After writing about ‘Ardi’, I remembered that this wasn’t the only paper I’d read recently suggesting that our ancestors were not knucklewalkers and that knuckle-walking must have evolved independently in the gorilla & chimp lineages. (We can say this because, if the last common ancestor of chimps & humans didn’t get around by knuckle-walking, then neither did the earlier last common ancestor of chimps-humans & gorillas.) Also in my ‘to write about’ folder is a paper by Tracy Kivell & Daniel Schmitt (2009), entitled Independent evolution of knuckle-walking in African apes shows that humans did not evolve from a knuckle-walking ancestor.
As Kivell & Schmitt note, scientists have been debating the origins of bipedalism in humans for many years, focusing on whether we evolved from some knuckle-walking ancestor that must have spent much of its time on the ground, or from an animal that still lived in the trees. I suspect that knuckle-walking has had the upper hand (sorry!) for a while, although the relatively early development of bipedalism must have given some pause for thought - even before the latest flurry of publications on Ardipithecus ramidus, there were suggestions that this species was probably bipedal.
Up until fairly recently, the arguments were based around evidence (or lack of it) for adaptations for knuckle-walking in the various members of the human lineage. The other side of the coin, of course, would be evidence for this behaviour in our closest living relatives, the chimps & gorillas, and in other primates. Kivell & Schmitt decided to evaluate this evidence, collecting data on supposed knuckle-walking characteristics in apes & monkeys & then "[using] these data to test the hypothesis that all knuckle-walking apes share similar anatomical features and that these features can be used to reliably infer locomotor behaviour in our extinct ancestors" (2009). In other words, if the same suite of physical features were found in all African apes, then we could safely assume that they were knuckle-walking adaptations that were also present in the last common ancestor of chimps & hominins. If, however, these features differed between chimps & gorillas, then all bets would be off – it would suggest convergent evolution in these groups & make it much less likely that the earliest hominins also had this trait.
Kivell & Schmitt found that a number of earlier papers had listed features linked to knuckle-walking, largely to do with the bones of the wrist, which limited extension of the wrist. It turned out, though, that many of the studies involved rather small sample sizes & were often restricted to chimpanzees alone; nor did these studies look at how the features changed during development (eg the amount of curvature in chimp finger bones changes as individuals grow & spend more time actively moving around in the trees.) So Kivell & Schmitt made sure that they examined developmental changes in the wrists of chimps, gorillas, & other primates, in addition to simply comparing the presence & extent of the various features they were interested in.
Rather to their surprise, the authors found that there wa a lot of variation in wrist structures of adult African apes, to a degree that "casts doubt on the assumed functional link between specific aspects of wrist morphology and knuckle-walking behaviour" (Kivell & Schmitt 2009). What’s more, most of the gorillas in their sample didn’t have the key features that scientists had always thought were needed to restrict wrist extension while knuckle-walking. Other supposed knuckle-walking features, while present in adult chimps, were also found in non-knuckle-walking monkeys – & were rare in gorillas. When they looked at the biomechanics of this form of locomotion, they found that chimps knucle-walk with their wrists in a relatively extended position, while gorillas have what they described as a "neutral, columnar hand posture" – the differences appear to be related to environment, with gorillas being more terrestrial.
These differences strongly suggest that knuckle-walking evolved independently in chimps & gorillas. Kivell & Schmitt concluded that "[the] presence of purported knuckle-walking features in the hominin wrist can thus be viewed as evidence of arboreality, not terrestriality, and provide evidence that human bipedalism evolved from a more arboreal ancestor…" The new descriptions of Ardipithecus certainly provide strong support for that conclusion.
T.L.Kivell & D.Schmitt (2009) Independent evolution of knuckle-walking in African apes shows that humans did not evolve from a knuckle-walking ancestor. Proceedings of the National Academy of Sciences 106 (34):14241-14246 doi 10.1073.pnas.0901280106