Some time ago, I noted that a key human feature, the opposable thumb, might be nudged into existence simply with the emergence of the pentadactyl limb and the appearance of trees. In fact, there is evidence to support the idea that the origin of our bipedalism occurred while our genetic ancestors lived among the trees.
Now there is a new study to suggest that this bipedalism was a preadaptation:
More than three million years ago, the ancestors of modern humans were still spending a considerable amount of their lives in trees, but something new was happening.
David Raichlen, an assistant professor in the University of Arizona School of Anthropology, and his colleagues at the University at Albany and City University of New York’s Lehman College have developed new experimental evidence indicating that these early hominins were walking with a human-like striding gait as long as 3.6 million years ago.
“Based on previous analyses of the skeletons of Australopithecus afarensis, we expected that the Laetoli footprints would resemble those of someone walking with a bent knee, bent hip gait typical of chimpanzees, and not the striding gait normally used by modern humans,” Raichlen said. “But to our surprise, the Laetoli footprints fall completely within the range of normal human footprints.”
The fossil footprints at Laetoli preserve a remarkably even depth at the toe and heel, just like those of modern humans. “This more human-like form of walking is incredibly energetically efficient, suggesting that reduced energy costs were very important in the evolution of bipedalism prior to the origins of our own genus, Homo,” Raichlen said.
If the Laetoli footprints were made by Lucy’s species, as most scientists agree to be the case, these experimental results have interesting implications for the timing of evolutionary events.
“What is fascinating about this study is that it suggests that, at a time when our ancestors had an anatomy well-suited to spending a significant amount of time in the trees, they had already developed a highly efficient, modern human-like mode of bipedalism,” said Adam Gordon.
It would seem to me that once a body had acquired the opposable thumb and human-like bipedalism, these would serve to nudge the emergence of a human-like brain that could fully exploit the architecture of the body. Simon Conway Morris speaks of the high likelihood of the evolutionary emergence of intelligence (a view shared by SETI advocates) and points to crows and dolphins as examples of the independent evolution of such a trait. However, given that crows and dolphins do not have the best body plan to fully facilitate the emergence of intelligence, this might explain why they have not reached the same level of intelligence as we see in humans.