The first evidence of human relatives walking on two feet comes from about 70 footprints left by at least two Australopithecus afarensis walking across soft volcanic ash about 3.6 million years ago. A. afarensis was a short hominin with a jutting lower jaw, which walked upright but may also have spent some time in the trees; the most famous member of the species is a fossil woman now called Lucy.
Not far from that site, another set of footprints reveals that Lucy and her kin may have lived alongside another bipedal hominin species, one that moved very differently.
The forgotten footprints
When the footprints were first spotted in 1976, the paleoanthropologists who unearthed them from what’s called Site A weren’t sure what to make of them. Paleoanthropologist Mary Leakey suggested they could be hominin tracks, but others weren’t so sure. One anthropologist even suggested the tracks could have been left by a young bear walking on its hind legs for a few steps.
Two years later, Leakey and her colleagues discovered the two sets of A. afarensis tracks at Site G; the now-famous trackway was a sensational-enough find to completely overshadow Site A. Ohio University anthropologist Ellison McNutt and colleagues recently revisited Site A and its five footprints, using Leakey and her colleagues’ field notes to find the site (it’s not far from a trail of equally old elephant tracks). They re-excavated the footprints, carefully cleaned them, and recorded them with 3D photogrammetry, a technique that uses multiple two-dimensional photos of an object to map the object in three-dimensional space.
After measuring the prints and comparing them to the tracks of modern people, ancient hominins like A. afarensis, chimpanzees, and bears, McNutt and colleagues concluded that the footprints belonged to a hominin—but a member of a different species than Lucy and her kin.
Bipedalism wasn’t a one-off
McNutt and colleagues don’t know which hominin species might have left the five footprints at Site A, and they don’t offer any hypotheses in their recent paper about it. However, the footprints offer some clues about how the individual moved and what its feet looked like. Whoever left the footprints at Site A was noticeably shorter than an average A. afarensis, and the trackmaker’s feet were wider and differently shaped than those of Lucy and company.
Unlike our big toes, apes’ big toes stick out to the side and move more freely than the other four, almost like a thumb; A. afarensis‘ big toes still very noticeably stuck out to the side, although not as much as an ape’s. And the hominin who left the tracks at Site A had an even more thumb-like big toe than A. afarensis.
But while apes can bend and curl their feet at the middle, for more flexibility while climbing, A. afarensis seems to have had a stiffer foot, more like ours, which works better for walking. Based on the impressions left behind, the Site A hominin may have had a more flexible, ape-like midfoot than A. afarensis.
“A minimum of 2 hominin taxa with different feet and gaits co-existed at Laetoli,” wrote McNutt and colleagues. This species was adapting to the bipedal life at around the same time as Lucy and her A. afarensis relatives, but its footprints suggest that it was taking a different evolutionary route.
(A 2018 study went into more detail on the mechanics of our early relatives’ strides.)
Walk this way?
Overall, the prints look more like they were made by a chimpanzee foot than a modern human foot, except that the heels are too wide for a chimpanzee. And, despite its chimp-like feet, this hominin didn’t actually walk like a modern ape.
When a chimpanzee walks on two legs, the wide set of its hips keeps its legs held apart. As a result, chimpanzee steps are wider and shorter than a human’s would be. But the hominin at Site A moved differently, crossing one foot in front of the other, as if it were trying to pass a Pliocene field sobriety test.
“The gait seems somewhat shambling, with one foot crossing in front of the other,” wrote Mary Leakey in 1976. A chimpanzee—or a bear, for that matter—would have found the cross-stepping feat nearly impossible, even for just a few steps. Modern humans don’t walk this way very often; usually it’s a way of regaining balance after a stumble (your faithful correspondent sustained only minor injuries in the attempt to replicate it).
If the hominin at Site A was able to walk this way, say McNutt and colleagues, it must have had one or both of two key features that help modern humans walk upright. Compared to a chimpanzee, our hips are adducted, or turned inward, so that we move with our legs closer together. And our knees are set at an angle that puts our feet just below our body’s center of mass. So these traits had appeared in more than one species by 3.6 million years ago, although their feet still looked quite different.