I really wish I still had the video, but as a child I distinctly remember seeing a stop-motion animation documentary featuring extinct North American mammals. The most memorable scene (if for no other reason than it’s the only one I remember) involved a Smilodon jumping onto the back of a Megatherium, attempting to bring down the beast. Ultimately, the extinct cat fails, the narrator going on to explain that the saber-toothed “tigers” went extinct because their tooth grew so large as to prevent their mouths from closing properly, and they simply could no longer eat. This child-oriented documentary was aired in the early 90′s, and yet scientists knew this idea was hopelessly wrong decades before! Here’s a passage from G.G. Simpson’s The Meaning of Evolution (3rd ed., 1950);
The sabertooth is one of the most famous of animals just because it is often innocently suppossed to be an indisputable example of an inadaptive trend. In fields far remote from paleontology the poor sabertooth has some to figure as a horrible example, a pathetic case history of evolution gone wrong. Its supposed evidence is thus characteristically summarized in a book on (human) personality: “The long canine tooth of the saber-toothed tiger grew more and more into an impossible occlusion. Finally, it was so long that the tiger could not bite effectively, and the animal became extinct.” Now, like so many things that everyone seems to know, this is not true… Throughout their history the size of sabertooth canines varied considerably from one group to another but varied about a fairly constant average size, which is exactly what would be expected if the size were adaptive at all times and there were no secular trend in adaptive advantage but only local and temporary differences in its details. The biting mechanism in the last sabertooths was still perfectly effective, no less and probably no more so than in the Oligocene. To characterize a finally ineffective a mechanism that persisted without essential change in a group abundant and obviously highly successful for some 40,000,000 years seems quaintly illogical! In short, the “inadaptive trend” of the sabertooth is a mere fairy tale, or more fairly, it was an error based on too facile conclusion from imperfect information and it has since been perpetuated as a scientific legend.
While there were some differences in the characteristics of saber-toothed cat canines (length, thickness, etc.), the central point of what Simpson writes is true; the idea that these cats kept evolving longer and longer teeth “just because” is utter nonsense, but yet because it appeared in so many popular works, it became accepted mythology. It is also important to note that things have changed since Simpson initially wrote this piece, being that on the next page he gives a comparison between a skull of Eusmilus and Smilodon to show that comparative tooth-size of these mammals was essentially the same at the beginning and the end. What we now understand, however, is that Eusmilus was a Nimravid and belonged to the family Nimravidae which are considered to have evolved a parallel body form to the true saber-tooths like Smilodon. While this does put a dent in Simpsons comparison between the two skulls, the central point still holds true; an earlier carnivore had canines similar in size to a much later one, neither skull giving us any reason to think the teeth prevented these animals from feeding.
This particular point in the book also illuminates a general theme of Simpsons book; the search for general trends in evolutionary change, if they even exist to begin with. While such ideas may be unheard of today, Simpsons gives considerable space to debunking orthogenesis and autogenic concepts of evolution, the beliefs that evolution always proceeds in a straight line and that there’s some kind internal driving force in organisms that makes them evolve, respectively. While there may still be some ancient adherents to these ideas, it is interesting to note how seriously they are discussed and refuted in a book little more than half a century old. Just as well, in reading Evolution’s Workshop I was surprised to find such a diversity of evolutionary ideas that were designed to somehow debase Darwin’s brand of evolution, i.e. the mixed catastrophism/evolution/secular creationism of Louis Agassiz. Evolutionary thought has itself evolved at frighteningly fast pace, and I think it’s as important to know the history of this brand of science as it is to keep up with the latest phylogenetic studies or fossil discoveries. Sometimes I even wonder about what books 50 years from now will say about this time in history pertaining to evolutionary science; what will the 20/20 hindsight of history reveal?