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
It sounds fascinating--and as though film makers might be of assistance in devising a battery operated hominid to resemble as much as is known about Laetoli.
I'm curious what could be gained through analysis of tracks made from a variety of human-possible gaits. It seems that, at this point, we should be casting a pretty wide net. At the very least it could get the investigation pointed in a good direction(?).
Gaits are as individual as faces, and as characteristic as nationalities. I (and you, perhaps with training) can recognize an individual from afar based on nothing but characteristic details of movement, and can distinguish unfamiliar individuals on the basis of their (e.g.) walking like a German vs. walking like a Frenchman. Young Swedes walk more like Americans than like their parents.
The gaits of the people who walked through that ashfall are a complex product of their anatomy, their individual histories, habits of their band, and their momentary mood. You can't get much by comparing to impressions from aping an ape's (which ape's?) gait, but perhaps it's a start.
I'd like to see what sort of tracks gibbons make. Unlike the non-human great apes, gibbons (and siamangs) have as much torsional flexibility in their spines as humans (and probably Australopithecenes), although their backbones lack the load-bearing recurve possessed by great apes (including Australopithecenes).
I don't think that having an ape walk bipedally would solve the problem. I seem to remember that somebody (unfortunately I don't know who and didn't save the abstracts) did that type of study at the last AAPA conference I went to, and the forceplates showed that humans left a nice, consistent mark; the apes were very variable, and nobody left the same mark twice.
But, even if we don't have a nice living equivalent to the Australopithecine, we DO have the anatomy of the most likely thing that made the Laetoli trackway, which is afarensis. That pelvis, sacrum, and the spine show without a doubt that Lucy and her folks were perfectly committed bipeds. The main changes in the pelvis that have occurred since then have been in the areas dealing with birth, and a little bit of adjustment for an increase in height.
Frankly, if the footprint analysis had come back "Bent-knee, bent-hip," I'd be more inclined to conclude that they had been left by something other than Lucy, than that Lucy was a BK/BH walker.
"Deeper and more pressure on the arch" suggests they were carrying a heavy load.
There are several bits here I take issue with.
"This means that upright, bipedal walking is best understood not as an adaptation, but the result of anatomical exaptations " - hold on, the evidence does not go anywhere near that conclusion. While it may well be that Homo did not originate from knuckle-walking ancestors (I tend to agree with that)none of the evidence gives any reason to consider it more likely an exaptation rather than an adaptation. Assertions do not constitute evidence.
Zinjanthropus states that "pelvis, sacrum, and the spine show without a doubt that Lucy and her folks were perfectly committed bipeds" - I know that has been asserted, personally I find the evidence for that entirely unconvincing. Facultative bipeds yes, 'committed bipeds' no.
To me this just adds more evidence to the idea that these footprints were not produced by Australopithecus afarensis.
Personally I feel that the logic that these footprints were made by an australopithecine based on the fact that australopithecine remains are known from the same area and period, is somewhat flawed. It is a conclusion reached despite the fact that the footprints have so much in common with modern human footprints, and that it is not a good fit with australopithecine anatomy (I know authorities do differ on this point, but go look at the evidence yourself). Over all I think it would tend to suggest that that they were not produced by an australopithecine, and that a hominid closer to (or assignable to) Homo was present.
It's good to have this new information, since this is a subject that's been debated for a long time, sometimes quite viciously.
"perfectly committed bipeds" (whatever "perfectly committed" is supposed to mean)???
only in your (and Lovejoy's) wildest dreams...
Assuming these prints belong to Au. afarensis for the sake of discussion:
relatively long lateral toes, chimp-like curved pedal phalanges, unstable 1st metatarso-phalangeal (big toe) joint, a whopping navicular tuberosity and a mobile calcaneo-cuboid joint (probably flat-footed), relatively short hind limbs, coronally oriented iliac blades with no evidence of a posterior gluteal line, etc.
yep, "perfect"...for an autralopithecine perhaps that still frequented the trees for sleeping sites, to forage and to escape from predators.
Could the depth of the footprints have any relationship with the consistency of volcanic ash versus sand? In my experience, which is quite limited, volcanic ash is rather finer grained (more like silt or mud)than are sands.