One of the most basic differences separating us from other apes is our ability to walk fully upright, and that goes back to the emergence of the Homo genus 1.9 million years ago. At least...that's what we used to think.
The human gait is unique not just in the fact that we walk fully upright on two legs, but precisely how we do it - we push off the ground using our big toe and by bending the sole of the foot, quite unlike other primates. These particular features are thought to distinguish the bipedalism of humans and our extinct relatives like Neanderthals. While our ancestors before 1.9 million years ago may have been bipedal, they were still in a transitional stage, and likely walked in a crouched position as a sort of throwback to their earlier, knuckle-walking gaits.
But now researchers at the Universities of Liverpool, Manchester, and Bournemouth have teamed up to re-examine the famous Laetoli footprints. These were left 3.7 million years ago by hominids walking through the volcanic ash of the nearby Sadiman Volcano, which then hardened and preserved the footprints for posterity. These human ancestors, who were most likely members of Australopithecus afarensis, were clearly bipedal - there were no marks from their knuckles, for a start - but previous researchers had assumed that was about where the similarities with the modern human gait came to an end.
By examining these prints with new technology that can determine what are genuine features of the ancient feet and what are just byproducts of erosion and other environmental factors, the researchers have figured out just what the hominids' gait would have looked like - and they're about as human as they could possibly be.
Professor Robin Crompton of the University of Liverpool explains:
"It was previously thought that Australopithecus afarensis walked in a crouched posture, and on the side of the foot, pushing off the ground with the middle part of the foot, as today's great apes do. We found, however, that the Laetoli prints represented a type of bipedal walking that was fully upright and driven by the front of the foot, particularly the big toe, much like humans today, and quite different to bipedal walking of chimpanzees and other apes.
"Quite remarkably, we found that some healthy humans produce footprints that are more like those of other apes than the Laetoli prints. The foot function represented by the prints is therefore most likely to be similar to patterns seen in modern-humans. This is important because the development of the features of human foot function helped our ancestors to expand further out of Africa."
And, while the evolution of bipedalism would indeed be crucial to hominids' expansion out of Africa, that still was millions of years after when these footprints were made. Crompton continues:
"Our work demonstrates that many of these features evolved nearly four million years ago in a species that most consider to be partially tree-dwelling. These findings show support for a previous study at Liverpool that showed upright bipedal walking originally evolved in a tree-living ancestor of living great apes and humans. Australopithecus afarensis, however, was not modern in body proportions of the limbs and torso.
"The characteristic long-legged, short body form of the modern human allows us to walk and run great distances, even when carrying heavy loads. Australopithecus afarensis had the reverse physical build, short legs and a long body, which makes it probable that it could only walk or run effectively over short distances. We now need to determine when our ancestors first became able to walk or run over the very long distances that enabled humans to colonise the world."