A comparison of three-dimensional scans of hominin footprints. Top) A footprint made by an experimental subject using a normal, “extended” gait. Middle) A footprint made by an experimental subject using a “bent-knee, bent-hip” gait. Bottom) A Laetoli footprints. From Raichlen et al., 2010.
About 3.6 million years ago, at a spot now in Laetoli, Tanzania, a pair of hominins trudged through the ashfall dumped onto the landscape by a nearby volcano. We don’t know for certain what they looked like (it is generally believed that they were Australopithecus afarensis from the presence of fossils found at the site), but the fossil trackway they left behind has provided scientists with a narrow glimpse into the life and behavior of these individuals. The big question has been what these tracks say about how the prehistoric humans moved. Did they walk like us, like apes forced to stand up, or in an entirely different fashion?
According to a new study published this week in PLoS One, the famous Laetoli trackway preserves the footprints of upright, bipedal hominins that walked in a manner extremely similar to us. After making eight modern human subjects walk through a 5 meter long sand trackway (both normally and using a bent-knee, bent-hip posture thought to simulate the gait of apes) David Raichlen, Adam Gordon, and colleagues compared the footprints to the fossil tracks from Laetoli using three-dimensional scans. What they found was that the Laetoli tracks most closely resembled those made by people walking using a normal, “extended limb” gait and not the bent-knee, bent-hip gait.
What this suggests is that 3.6 million years ago, before the evolution of the first members of our genus (Homo), hominins were walking with a posture and gait very similar to our own. This appears to fit well with the growing body of evidence that the first hominins did not evolve from knuckle-walking ancestors, but instead had a unique mode of locomotion which was modified into bipedalism while walking on the ground. This means that upright, bipedal walking is best understood not as an adaptation, but the result of anatomical exaptations (or the consequence of other changes which initially had nothing to do with upright walking but were shifted into new roles).
Unfortunately, the new PLoS One study is not as conclusive as some of the press releases surrounding it have made it seem. It is true that the Laetoli footprints scrutinized in the study more generally resembled those made by humans using an extended limb posture, but they are not exactly alike, either. The Laetoli prints are deeper than the ones made by modern humans, and the middle of the Laetoli prints, especially, seem to indicate that more pressure was placed on the arch of the foot.
Additionally, having modern humans walk with an “ape-like” bent-knee, bent-hip posture may have created a false alternative model. Our anatomy is not well-suited to this kind of walking, so it is little wonder that the tracks made in such circumstances would not match well with those made by a bipedal hominin. It might have been better to have apes walk bipedally over the same trackway and then analyze those footprints, but then again the foot anatomy of living apes differs greatly from those of the trackmakers and us. It is not as if the Laetoli trackmakers were intermediate between living apes and our species, anyway, and so even this experimental approach would have its flaws.
The trouble with figuring out how early hominins walked is that they have no living equivalent. They were anatomically distinct from both living apes and our species, and so using the goalposts of more “ape-like” or more “human-like” is not especially informative. Based upon what we know of early hominins around 3.6 million years ago I think the authors are correct that the Laetoli trackmakers were not walking with a bent-knee, bent-hip posture, but neither do I think that they were yet walking just like we do. Further studies will be required to resolve precisely how the Laetoli trackmakers moved around the landscape.
Raichlen, D., Gordon, A., Harcourt-Smith, W., Foster, A., & Haas, W. (2010). Laetoli Footprints Preserve Earliest Direct Evidence of Human-Like Bipedal Biomechanics PLoS ONE, 5 (3) DOI: 10.1371/journal.pone.0009769