Holy Hominids Batman! There has been a flood of news over the past few days about human evolution, especially in terms of when we became obligate bipeds.
While walking on our knuckles or stooping down to some other posture would not be comfortable, virtually everyone experiences some variety of back problems at some point, and Dr. Aaron Filler, writing in the journal Neurosurgical Focus, attempts to explain the origins of the back pain we experience today through evolutionary time. The paper, “Emergence and optimization of upright posture among hominiform hominoids and the evolutionary pathophysiology of back pain,” explains that the vast majority of back pain cases have to do with our lumbar region of our spine, or the vertebrae that extend from our sacrum (pelvis) to our diaphragm (just below the rib cage). This part of our spine, when viewed from the side, is curved inwards, a condition that (as far as I am aware) is unique to bipedal primates. There is more to the evolution of the spine than a change from relatively straight to curved, however; two prongs of bone, one on each side of an individual vertebra called the lumbar transverse process, moved from pointing forward (ventrally) to sticking out on the sides at the most dorsal part of the vertebra (see the paper for an illustration). This migration may seem insignificant at first, but the placement of the lumbar transverse processes help to determine muscle attachment and how far the animal and bend/extend its back, and so the move from ventral to dorsal of these bits of bone was surprisingly important to enabling primates the ability to adopt a more upright posture. This change, however, was not 100% beneficial; more strain would be put on the lumbar vertebrae and the muscles being that both the bone and muscle would become important for support and movement, much of the movement of the upper body relating to muscles in the lumbar region. When did this change start to occur? According to Filler, approximiately 21 million years ago in what is now Uganda, the ape Morotopithecus bishopi giving us perhaps the earliest evidence of a change in LTP structure (perhaps pushing back the possibility of “upright apes” further than previously thought). Given that people are not being prevented from having children due to back problems (especially experienced in middle age), our lumbar problems will likely continue as long as our species continues to exist.
While Filler’s study may give us some clues as to the variations that gave rise to the changes, the pressures that would favor being fully bipedal (and not just some of the time) are still mysterious. A new PNAS study, however, suggests that it is more efficient to be an obligate biped than a knuckle-walker, however. The study, “Chimpanzee locomotor energetics and the origin of human bipedalism” by Sockol, Raichlen, and Pontzer points out that modern humans have a far more efficient way of getting around than extant chimpanzees, suggesting that a move towards bipedalism would be favored during time when apes may have had to go long distances to procure food at the end of the Miocene. Part of the problem, however, is that chimpanzees were about 10% less energy efficient when walking upright than when using all four limbs, so (as Filler suggested in the article previously discussed), anatomical changes in the spine and lumbar muscles would likely have to anticipate the behavioral attempts to walk longer distances upright. Sockol, Raichlen, and Pontzer point out that it’s not all about the spine, however; given their data we would expect early hominids to have longer relative leg lengths, as the length of stride has a lot to do with being able to walk upright over long distances. Part of the reason the chimpanzees were likely inefficient bipeds is because it is uncomfortable to a degree and their anatomy has not been adapted to standing upright for a long amount of time, probably experiencing some amount of back discomfort and unable to take comfortable strides with their comparatively shorter legs. Thus, not only would the bones and muscles along the spine have to change, but also limb structure, especially a lengthening of the legs.
Still, the study does not necessarily answer the question of why we are bipedal and chimpanzees are not. Chimpanzees, bonobos, and gorillas call can stand up on two legs, but their main mode of locomotion is to knuckle-walk; what preserved such anatomy and behavior in our closest living relatives but favored a bipedal gait in us? What sort of stages/variations did bipedalism go through before arriving at us? What sort of locomotion did our ancestors employ? What was the environment like? This last question, to me, is of the utmost importance; we’ll need to more fully understand the ecology and biogeography involved in hominid evolution if we are to correctly identify the pressures that led to our current state. While the new study is important, I still think the bigger question is still unanswered, a change in energy efficiency not being enough to explain the move from knuckle-walking to obligate bipedalism (I would imagine that some of the transitional forms might have even experienced some disadvantages). Hopefully the fossil record will be kind to us and yield more post-cranial skeletons for us to study, but until then hypotheses will abound.
Speaking of fossil hominds, a new press release offers some tantalizing (but scant) details about some 3.5-3.8 million year old remains found in Ethiopia, perhaps providing some insight into the relationships of Australopithicene hominids. A jaw, a partial skeleton, and other fossils are said to have been recovered, a 2005 report mentioning that some of the bones might be a link between Australopithecene species. We’ll do well to keep our eyes open for when the fossils are fully described and analyzed as they seem to be from a key point in hominid evolution.