Bipedalism: From the ground-up or trees-down?

The origin of human bipedalism has long been a hot topic among paleoanthropologists. At the very least it is seen as something of a marker for the emergence of the first hominin, yet it remains unclear whether the earliest hominins evolved from a terrestrial, knuckle-walking ancestor or a more arboreal ape. A common interpretation is that since our closest living relatives, gorillas and chimpanzees, are both knuckle-walkers then the first hominins, too, evolved from a knuckle-walking ancestor. As Tracy Kivell and Daniel Schmitt explain in a new PNAS paper, though, this idea may overlook subtle differences between chimpanzees and gorillas that may help us understand the evolution of the earliest hominins.

Gorillas are physically larger than chimpanzees, so it might be expected that they would have more rigid wrists that would help stabilize them as they walked around on their knuckles. This is not what Kivell and Schmitt found. Not only did gorillas have a much greater range of wrist motion than chimpanzees, but it was the chimpanzees that had adaptations in their wrists to increase stability. Despite being more terrestrial and knuckle-walking on the ground more often gorillas actually showed fewer "classic" knuckle-walking adaptations in their wrist bones than chimpanzees did!

As Kivell and Schmitt note, gorillas and chimpanzees are not the same when it comes to knuckle-walking. It seems that gorillas have more wrist flexibility because they hold their lower arms and wrists in a columnar fashion which gives them the support they need. Chimpanzees, on the other hand, bend their wrists backwards in an "extended" position when supporting themselves on their knuckles, and their range of motion is restricted by the shape of the wrist bones. Why do gorillas and chimpanzees differ in this way?

Surprisingly, some of the "knuckle-walking features" of the chimpanzee wrist are seen in arboreal, quadrupedal primates. It may be, then, that the traits of the chimpanzee wrist commonly associated with knuckle-walking are actually adaptations to climbing in the trees. Perhaps, the authors hypothesize, this is because the primates that exhibit this wrist-limiting morphology extend their wrists to grasp with their hands while moving on all fours in trees. If they are correct the wrist morphology of chimpanzees may have more to do with what the apes do in the trees than what they do on the ground.

There are other traits of the wrist and lower arm associated with knuckle-walking the authors did not investigate, but it is significant that 1) not all knuckle-walking apes had all the "knuckle-walking traits", and 2) at least some of those traits are probably more associated with life in the trees than on the ground. The authors state;

The results of this study show that researchers need to reevaluate all posited knuckle-walking features and reconsider their efficacy as indicators of knuckle-walking behavior in extant and extinct primates. In this context, the absence of several posited knuckle-walking features in extant knuckle-walkers (and the presence of some of these features in nonknuckle-walkers) makes it difficult to argue that there is unambiguous evidence that bipedalism evolved from a terrestrial knuckle-walking ancestor. Instead, our data support the opposite notion, that features of the hand and wrist found in the human fossil record that have traditionally been treated as indicators of knucklewalking behavior are in fact evidence of arboreality and not terrestriality.

It cannot be assumed that the earliest hominins evolved from an ancestor that knuckle-walked on the ground, and the differences in the knuckle-walking traits in gorillas and chimpanzees suggests that it could have independently evolved in each lineage. At the very least the authors have indicated that knuckle-walking in chimpanzees and gorillas cannot be considered to be identical, and it is very interesting that some traits thought to indicate life on the ground may in fact be adaptations to life in the trees. More fossil evidence and comparative studies will be required to test the hypotheses the authors propose, but this paper provides a good starting point to reconsider how the earliest hominins evolved.

See Afarensis for another take.

Kivell TL, & Schmitt D (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 of the United States of America PMID: 19667206