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ResearchBlogging.orgOnce upon a time a volcano erupted, spewing ash into the air which fell onto the surrounding grassland. But amidst this grey and alien landscape a few pillars of green remained, clumps of forest whose leaves the ash simply fell off. Swinging through this forest was an ape, at first glance like any other. She had a face which sticks out and was covered in hair, with long arms and curved fingers so she can better climb the few trees in her grove. However, when she drops to the ground the similarities suddenly disappear, for she stands up.

It’s unnerving to see something so non-human assume a distinctly human pose. It becomes even more unsettling when she begins to walk, striding forward in the same manner you or I might. She does not waddle from side to side or do something else noticeably different from a normal person, she just walks. And she must, for her patch of forest has run out of food and she has to travel to a new forest on the other side of the ashy grassland in order to survive. She does not want to go, for the journey is long on an open and exposed grassland where they would be easy pickings for a predator. Besides, the grey landscape is not appealing. It’s as unnerving to her as her locomotion is to us.

But eventually she does leave the forest and travel across the plains, leaving behind a trail of footprints. Rain falls and hardens the ash, which is soon buried below more volcanic detritus and preserved. They lay undiscovered for 3 and a half million years until 1978, when the famous Leakys excavated the “Laetoli footprints” as they are now called. At the time they were the oldest evidence of human-like locomotion (Ardipithecus ramidus remains have pushed back the earliest date for bipedal movement) which immediately made them controversial. For something so ancient and so ape-like to be able to walk like a person it’s…..well, it’s unnerviving.

Thus the foundation was laid for a great debate regarding who made it, revolving around whether Australopithecus afarensis was a biped like us. Au. afarensis lived at the same time the tracks were made and – with all contemporary hominins lacking their bipedal adaptations – was the best candidate. Two camps emerged, one arguing that they weren’t the culprit and lacked the ability to walk like a human, the other that they could and they were. Neither side denied that they were a biped – the evidence for that is indisputable – but they argued how they walked bipedally.

Being a scientific debate it was firmly rooted in the evidence, with neither side disputing the traits the fossil had. The disagreement stemmed from the fact that Au. afarensis differed from modern humans in several ways. Those disputing that they made the Laetoli footprints believed that these differences precluded the possibility the species walked like us. On the other hand, those in favour of Au. afarensis being the culprit argued that these differences were mostly related to efficiency, speed and childbirth. They claimed Au. afarensis wasn’t as fast or as efficient as humans (and couldn’t give birth to babies with as large a head as us) but didn’t move in a fundamentally different manner to us. The difference was one of degree.

For decades the debate went back and forth, neither side really able to achieve victory because they couldn’t rebuild an Au. afarensis and check how it walked. At least, they couldn’t until computers came to town. Suddenly researchers had the ability to recreate Lucy et al. and work out how they would’ve actually walked. One of the best such models was created in 2005 by a team of researchers from across the United Kingdom (including one of the lecturers that’ll be teaching me next year). They created a simulation which kept trying different ways of walking until it arrived at the most efficient. When they tested it on living things, this “most efficient” method was almost identical to the way the animals actually walked.

The gait the simulation produced for Au. afarensis

Of course, the simulation results weren’t identical to the manner in which, for example, humans walked. However the differences stemmed from the fact the simulation was simplified rather than any inherent flaw with their technique. Notably it produced much more “stiff” reconstructions because it was a 2D simulation and so couldn’t take into account pelvic rotation. However, this meant that when they reconstructed Au. afarensis and noted such stiff walking they knew that this was an artefact of the simulation and not because Au. afarensis actually walked like that. When all of this was taken into account it revealed that Lucy et al. walked with a natural, human-like gait at speeds of between 0.9 – 1.3 metres/second. For contrast, humans walk between 1.0 and 1.7 m/s.

A human footprint (top), chimp-like footprint (middle) and the Laetoli footprint (bottom). The key difference between top and bottom is that the toes make a deeper impression at Laetoli but this is because they were made on an incline.

So it turns out Australopithecus afarensis could walk like us and – despite being overall slower and less efficient – even fall within the lower bounds of the human norm! Suddenly Lucy et al. look like very viable candidates for the creators of the Laetoli tracks. Now new research has helped cement their claim further by showing that the predicted Au. afarensis speed of between 0.9 – 1.3 m/s almost exactly matches the speed at which the Laetoli hominins were travelling at. A team from across the USA recreated the ash conditions present when the tracks were made and got people to walk across it like a chimp and like a human at different speeds. They found that someone walking like a human at 1 m/s was an almost perfect match for the Laetoli prints (after the fact humans weigh more had been taken into account).

Australopithecus afarensis was a capable upright biped, much like the creature which made the Laetoli footprint and, crucially, like modern humans. It would seem that our lineage developed competent bipedalism early in its history even if it was ultimately slower and less efficient than it is today. The speed at which bipedalism evolved is probably because our common ancestor with chimps was already partway there and not much evolution was needed to create a being capable of leaving the Laetoli footprints.

Now the only real question which remains about those tracks is whether the upright ape who made them ever reached their destination.

Kimbel, W.H. & Delezene, L.K., 2009. ‘Lucy’ redux: A review of research on Australopithecus afarensis. American Journal of Physical Anthropology, 140(S49), pp.2–48.
Raichlen, D.A. et al., 2010. Laetoli Footprints Preserve Earliest Direct Evidence of Human-Like Bipedal Biomechanics. PLoS ONE, 5(3), p.e9769.
Sellers WI, Cain GM, Wang W, & Crompton RH (2005). Stride lengths, speed and energy costs in walking of Australopithecus afarensis: using evolutionary robotics to predict locomotion of early human ancestors. Journal of the Royal Society, Interface / the Royal Society, 2 (5), 431-41 PMID: 16849203

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