thylacoleo on display at the AMNH.

In order to understand why Thylacoleo is relevant to our discussion of convergence we need to first understand what makes living placental Carnivores so special. Many carnivores, especially cats, have a rather specialized dentition, certain molars and premolars making up what is known as the “carnissal shear.” These teeth are pointed and act like scissors, easily cutting up flesh or crushing bone. The molars behind the shear are often reduced (some groups have retained their molars in order to incorporate a more generalized diet, like dogs and bears), the dental specialization perhaps being one of the keys to the success of this group. Earlier predators of now-extinct lines like Mesonychids lacked such specialized cutting teeth, and the teeth behind the canines of the large Andrewsarchus show that their oral tool-kit was a bit more blunted.

Andrewsarchus
The skull of Andrewsarchus, on display at the AMNH

Thylacoleo, a carnivorous marsupial not descended from the Miacids that gave rise to living carnivores, also developed something of a “carnissal shear” but in a different way. Rather than a battery of teeth that became sharpened, one of the upper and lower premolars of Thylacoleo became elongated and blade-like, and the cleaver-like teeth helped to sharpen each other as they moved past each other when opening or closing the jaw. Thylacoleo also had a terrible bite, the attachments for the muscles that opened and shut the jaw were massive, somewhat constricting the amount of space the brain could take up, but giving Thylacoleo what was perhaps the most powerful bite forces amongst mammalian predators, especially given it’s relatively small body size (it was only about four feet long and 220 pounds).

Thylacoleo is an odd marsupial in another respect; the claw on its thumb was retractable like that of a big cat. This sort of adaptation is especially useful in keeping claws sharp, and perhaps keeping the claws sharp would allow Thylacoleo to get a good hold on its prey before going to work on it with its teeth. At this point I should probably mention that some scholars in the past have thought that Thylacoleo was an herbivore, not unlike the extant marsupial Phalangers. I will leave the response to such an argument to Richard Owen;

These eminent authors received the support, in reference to objections to my conclusions, of the (then) Curator of the Australian Museum, Sydney, Mr. GERARD KREFFT, who, in his contribution to the ‘Annals and Magazine of Natural History,’ series 3, vol. 18, 1866, p. 148, records his opinion that “the famous marsupial Lion was not much more carnivorous than the Phalangers of the present time.”

The species of carnivorous Phalanger is not named. No evidence of such by fossil specimens has reached me, nor have I found such exceptional habit of an existing species of Phalangista elsewhere noted.

As my friend Zach has noted, however, calling Thylacoleo a “marsupial lion” is a bit misleading. Even though some lion-like aspects of the skull (the results of convergence on a hypercarnivorous lifestyle, and Thylacoleo means “pouched lion”) led the anatomist Richard Owen to name the creature on the basis of such resemblances, the ways in which Thylacoleo shows its marsupial affinities are much more important. Referring to this animal as the “marsupial lion” without qualifications (as well as calling the extinct Tasmanian Tiger the “marsupial wolf”) usually confuses more than illuminates, and creationists often take the names and superficial resemblances to mean that evolution didn’t occur. Instead, they propose that God made the beginning of a “kind” of carnivorous mammal which was preserved on Noah’s Ark and gave rise to all later forms, important reproductive habits deemed to be of little consequence.

Even so, Thylacoleo carnifex and its relatives represent a branch of marsupials that became fairly specialized predators, and given the plasticity of tooth structure, it’s not hard to see how sharp premolars could be adapted into a blade to cut flesh. While it may be easy to draw connections between this animal and living carnivores, however, perhaps we should be more measured in our descriptions; both groups met the same challenges in similar ways, but the differences are far more striking and important in this example of convergence on a particular niche.

On what day were the Ptero-Bats created?

Pterodactylus
An engraving of the creature now known as Pterodactylus antiquus, the very one described by Collini.

Before there were natural history museums, there were motley assortments of organic odds and ends known as curiosity cabinets, and in the cabinet of Karl Theodor there would eventually come to be a petrified treasure. Although it was probably collected around 1767, the first known pterosaur fossil was not described until 1784, when the appointed caretaker of the collection, Cosimo Alessandro Collini, attempted to determine the nature of the strange creature that came to him from the limestone of Bavaria (the same deposits that later yeilded Archaeopteryx). Although certain that he was the remains of an animal from an earlier time, Collini was agnostic about what kind of animal he had come to possess. Years later, the famed anatomist Georges Cuvier investigated Collini’s paper and illustrations, noting that the creature was certainly a reptile. Still, the fossil would remain without a proper name until Cuvier would write a more detailed analysis in 1809, dubbing the fossil “Ptero-dactyle.”

Not everyone agreed with the analysis of Cuvier, however, especially since Cuvier did not get to see the fossil himself and had to work from the drawings in Collini’s paper. Samuel Thomas von Soemmerring, of the Bavarian Academy of Science, thought that the pterosaur was some unknown type of bat, a view that would remain entrenched in the minds of some scientists for many years. Indeed, one restoration by Edward Newmann in 1843 (and “re-drawn” for Gosse’s work Omphalos, as shown below), depicted the two known types of pterodactyl known at that time as fuzzy bats, complete with cute little ears. It is clear from the drawing that pterosaurs do not make good bats, although this didn’t stop many German paleontologists from taking such a stance through the first half of the 19th century.

Ptero bats
Newmann’s “marsupial bats”, conspicuously missing their ears, from Gosse’s Omphalos. It’s likely that Gosse recognized the reptilian nature of these Pterodactyl by the time he wrote his book, so Newmann’s work was copied minus the more mammalian aspects.

But why was there such confusion? It is likely because there is something familiar about pterosaurs that had been seen in living bats; the extension of digits to hold a membraneous wing. While the first fossil, despite wonderful preservation, did not preserve a membrane impression, it is hard to look at it and not recognize the superficially similar structure of a bat’s wing, which also carries a membrane to enable flight. In fact, birds seem a bit unusual in developing feathers for flight; many varieties of gliding and flying creatures have taken to the air (regardless of whether they engage in powered flight or glide) by the use of membranes. Indeed, gliding may often precede powered flight, and once an animal has developed a membrane that can be stretched between its limbs to glide, the extension of the digits at the point(s) of attachment can help to expand the wing size. Such changes likely occur as a result of changes in development, natural selection favoring the invasion of a new niche based upon variations that exist in a population, although in the case of pterosaurs we can no longer test to see if this is correct.

As we just saw with Thylacoleo, however, the convergences of pterosaurs and bats are rather slight, overall. While both acheived flight on membraneous wings attached to extended digits (many more in the case of bats) and have relatively compressed bodies, pterosaurs had a much greater diversity in shape and size than modern bats. Likewise, they did not elongate the rest of their fingers, suggesting that there was some situation (be it climbing or hanging on to a perch) that the pterosaurs still needed their other fingers for (although bats can climb pretty well with their thumbs, and some have even evolved suction disks). Still, it can be said that both took to the air by similar means and had to deal with similar constraints, but their evolutionary paths are far more divergent than that of the aforementioned sharks, ichthyosaurs, and cetaceans.

It doesn’t look like much of a planet-eater to me

Gharial
A female Gharial at the National Zoo in Washington, D.C.

Perhaps one of the most unnecessarily confusing groups of extinct animals are the phytosaurs. Filling the niche now occupied by reptiles like the Saltwater Crocodile, the water-dwelling archosaurs have left no living descendants despite their past diversity. At first glance, the phytosaur Rutidon looks just like a modern-day Gharial, and even though it shares a common ancestor with the reptiles that now exist in tropical watery habitats all over the world, it is not otherwise related. The most prominent phytosaur feature is that their noses are over or just anterior to their eyes on their head, not at the end of their snouts. This would allow them to breathe while completely submerged, although their eyes might not have been above water when hiding in such a manner. Even beyond this feature, their jaws seem to be fairly simple, merely having a hinge at the back to open-and-close. Compare this arrangement, here represented by the giant Machaeroprosopus gregorii, with the more complex reconstruction of the true crocodilian Deinosuchus (although, admittedly, this reconstruction was heavily based upon the living Cuban Crocodile and may not be fully accurate. It still serves to show the differences between the groups, however).

phyto
Machaeroprosopus, currently on display at the AMNH

Deino
Deinosuchus reconstruction, formerly on display at the AMNH

The most notable difference are the complex bones at the back of the throat of Deinosuchus which are arranged to slide past each other as the jaw opened and closed. No such feature is seen in the giant phytosaur. Still, even after the phytosaurs died out, crocodilians did not return to the water until about the Cretaceous period, many forms being absolutely terrifying land predators that have also long been extinct. One of the early forms was Protosuchus, a small true crocodilian that represented a line that changed little during its tenure on the earth.

Protosuchus
Reconstruction of Protosuchus

Outside of walking relatively high off the ground, Protosuchus had a foreshortened snout which was lower than its eyes, quite different from the arrangement in living crocodilians. As seen in the Dwarf Caiman photograph, below, living crocodilians have their eye sockets on the top of their head, their eyes sticking out on the surface as well as the tip of their nose when they lie in wait for prey (or just rest, for those who would like a less sensationalist tone). Protosuchus, by contrast, has eyes to the sides of the head, even facing somewhat forward, showing that it was much more well-adapted to the land than any swamp or shallow pool. Crocodilans did eventually enter the water, however, and their fossils are among the most common of any vertebrates. Some, like New Jersey’s very own Thoracosaurus, even became marine species, and a few varieties evolved crescent-shaped caudal fins on the ends of their tails to help them swim. The common belief, however, is that crocodiles have always been crocodiles, “changing little since the time of the dinosaurs,” and such generalized half-truths do little justice to crocodilians or their distant phytosaur cousins.

Dwarf Caiman

Saber-toothed Nimravid doesn’t sound quite the same…

Many museums have cases devoted to the great saber-toothed cats of epochs long gone, but it would take someone with more than just a cursory understanding of paleontology to sort out what is really being displayed. Saber-teeth, or elongated canines, have evolved many times over in the course of mammalian history, showing up in herbivores like the living Musk Deer as well as extinct groups like the gorgonopsids. Animals as different as a Musk Deer and Inostrancevia are fairly easy to tell apart, even for the non-specialist, but what about nimravids and the “true” saber-toothed cats?

Nimravids
A diagram of the three ideas of Nimravid/Felid evolution.

James Whitcomb Riley is purported to have once written “When I see a bird that walks like a duck and swims like a duck and quacks like a duck, I call that bird a duck.” Unfortunately, this argument is quite popular (even being utilized by the likes of prominent Intelligent Design advocate Michael Behe) despite being very superficial and even vapid. Needless to say, it doesn’t apply to our discussion of Nimravids and true felid saber-toothed cats, but in decades past the two groups were lumped together.

<img src=”Skulls” alt=”Skulls” />

So, what makes a nimravid a nimravid? They look awfully like cats, so why aren’t they included in the Family Felidae? What makes such distinctions so difficult is that those investigating the skull of Smilodon and Eusmilus would have to be relatively well-versed in scientific jargon and anatomy in order to point out the most important differences. While some nimravids (like Eusmilus) had large canines, their teeth alone are not diagnostic, and the original factors used by E.D. Cope that differentiated these animals from “true” cats were the “alisphenoid canal, postglenoid foramen, carotid, posterior lacerate, and condyloid foramina, postparietal foramina” in the skull (Hunt, 1987). The various canals and foramina listed dictate the paths of various nerves and blood vessels in the skull, and the arrangement in nimravid skulls seem to be more primitive compared with true felids. Likewise, nimravids lack a two-chambered auditory bulla, which is a rounded bit of bone associated with the ear which true cats posess.

There are a few more obvious giveaways when dealing with some nimravids, however. Nimravids equipped with long canines often have more cone-shaped canines than those of saber-toothed cats (which are flatter in cross-section), and many have bony “sheaths” extending from the lower jaw into which the massive teeth fit. Perhaps the most famous example of this kind of arrangement is the genus Barbourofelis, an animal that has actually been assigned to its own family as it is likely more closely related to true cats than nimravids (Barbourofelis was previously classified as a nimravid). Because of this (and the fact that another cat-like offshoot, the marsupial Thylacosmilus) the tooth-sheath shouldn’t be considered diagnostic of nimravids only, but it does give you a substantial clue that you’re probably not dealing with an actual saber-toothed felid.

Despite these differences, it has often been difficult to differentiate the groups (and debate still continues). The diagram above, based upon one in Robert Hunt’s 1987 paper “Evolution of the aeluroid Carnivora. Significance of auditory structure in the nimravid cat Dinictis,” offers three simplified versions of the hypotheses about the relationships of nimravids and felids. Initially it was thought that there was a progressive evolution from ancestor to descendant in a straight line, the nimravids being the direct ancestors to the saber-toothed cats. This view does not represent how evolution truly works, however, and was found to be incorrect. In its place came a view that nimravids and saber-toothed cats diverged from a common ancestor at about the same time, going off in separate directions. This is better, and is more consonant with the data, but again it suggests that the line representing the common ancestor went extinct, either in becoming nimravids or saber-toothed cats. What seems to be the case based upon current data is that the nimravids split off from a common ancestor somewhat before the saber-toothed cats, the line containing their common ancestor continuing its own evolution as both groups evolved. Such a branching pattern is not unusual, and should even be expected, especially since there are living primates like tarsiers and lemurs that represent the overall kind of animal our ancestors once were, but still quite different and undergoing their own evolution alongside our own lineage.

Thylacosmilus
The skull of Thylacosmilus, the marsupial answer to the saber-toothed cat, on display at the American Museum of Natural History in New York City. Note how far back in the skull the roots of its massive canines extend.

Saber
The skull of Megantereon on display at the AMNH. It was one of the “true” saber-toothed cats.

To complicate things even further, the skull or skeleton of the marsupial Thylacosmilus is also often thrown into the mix. Although totally unrelated to nimravids or felid saber-toothed cats, the South-American Thylacosmilus converges closely on the appearance of the placental predators, although there are some important differences. As can be seen from the above photographs, the eye of Thylacosmilus is entirely enclosed by bone on the side of the head, while in many felids and nimravids the eye socket is not entirely ringed-in by bone as if someone had bored a hole in the skull (compare the skull of Thylacosmilus with that of Thylacoleo, above). Further, the teeth of Thylacosmilus have very deep roots, going back in the skull almost over the eye. Originally it was thought that the teeth faced outward, but this was based upon a distorted skull and later finds showed the true position of the long canines.

Nimravid
Hoplophoneus

Now that we have elimated Thylacosmilus from the running as another case of marsupial convergence, we must ask why the nimravids and felid saber-toothed cats are so close to each other in appearance. While many of the instances I’ve discussed previously have been instances of convergence, be it throughout the entire body or merely certain aspects of it, the nimravid-felid connection is a wonerful example of parallel evolution. W.E. le Gros Clark provides an excellent summation of understanding the difference in his 1959 book The Antecedents of Man;

From what has already been said, it is clear that, in assessing degrees of phylogenetic affinity, it is always necessary to take into account the factors of parallelism and convergence in the evolutionary development of related or unrelated groups. These processes can lead to structural similarities, which, taken by themselves, may be misleading. The term convergence is applied to the occasion in general proportions or in the development of analogous adaptations in response to similar functional needs. But such similarities are superficial and easily distinguishable by a detailed comparative study of the animal as a whole. For example, the resemblance in general appearance, even in a number of morphological features, of the Tasmanian wolf to a dog does not obscure the fact that in fundamental details of their anatomical construction they belong to quite different mammalian groups. On the other hand, the potentialities of parallelism seem often to have been much overestimated by some anatomists, for this phenomenon has sometimes been invoked in support of extreme claims for independant evolution of groups which are almost certainly quite closely related. We can agree with G.G. Simpson that the whole basis of parallelism depends on an initial similarity of structure and the inheritance of a common potentiality for reproducing homologous mutations, and that, this being so, the initial similarity and the homology of mutations themselves imply an evolutionary relationship. Expressed in another way, it may be said that convergence increases resemblances (which are, however, no more than superficial), while parallelism does not so much increase resemblances as maintain and perpetuate (by development ‘in parallel’ so to speak) similarities which have already existed ab initio in the genetic make-up of related types. Thus, ‘closeness of parallelism tends to be proportional to closeness of affinity.’

There are a few problems with this reasoning, namely that it seems to give credence to an almost pre-determined genetic course for the lines to evolve in parallel, although le Gros Clark makes it clear in the work that he does not support in any way the notion of orthogenesis. Still, the passage makes the important distinction that in order to undergo parallel evolution groups need to be somewhat closely related and already bear similar structures, evolution preserving many of the similar traits instead of working to the same end from two disparate points. In the case of the nimravids and the felids it seems that they evolved from a common ancestor which was probably taken to carnivory. Nimravids branched off earlier, being more “primitive,” while the felids came off the same line (or a very similar one) after it had accumulated a few more evolutionary changes. Indeed, even if form seems to be static or change little, it’s hard for me to believe that designs are not slightly adapted this way or that as if the creature was an already perfect creation not influenced by changing ecological circumstances. Still, it seems that the nimravids and felids were adapted in similar ways, their ancestral lines probably possessing at least semi-retractable claws, long and sharp canines (although not long to the extreme like its descendants), a shortened face, and a developed carnissal shear. It is really not that difficult to change a civet-like creature (or in the case of our hypothetical common ancestor, a creature a bit closer to a cat) into a saber-toothed Smilodon, the changes being modifications of existing structures more than the creation of something entirely new out of nowhere. In fact, the vertebrate tetrapod skeleton has proven to be quite versatile, and most of the major bones in any vertebrate skeleton can be found to correspond with those in another vertebrate, allowing us to compare rhinos with ceratopsians, dromeosaurs with birds, cats with dogs, ichthyosaurs with cetaceans, and humans with primates.

Of constraints and convergence

I hope that is has become clear why convergence is such a strong theme in the evolution of vertebrates. At this point in the history of evolution, vertebrates have had a chance to fill nearly every niche imaginable in a large variety of habitats over millions of years, and so common themes are bound to arise. When groups return to the ocean, the environmental constraints shape them in ways peculiar to their new way of life that would not be advantageous in other situtations (i.e. being such a large aquatic animal that you’d be crushed by your own weight if you came onto land). When mammals become adapted to be predators, their dentition and morphology must be altered if they are to be successful hunters, carnivores past and present showing some suprising similarities despite being only distantly related. Even when taking to the air, laws of physics still apply, and natural selection often works through physical and chemical constraints to produce new forms.

It is of little doubt that the tetrapod design is a versatile one, retaining its overall character through the various changes that it has endured. Indeed, even when a lineage dies out and may seem gone forever, there is no law that says a similar situtation in the future will not produce forms that may be strikingly familiar, even if such organisms are not directly related to the last group that filled their new niche. Evolution has produced “endless forms most beautiful and most wonderful” and will continue to do so long after I am gone, but random mutation/natural selection do not work in isolation from the rest of the natural world. Evolution has produced so many amazing creatures precisely because ecology, physics, and chemistry have offered up both opportunities and challenges, and I only regret that I will not be able to witness the familiar and unfamiliar about what is swimming in the seas, flying in the air, or stalking the land 500 million years from now.





Icon of Delusion: Jonathan Wells

31 08 2007

If ever there were an unsavory, real-life creationist character that should be cryptically referred to as “You Know Who” as in the Harry Potter series, it would have to be Jonathan Wells. While I have not actually tested this as yet, the very mention of his name seems to make my blood pressure rise, and perhaps his goal is to be so annoying and deceitful that evolutionary scientists all die of stress-induced heart attacks because they can not even stand the mention of his name. If this is indeed the plan, then Wells has certainly taken another step towards its fruition. On the dubiously titled “Evolution News & Views” blog run by the Disco. Institute, Wells has issued a rather hateful screed about the terms “Darwinist” and “Darwinian.”

Starting out with a rather hateful attack on the quality of The Seattle Weekly, stating that “as [a source] of news [it’s] probably about as reliable as Minju Choson, the official organ of the Democratic People’s Republic of [North] Korea. But homeless people make good use of [it],” Wells quotes a recent article in the paper in which Eugenie Scott of the NCSE as saying “a real follower of modern science would never call himself a ‘Darwinist’,” because “evolutionary biology has advanced way beyond Darwin’s 19th-century tracts.” This is rather strange, especially the quotation marks that Wells decided to “helpfully” insert as they do not appear in the original article. The original sentence reads as follows;

Scott isn’t buying it, not least because she says evolutionary biology has advanced way beyond Darwin’s 19th-century tracts, so that a real follower of modern science would never call himself a “Darwinist.”

Sounds more like the reporter, Nina Shapiro, tried to condense Scott’s argument down into a shorter sentence and was not quoting Scott directly, so once again Wells has shown us that he is either being deceitful or ignorant of how to properly use the copy-paste function on a computer. Still, he uses his doctored quote as a set-up to try and muddy the waters with cherrypicked examples from historical science, and Wells misses the entire point of the now outdated term “Darwinist.” Wells writes;

The reason that “Darwinism” and “Darwinian” – even “Darwinist” – are used by modern evolutionary biologists is that they are more precise than “evolution” and “evolutionist.” The latter have many meanings, most of them uncontroversial. For example, “evolution” can refer simply to change over time, something no sane person would deny. Or it can refer to minor changes within existing species, which breeders have known about for centuries.

Actually, the reason why “Darwinist” was the most popular term in years past is because Darwin’s mechanism of natural selection (and later sexual selection) proved to be the correct one. Prior to Darwin and even during his time “transmutation” of species was a hot topic, and there were various schools of thought as to how creatures evolved. Thus a more exact term for the school of evolutionary thought Darwin founded, “Darwinism,” was necessary to distinguish it from the competing hypotheses of Lamarck, Agassiz, the thoughts put forth in the popular book Vestiges of the Natural History of Creation by Robert Chambers, and the later criticisms of Darwin put forth by the likes of St. George Mivart. In fact, especially in America, evolution by natural selection was not immediately and fully embraced, the famed scientist Louis Agassiz being a critical of Darwin so vociferous that even Wells would have been likely to get his admiration. Even the noted paleontologist E.D. Cope ascribed to Neo-Lamarckian ideas of evolution, a evolutionary framework that has been long known to be insufficient.

But if Darwin was right, why would Eugenie Scott say his “tracts are outdated”? Well, Darwin was right in terms of his big ideas of natural & sexual selection (as well as many other points), but he did get some things wrong. Heredity was vastly unknown during his time, and even Darwin threw in a pinch of Lamarckism into his writings. In the 2nd edition of The Descent of Man, Darwin wrote the following in the preface;

I may take this opportunity of remarking that my critics frequently assume that I attribute all changes of corporeal structure and mental power exclusively to the natural selection of such variations as are often called spontaneous; whereas, even in the first edition of the ‘Origin of Species,’ I distinctly stated that great weight must be attributed to the inherited effects of use and disuse, with respect both to the body and mind. I also attributed some amount of modification to the direct and prolonged action of changed conditions of life. Some allowance, too, must be made for occasional reversions of structure; nor must we forget what I have called “correlated” growth, meaning, thereby, that various parts of the organization are in some unknown manner so connected, that when one part varies, so do others; and if variations in the one are accumulated by selection, other parts will be modified. Again, it has been said by several critics, that when I found that many details of structure in man could not be explained through natural selection, I invented sexual selection; I gave, however, a tolerably clear sketch of this principle in the first edition of the ‘Origin of Species,’ and I there stated that it was applicable to man.

And so I still cringe when I heard scientists refer to themselves as “Darwinists” (or even worse, “orthodox Darwinists,” as I once heard Ken Miller opine). The term is no longer necessary or even accurate because in scientific understanding Darwin’s big ideas won the day ages ago while some of his subjects he did not fully understand have become better known, the science we now have being based on Darwin but not adhering only to the thoughts within his published works. If we’re going to start tagging schools of thoughts with names, we could very well have “Gouldists,” “Dawkinsists,” “Simpsonists,” “Mayrists,” “Morganists,” “Copeists,” “Agassizists,” etc. etc. etc. The distinction that the term “Darwinist” used to have is now largely lost because of our greater understanding, time proving Darwin to be the victor in the battle that took place in evolutionary through between the late 19th and early 20th centuries, but I know of no scientists who holds On the Origin of Species by Natural Selection who holds the great work to be a holy book that may not be added to or contradicted in any fashion whatsoever. Just like paleontology requires a historical understanding and a long-view of the subject at hand, so does the topic of the evolution of how we think about biological evolution.

After some more jabs at Scott and confusion of the origins of the words he’s talking about, Wells concludes;

So rather than learn Scott’s word games, biology students should begin by learning to distinguish “evolution” from “Darwinism” and “evolutionist” from “Darwinist.” Or “Darwinian” – it’s one and the same.

I assume that he’s not suggesting that school boards should hire Neo-Lamarckian staff to “Teach the Controversy!” about evolution. I think a biology class would largely benefit from understanding the historical aspect of the evolution idea, starting with the Ionians like Thales and Anaximander and working through Darwin and the Modern Synthesis to today. That way it can be clearly seen that there indeed was a time when evolution did not necessarily mean “natural selection, common descent, etc.” in the minds of some notable scientists, and how eventually their ideas (often influenced by their adherence to religious doctrine) came crumbling down. “Teach the Controversy!” seems like it could have just as well been the battle cry of Louis Agassiz or St. George Mivart as that of current ID thinkers, but apparently they cannot be bothered to go back and try and uncover the history of the evolution idea.

To put it concisely, Darwin was a highly intelligent man who uncovered the beginnings of one of nature’s greatest mysteries, but we would be fools to think that he was somehow all-knowing or that no data would later be found that would clarify or possibly refute his ideas. Nearly 150 years after On the Origin of Species by Natural Selection was published, however, natural selection working on variations in organisms is still a major mechanism of evolution, exemplified by Stephen J. Gould in his coral-branch analogy in The Structure of Evolutionary Theory. Can we expect any scientist never to make a mistake or never have their ideas overturned? For every important idea that has been put forth by great minds, how many ideas ended up being stillborn or eventually refuted? Are we to remember scientists for their failures only, disregarding their successes? To do so would mark us imbeciles, and we would pay a heavy price for judging those who strove to bring enlightenment to the work by sharing their ideas. Darwin was one of those great minds, and even though “the long argument” will likely continue, I see no reason why we should exhume the corpses of long-dead competing hypotheses of evolution when Charles so eloquently put laid to rest.





Waterlogged Weblog

30 08 2007

Evolutionary transitions into and out of the water (imaginary or real) have been a hot topic on this blog lately, so I thought I would provide the links to some of the posts from the past year on that theme. Hopefully I’ll be able to cover icthyosaurs, mosasaurs, plesiosaurs, and other critters in due time, but for now here are some aquatic-oriented works to keep you occupied;

Scuttling the Aquatic Ape Hypothesis

Everyone back in the pool!: From artiodactyl to cetacean

Protosuchus

Alligators? In the sewers?

A different kind of White Shark

Carnivory in Hippos

You can bring a Rhipidistid to land but you can’t make it walk

Idiocy beyond belief: Sharks don’t have webbed feet!

Bluffing in crayfish arm-wrestling

Skimming for supper, or not…

Giant Swimming Sloths of South America

An Iguanodon with flippers





Scuttling the Aquatic Ape Hypothesis

29 08 2007

On our occasional trips to the New Jersey shore, my wife is always the first one in the water. While I’m cautiously wading in, dreading that final slap of cold water just below my waist, she’s already frolicking in the waves, egging me on to just jump in and get it over with. Eventually I too become submerged (either willfully or by force of a wave I never saw coming), salt water inevitably shooting up my nose. Don’t get me wrong, I do enjoy warm days at the beach, but on each visit it seems that I as an individual, if not a representative of a population or species, am not well-adapted to a near-shore marine existence. Followers of the Aquatic Ape Hypothesis* (AAH), however, beg to differ.

[* I say “hypothesis” and not “theory” (AAT) because the writings of Elaine Morgan and others do not have enough supporting evidence to garner it the more prestigious title of “theory.” Given the current paucity of evidence and research, the Aquatic Ape Hypothesis is precisely that and no more.]

Before discussing the current manifestation of the AAH, we need to go back to a time when the truth of evolution had yet to fully take hold in the minds of scientists and philosophers. The Ionian philosopher Anaximander (610-546 BCE), student of Thales, suggested that the world first existed in an entirely aquatic state, the recession of the globe-consuming waters creating life. In From the Greeks to Darwin (1905), famed American Museum of Natural History president Henry Fairfield Osborn described the views of Anaximander as follows (a similar treatment is given in Osborn’s Man Rises to Parnassus, as well);

He conceived of the earth as first existing in a fluid state. From its gradual drying up all living creatures were produced, beginning with men. These aquatic men first appeared in the form of fishes in the water, and they emerged from this element only after they had progressed so far as to be able to further develop and sustain themselves upon land. This is rather analogous to the bursting of a chrysalis, then to progressive development from a simpler to a more advanced structure by a change of organs, yet a germ of the Evolution idea is found here.

We find that Anaximander advanced some reasons for this view. He pointed to man’s long helplessness after birth as one of the proofs that he cannot be in his original condition. His hypothetical ancestors of man were supposed to be first encased in horny capsules, floating and feeding in water; as soon as these ‘fish-men’ were in a condition to emerge, they came on land, the capsule burst, and they took their human form.

Like the works of many Ionian philosophers, the ideas and opinions of Anaximander do not seem to have taken hold (Aristotle ultimately becoming the preferred scientific and philosophical source for further consideration in Europe in centuries to come), and not much of his work remains. It is curious to note, though, that wrong as Anaximander was about the origins of humans, the reasons he uses to support his ideas (as relayed by Osborn) are very similar in approach to those of Elaine Morgan and some modern-day AAH adherents, as we shall soon see.

To the best of my current understanding, the hypothesis that man was a product of the sea did surface again until 1942 when Max Westenhofer of the University of Berlin published the book The Unique Road to Man. According to Donna Kossy’s book Strange Creations, Westenhofer’s treatment of an aquatic origin of mankind consisted of little more than mention of it as a promising hypothesis, however, and the outbreak of war prevented the professor from pursuing the line of inquiry further. The hypothesis would have to wait until March 5, 1960, when marine biologist Sir Alister Hardy presented a lecture on “Aquatic Man: Past, Present, and Future” to the British Sub-Aqua Club. The address caused quite a stir and led Hardy, who had been inspired by the layers of sub-cutaneous present in humans and some marine mammals he had seen skinned on a journey to the Antarctic in 1927, to write a series of articles in the magazine New Scientist to clarify his position on the subject. Kossy relates the words of Hardy from an April issue of the magazine (although the year is not specified);

My thesis is that a branch of this primitive ape-stock was forced by competition from life in the trees to feed on the seashores and to hunt for food, shell fish, sea-urchins, etc., in the shallow waters of the coast. I suppose that they were forced into the water just as we have seen happen in so many other groups of terrestrial animals. I am imagining this happening in the warmer parts of the world, in the tropical seas where Man could stand being in the water for relatively long periods, that is, several hours at a stretch. I imagine him wading, at first perhaps still crouching, almost on all fours, groping about in the water, digging for shellfish, but becoming gradually more adept at swimming. Then, in time, I see him becoming more and more of an aquatic animal going further out from the shore; I see him diving for shell fish, prising out worms, burrowing crabs and bivalves from the sands at the bottom of shallow seas, and breaking open sea-urchins, and then, with increasing skill, capturing fish with his hands.

Thus the more familiar image of the amorphous “Aquatic Ape” was born, wading out into the surf and feeling in the shallow sands for food. The early stage of such a transformation is awfully raccoon-like, as raccoons have incredibly sensitive hands that they use to feel about in streams and shallow waters for mussels, crayfish, and other morsels without being driven to become fully aquatic themselves. Nevertheless, the idea that man had his origins in a shallow sea rather than on a hot and brutal savanna was certainly controversial. Ever since Raymond Dart described the skull of the Taung Child in 1925 (shifting attention away from Europe and Asia for the origins of man) and the fossil assemblages of the South African caves were discovered, humans were thought to have evolved through a hunting culture, nearly every specialization that separates us from living primate relatives being due to our meat-craving societies. Indeed, the remains of Australopithecus found in South African caves (especially the jaw of a 12-year old child whose jaw appeared to have been fractured by a direct and accurate blow) like those Makapansgat suggested to Dart that these “proto-men” were not only skilled hunters, but also murderers and cannibals. Even though our understanding of these assemblages has greatly changed since Dart’s time (see C.K. Brain’s The Hunters or the Hunted?), the overall image of human evolution being intricately linked to meat-eating and hunting has dominated the discussion of our origins. Even more specifically, the considerations of our ancestors have nearly always focused on the male of the species, and even Hardy’s early ideas of an aquatic ape focused primarily on males.

In 1964, zoologist Desmond Morris published the bestseller The Naked Ape. Today the book is nearly useless outside of understanding the history of thought about human evolution, but when it was first published a short discussion of the AAH caught the attention of a woman named Elaine Morgan. On page 37 of the 1967 paperback edition, Morris states;

Another, more ingenious theory is that, before he became a hunting ape, the original ground ape that had left the forests went through a long phase as an aquatic ape. He is envisaged as moving to the tropical sea-shores in search of food. There he will have found shellfish and other sea-shore creatures in comparative abundance, a food supply much richer and more attractive than that on the open plains. At first he will have groped around in the rock pools and the shallow water, but gradually he will have started to swim out to greater depths and dive for food. During this process, it is argued, he will have lost his hair like other mammals that have returned to the sea. Only his head, protruding from the surface of the water, would retain the hairy coat to protect him from the direct glare of the sun. Then, later on, when his tools (originally developed for cracking open shells) became sufficiently advanced, he will have spread away from the cradle of the sea-shore and out into the open land spaces as an emerging hunter.

Unfortunately, [searching for fossils in marine or fluvial deposits or further research into the AAH] has yet to be done and, despite its most appealing indirect evidence, the aquatic theory lacks solid support. It neatly accounts for a number of special features, but it demands in exchange the acceptance of a hypothetical major evolutionary phase for which there is no direct evidence. (Even if eventually it does turn out to be true, it will not clash seriously with the general picture of the hunting ape’s evolution out of a ground ape. It will simply mean that the ground ape went through a rather salutary christening ceremony.)

[Emphasis mine]

Elaine Morgan read the brief treatment and qualifications (some of which has been omitted here for the sake of brevity) and wanted to know more about the possibility of our ancestors going through an aquatic stage of evolution. No information seemed to be available, and so Morgan wrote to Hardy in 1970, and he encouraged Morgan to push ahead with her research and desire to write a book about the AAH. The result was the bestselling The Descent of Woman, published in 1972. My copy is a little bit newer than that, being the Bantam 1973 edition, and featuring what appears to be a nude mother and child on the cover. Closer inspection reveals that something isn’t quite right, however; the mother I previously assumed was a representative of Homo sapiens looks like she’s been hit in the face with a frying pan. I didn’t know it at the time, but the text would reveal that the plump, nude, and long haired female on the cover was not drawn from life, but rather was Morgan’s idea of the Australopithecus specimen “Lucy” as Aphrodite.

Morgan’s first book is certainly a unique one, weaving in between “Just-so story” type paleo-fiction and long arguments about the female orgasm, including it’s fallacious mythical status. Indeed, the AAH only seems to occupy the first 1/3 of the book, only cropping up here and there in the following chapters, and receiving only a brief mention in the conclusion. Still, the way Morgan structures her argument in her first book will tell us much about her later works and the rise of the AAH as a popular idea. Early on in the work, we are introduced to a hypothetical female ape, not unlike Proconsul, living in Africa sometime during the Pliocene (~5.3-1.8 million years ago). During this time a hard life trying to find food and avoid predators was becoming even harder, Morgan hypothesizing that a terrible heat wave would change the way of life for many populations of these unnamed apes (from what I understand, however, the climate of the Pliocene began to approach our own and became cooler, drier, and had more seasonal distinctions rather than being a global hot-house).

Morgan’s ape was in a bit of a jam, that’s for sure. The water holes are said to be stalked by hungry cats and food was becoming scarce, and the imaginary female was not as fearsome or powerful as the males in her group. Eventually she was chased into the water by a large cat, and decides that, despite her distaste for water, “the seaside not at all a bad place to be. She found to her delight that almost everything on the beach and in the water was either smaller or more timid than she was herself.” Indeed, Morgan’s ape appeared to have found paradise. While other animals cooked during the “dog days of the Pliocene”, her ape (and by extension the population of apes) found what seems to be a sheltered and peaceful lagoon devoid of predators, scavengers, or other threats from either land or sea. “Leopards don’t come so far into the sea, nor sharks so near to the land,” we are told, and while leopards may not be attracted to water, sharks are well-known for their shallow water hunting habits. Crocodiles are not even considered, nor are stingrays, poisonous urchins, jellyfish, disease, infection, or any of the other biological problems that may come with an aquatic existence or change in ecological setting. The new home of the apes sounds better than Club Med, a watery Eden lacking in devious serpents and forbidden fruit.

As suggested by Morris and Hardy, the population of apes gets by on a diet of shellfish and relatively stupid sirenians that happen to come by, males making short work of the water-going creatures with rocks found along the shore. Given the amount of time that the apes would be spending in the water (they couldn’t have just subsisted by wading in or eating what washed up, or at least this is what is implied), bodies started to change. Males are paid little attention by Morgan, and the warm relationship between mother and child takes center stage. While most of the hair on the body would be lost as an adaptation to water (an odd conclusion given that otters, seals, and sea lions all are covered in hair), the hair on the head would be allowed to grow long, the water babies being able to curl their fingers into it and stay close to mom for a nap when they got tired of exploring off on their own. Conversely, breast feeding would still have to take place on shore, but the upright posture of the females (acquired from so much time in the water) would require the baby to be held at an awkward position in which they could not reach their mother’s nipples. This was solved by developing larger “hemispherical” breasts to reach down to the infant, even though larger breasts may cause infants problems when they try to get their mouths on them to breast feed (if the breast is so large that the infant’s nose is covered by it, breathing and feeding becomes difficult).

In searching for an aquatic example of such a striking characteristic, Morgan turns to the Florida manatee and other sirenians, many who have seen females with young noting the presence of “breasts” on the aquatic mammals. Interestingly enough, however, the manatee shares it’s ancestry with living elephants, the females of which also exhibit some rather sizable swellings when lactating. Robert Sapolsky, in his book A Primate’s Memoir, describes seeing such an unexpected shape on the chest of a female elephant for the first time;

Did you know that female elephants have breasts? I do not mean rows of teats, a mama elephant lying on her side with dozens of little piglet elephants nursing with their eyes still closed. I mean breasts, two huge voluptuous billowy mounds, complete with cleavage. I bet you had no idea, did you? Nor did I – it is a subject rarely broached in our public schools. I’m out in the bush that first month, armed with binoculars and stopwatch and notepad, spending the days carefully watching baboons mating left and right. And then, suddenly, some pachyderms come cruising past, and I see some elephant with these, well, breasts. And the natural first reaction is to think, Oh, great, I’m such a horny lascivious pathetic adolescent that after a mere month of isolation in the bush I’ve already cracked, I’m hallucinating breasts the size of Volkswagens on the elephants. Horrors, to have one’s psychotic break occur so soon, and to have it take the form of a puerile sexual obsession many embarrassing steps below gawking at National Geographic nudies. I was greatly relieved to eventually discover that the elephant’s breasts were real, that I was not having some Marlin Perkins wet dream.

It should be noted, however, that Morgan attributes an aquatic origin to elephants as well, primarily based upon their ability to shed tears (and therefore salt), as well as the ability of living Indian elephants to swim long distances in the ocean. Such considerations are a side trip from the main thrust of her argument, and no detail is given as to when, where, why, or how elephants arose from a water-dwelling species, only that a few characteristics in living animals point to an Aquatic Pachyderm Hypothesis.

Going back to the AHH, given about 10 million years in the water Morgan’s ape is substantially different than the one that was first driven into the waves by a predator. Referring to her as “Mrs. Australopithecus,” Morgan paints the following portrait (the artistic manifestation of which is found on the cover of the book);

So our hominid has a nose. I have no doubt that she also had fleshy nostrils, but considerable doubt that they evolved to make sex sexier for her mate. I think she was by no means the simian, cadaverous, lipless creature that artists sometimes reconstruct by covering her dug-up skull with a tightly fitting layer of hairy skin. The layer of fat which was rounding out her arms and legs and adding bulk to her breasts was also filling out her cheeks, and her nostrils, and her earlobes, and everting her lips… We would not have accounted her beautiful, with her low forehead and prognathous jaw, but the chances are that she was a chubby little creature with several superficial features resembling our own more nearly than they resembled any ape’s. And as for the expressions that flitted across that prehistoric countenance, her millions of years in the water had certainly left their mark on those also.

This is quite a different picture of “Lucy” than is often seen, but is there anything to it? Part of the advantage of the AAH is that Morgan doesn’t specify her ideas down to a scientific level, allowing her to poetically play with her ideas in any way she wishes, the female becoming more beautiful while the men continue to try and kill dugongs with rocks. This type of feminist reaction to the “Man the Hunter” narrative is the main connective feature throughout the book, and Morgan’s writing is far more concerned with the more graceful and beautiful evolution of woman, with sex ultimately bringing “sin” into the Garden.

In Morgan’s story, the genitals of the ancestral females went from facing backwards (making rear-mounting positions by the male easy) to facing downwards, a position that Morgan insists will not work for males, face-to-face mating being adopted as a must. Morgan’s reasoning for the change is that aquatic animals often undergo this type of genital shift (cetaceans are her primary example), but she generally ignores why the genitals should be shifted in the first place. In terms of cetaceans, the ancestors likely had their anal-genital openings in the position typical for quadrapeds; facing backwards at the location of the pelvis just under the tail, usually being at the most distal end of the body. As they evolved, the archaeocetes lost their hind limbs and their spines elongated, being the main source of propulsion, so rather than keep moving backwards with the spine the genitals stayed in the pelvic region “settled” on the ventral side of the body; where else they would have gone, I do not know. Given this morphological necessity, face-to-face mating became the only way cetaceans could copulate. Seals and sea lions, on the other hand, still have their anal-genital openings near the distal most parts of their bodies because that is where the pelvis is and there was no need to change mating styles, and males still mount females from behind. Even beyond such considerations, I do not see how the rear-mount strategy can be dogmatically ruled out, and I have a feeling that because such a position is considered “kinky” by some it was essentially ruled not to have happened. In fact, the retention of rear-mounting with the shift in female genitals could help explain elongation of the penis in males (they’d have to extend a bit farther), although this matter is far from settled. Curiously, Morgan generally ignores the bonobo and it’s face-to-face mating habits, even in her later books. She’s clearly aware of these apes (she does mention them and one graces the cover of The Descent of the Child), but they are conspicuously absent from discussions about sex.

Still, if we are to follow Morgan’s model, the apes would have to switch from mating using a rear-mount position to face-to-face (the males, we are told, couldn’t penetrate any other way), such a position causing much trauma for females. Males wouldn’t know how to calm the female for a face-to-face encounter, and it essentially led to either rape or an unfruitful attempt to mate. Morgan describes such a scene;

The primate was a totally different shape. Her new aquatic streamlining had been unable to prevent her becoming lumpy in the middle, and as a littoral biped her legs were developing in the opposite direction from the seal’s – they were becoming not smaller and thinner but farther apart, but longer and thicker and closer together. The seal’s solution was impossible for the aquatic apes. Their dilemma was unique.

So we left her on her back, kicking and struggling and frightened out of her tiny anthropoid mind, with her mate beginning to get irritated. When she saw him snarl and bare his canines she was finally convinced that he wanted her for dinner, and that her last hour had come. Further resistance was useless. She stopped fighting and signaled her submission, defeat, and appeasement as strongly as she could with so little room for maneuver.

Immediately, the incident was over. The male was a properly programmed animal, and it was impossible for him to go on clobbering a member of his own species that was giving clear indications that it had stopped fighting back. He moved a little way off, wearing a puzzled expression. He had thought for a moment that he was on to a good idea, but obviously there was a snag to it.

Such events removed us from our Eden along the shores, males taking up hunting on the plains soon after the eviction. Rains that quenched the African drought allowed the apes to leave the habitat that they had become so accustomed to (it seems like the males led the charge, being sick of their prolonged day at the beach), moving on to evolve in ways that fit the scientific orthodoxy of the times a bit more closely. Even so, Morgan suggests that women have retained the peace, beauty, and grace of their aquatic origins while males are more shaped by violence and hunting, her parting words being;

He is the most miraculous of all the creatures that God ever made or the earth ever spawned. All we need to do is hold out our loving arms to him and say: “Come on in, the water’s lovely.”

Oddly enough, such arguments seem more specific and in-depth than those in Morgan’s later works The Descent of the Child (1995) and The Scars of Evolution (1990). The Descent of the Child can largely be ignored, being that it’s primary focus is on doing for human babies (from conception through early childhood) what The Descent of Woman did for women, all-in-all being a string of facts presented to the reader in an easy-to-digest manner but without much further discussion. In covering past evolution, the “savanna hypothesis” and “man the hunter” are both alluded to or pointed out to be wrong, although no rigorous refutation is made. Instead the reader is referred to the earlier The Scars of Evolution for the “scientific” argument, but Morgan’s earlier poetry contains far more detail than the 1990 work. I breezed through the 178 pages of the book easily enough, but there was little positive evidence within it’s pages for the AAH. Certain physiological systems were pinpointed and deemed to be of aquatic origin since Morgan deemed no other hypotheses to be adequate (which, of course, assumes that all possibilities have been discovered and have received proper consideration).

I actually would love to write up a longer discussion of The Scars of Evolution but there is surprisingly little actual AAH evidence to be considered, and Morgan even makes some fairly basic mistakes about fossil preservation. Early on in the book she writes;

So if the prospecting had started in the north [of the Rift Valley] and worked down, popular illustrations of groups of Australopithecus would have shown them reclining under a shady tree at the water’s edge, living perhaps on fruit and greenery and fish. Instead, they are depicted as shaggy creatures trekking through parched grass and a scatter of stunted thorn bushes, turning to scavenging and hunting to supplement their diet.

This conclusion comes from Morgan’s assertion that some specimens of Australopithecus are found associated with fossils like crocodile remains and turtle eggs, suggesting an aquatic habitat. This largely ignores taphonomy, however, and an animal that dies in or near water being much more likely to be preserved and fossilized than one that drops out on the plains, the body undoubtedly being ripped apart by scavengers and leaving little or nothing to the fossil record. Most of the rest of the book covers material already mentioned in The Descent of Woman, like the fallacious notion that pheromones are essentially nonexistent or non-influential in humans because we went through an aquatic phase of evolution where scent wouldn’t have counted for much. Also curious is one of Morgan’s final statements about how evolution works, especially in regards to water. Rather than gaining specializations mentioned in so many of her works (i.e. the ability to cry and remove saline from the body, nostrils with possible flaps to keep water out, enlargement of the female breasts), a kind of de-volution of our ancestors is favored;

Conceivably, a species finding itself in a radically new environment (such as water) begins to shed the more advanced features which fitted it for its old environment. It back-tracks to a more unspecialized foetus-like form, before re-adapting to the new habitat. If that were the case, then our own ancestors, having moved from the land to the water and subsequently from water to land, would have been subjected to an impetus towards neoteny on two successive occasions. It would explain why in our case the trend was unusually powerful.

In all, Morgan’s work seem to be lacking of any rigorous research or hypotheses, and it led me to wonder why the AAH will simply not go away. Perhaps some of it is the mental appeal and the common error of linking correlation in evolutionary convergence to causation, working backwards to whatever ideal we hold most dear. Even if I’m incorrect as far as social motivation goes, the AAH has shown up in the scientific literature in the past few years, and it’s primary advocate seems to be Marc Verhaegen. Although the majority of his papers seem to be currently unavailable online, there is no name that more frequently appears in terms of AAH literature in scientific journals, giving the hypothesis some visibility (and credibility, as far as AAH advocates may be concerned). Some of the papers published on the AAH I could find are;

Bender R, Verhaegen M, & Oser N. “Acquisition of human bipedal gait from the viewpoint of the aquatic ape theoryAnthropol Anz. 1997 Mar;55(1):1-14.

Cunnane, S.C. “The Aquatic Ape Theory reconsideredMedical Hypotheses Volume 6, Issue 1, January 1980, Pages 49-58

Ellis, D.V. “Wetlands or aquatic ape? Availability of food resources.Nutr Health. 1993;9(3):205-17.

Rhŷs Evans, PH. “The paranasal sinuses and other enigmas: an aquatic evolutionary theoryJ Laryngol Otol. 1992 Mar;106(3):214-25.

Vaneechoutte, M. ” Report of the Symposium ‘Water and Human Evolution’, Gent, Belgium, April 30th 1999Human Evolution. 2000 Volume 15, Numbers 3-4

Verhaegen, M.J.B., Puech, P.F., & Munro, S. “Aquarboreal ancestors?Trends in ecology & evolution (Amsterdam). 2002 Vol. 17, Issue 5, page 212

Verhaegen, M.J.B. and Puech, P.F. “Hominid lifestyle and diet reconsidered: paleo-environmental and comparative dataHuman Evolution. 2000 Volume 15, Numbers 3-4

Verhaegen, M.J.B. “The Aquatic Ape Theory and some common diseases”. Medical Hypotheses
Volume 24, Issue 3, November 1987, Pages 293-299

Verhaegen, M.J.B. “The Aquatic Ape Theory: Evidence and a possible scenario
Medical Hypotheses Volume 16, Issue 1, January 1985, Pages 17-32

Verhaegen, M.J.B. “Aquatic ape theory and fossil hominidsMedical Hypotheses Volume 35, Issue 2, June 1991, Pages 108-114

Verhaegen, M.J.B. “Aquatic ape theory, speech origins, and brain differences with apes and monkeysMedical Hypotheses Volume 44, Issue 5, May 1995, Pages 409-413

[And for an opposing view see Langdon, J.H. “Umbrella hypotheses and parsimony in human evolution: a critique of the Aquatic Ape HypothesisHuman Evolution. 1997, Volume 33, Number 4, pp. 479-494(16)]

As is immediately apparent, the great majority of the papers have appeared in one journal (Medical Hypotheses) and can be attributed to one author, Verhaegen. Judging from what I was able to find, many of the arguments that Verhaegen employs are very similar to those of Morgan, working backwards from somewhat contested or enigmatic human features to an aquatic origin to the exclusion of other hypotheses. Where Verhaegen differs, however, is that his aquatic hypothesis is far more broad than that of Hardy or Morgan. While Morgan implied that the aquatic apes were an isolated group that ended up leading to man (what happened to populations elsewhere is never spelled out), Verhaegen suggests that the last common ancestor of living gorillas, chimpanzees, bonobos, and humans was at least semi-bipedal and semi-aquatic, likely living in a habitat like a mangrove swamp. From the paper “Aquaborel ancestors?”;

A vertical posture and an ability to climb with the arms raised above the head could have helped a wading primate to enter or leave the water by grasping overhanging branches or waterside vegetation, and to grasp fruits above the water. Body enlargement and tail reduction would hinder agile arborealism, whereas a larger body is more easily supported in water and helps reduce heat loss (explaining why aquatic mammals are larger than related terrestrial forms). Tails would be of little use for a wading and/or swimming primate and would cause both drag and heat loss.

Thus Verhaegen attempts to separate New and Old World monkeys from apes by making the ancestors of all living apes at least partially water-bound, standing up to wade through water. Ultimately humans would have stayed in the pool while gorillas and chimpanzees got out, although gorillas, chimpanzees, and bonobos do not seem to show the same signs of being adapted to water that are often associated with humans under the AAH. Of further note is the fact that living primates like baboons, macaques, and proboscis monkeys have been known to swim and stand upright in water, although none seem to show signs of becoming exclusively adapted to an aquatic lifestyle. In the recent BBC series Planet Earth, baboons of the Okovango Delta in Botswana were shown wading through the water;

The baboons are not especially comfortable in the water, just as many other animals in the delta like cheetahs and lions don’t especially like crossing the waterways. Indeed, crocodiles are the primary danger in the water, and many animals seem to know of the threat all too well (but must cross from time to time, anyway). Such modes of moving through water, also seen in chimpanzees (see the final episode of the BBC’s Life of Mammals, entitled “Food for Thought”), seem to constitute the “weak” version of the AAH, and isn’t entirely unreasonable in explain possible motivation to become bipedal. It does require a certain ecology, however, (i.e. flooded plains, a swamp, shallow mangrove forest, lagoon, etc.) and has little explaining power out of such a context. Still, there are even more aquatic primates that were also featured in Planet Earth; the crab-eating macaques.

If we’re looking for a model of what an aquatic ape would look and act like, surely these monkeys would be it, and Morgan does note some of the aquatic habits of macaques (especially the behavior of washing food in water, as seen in Japanese Macaques). The problem is that macaques are monkeys, not apes, although they seem to get along in the water just fine. Unfortunately the adaptation of these primates to water is going to be slow and take many generations, but the study of these animals could give us some clues as to what the AAH can and cannot explain, although it seems that many of the features explained by the AAH don’t fit with what we see in the macaques. Looking at the underwater behavior, it would seem that the monkeys would be adapted to swim in a matter similar to that of quadrupeds rather than to start wading in, becoming bipedal, and then doing a breast-stroke. Indeed, the video shows that becoming bipedal is not a necessary precursor to being able to swim or becoming semi-aquatic, and it is quite possible (even probable) that primates could abandon the upright stage altogether. Standing upright seems to be generally uncomfortable for many primates, and it’s hard to see how primates introduced to a fruitful aquatic habitat would want to stand up before just jumping in if there was really nothing to fear in the waters. Even in the weak version of the AAH, it is hard to see how standing upright while crossing a river would have selected for bipedalism as it seems that many primates are capable of doing it over short periods and it does not hold any strong advantage that would relate to mating success or overall survival. Unless the hypothetical apes lived in an area constantly flooded, requiring them to stand up much of the time, it is difficult for me to imagine how water could have helped to select for an upright posture.

The overall problem I have with Morgan’s hypothesis about apes becoming almost exclusively aquatic is that it forces us to make a choice of one habitat or another. Mentions are made of Proboscis Monkeys and Macaques enjoying a swim, washing off food, or living near water, but they don’t seem to be bent on the same path as the one Morgan proposes. Organisms certainly are plastic, and they don’t rigidly abide by the “rules” set down by those that describe them as to where to live, what to eat, and how to act properly. In fact, it seems more reasonable to me that primates past and present would take advantage of an aquatic resource if readily available, but still maintain their terrestrial life unless they were so isolated that they had no choice for food other than the water. Time will tell if some of today’s semi-aquatic primates ever become more fully at home in the water, but I see no reason to believe that our ancestors decided to take a prolonged summer vacation on the beach, proceeding in a way that just so happened to explain everything neatly (if un-parsimoniously).

The AAH hinges on apes willingly going into the water for safety from predators, but this is only a Just-So story without the details. It also ignores the fact that the water can be almost as dangerous, if not moreso, than the land, and there are predators in the water just as there are in terrestrial habitats, not to mention rip-tides and other problems inherent to the ocean itself. While Morgan, in The Descent of Woman, states that newborns could be left to paddle about on their own while mom went about her own business, such maternal inattention doesn’t seem like it would be especially effective in making sure that young made it to adulthood. Only the calmest, most sheltered, and safe of lagoons would have allowed for this. If the AAH is to be taken seriously in whatever form, it is going to require rigorous ecological study, and so far it seems that it relies far more on post hoc arguments than actual evidence.

While Jim Moore has already done a fantastic job dismantling the various problems with the AAH, I hope I have helped to illuminate the overall lack of evidence for the idea. As an idea it’s not a bad one, but it seems to have never gone beyond hypothetical situations and Just-So stories, and most of the ideas associated with the AAH seem to be criticisms of other hypotheses, therefore leaving the AAH as the only alternative. While I can certainly appreciate the frustration Morgan and others must have felt (and even still feel) towards a male-dominated field in science and consideration being mainly given to the strong, archetypal male, I feel that the AAH is taking things too far in the other extreme. It is hard to ignore the feminist underpinnings of Morgan’s writing and the overall disregard for the big picture in order to bring women and children into closer focus. Combating a hypothesis you don’t like with an equally narrow one, just reversed, is not the way to bring greater understanding of our evolutionary history, and given that hominids and apes are so close to us, it’s easy to fall into trapping of preference. Being that I am no expert on the matter, however, I will close with T.H. Huxley’s final words from his work Man’s Place in Nature, as they seem to resonate with the big questions about our origins that remain unknown;

Where, then, must we look for primeval man? Was the oldest Homo sapiens pliocene or miocene, or yet more ancient? In still older strata do the fossilized bones of an Ape more anthropoid, or a Man more pithecoid, than any yet known await the researches of some unborn paleontologist?

Time will show. But, in the meanwhile, if any form of the doctrine of progressive development is correct, we must extend by long epochs of the most liberal estimate that has yet been made of the antiquity of Man.