The ancient human species Australopithecus afarensis may have been the earliest hominin to run on two legs. Although it had relatively short, ape-like legs, A. afarensis may have had a long Achilles’ tendon just like modern humans do – a feature that helps us to run more efficiently.
Conventional thinking is that early hominins like A. afarensis – the species to which the famous Lucy fossil belonged – learned to walk long before they could run. Lucy was an ape-like bipedal hominin sometimes seen as a likely direct ancestor of the earliest species of human.
Some evidence places the origin of bipedal walking more than 10 million years ago. But many researchers think it was only with the appearance of the human genus Homo, between 2 and 3 million years ago, that hominins began to run.
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Ellison McNutt at the University of Southern California thinks the story is more complicated than that. Some earlier hominins should have had some ability to run when faced with a predator, for instance. McNutt looked for evidence of running ability in A. afarensis, because this species appeared about 3.9 million years ago and disappeared a million years later, about the same time as the first humans, such as Homo habilis, evolved.
She and Jeremy DeSilva at Dartmouth College in New Hampshire focused on the Achilles’ tendon, a band of tissue connecting the calf muscles to the heel.
Modern humans have a long Achilles’ tendon that extends more than halfway up our lower leg. It stretches when we run to store elastic energy that it then releases explosively. This helps us to save as much as 35 per cent of the energy we use when running. “A long Achilles’ tendon is helpful for efficient walking, but it is especially critical for efficient running,” says McNutt.
Tendons rarely fossilise. But by studying the shape of the heel bone in humans and 11 other living primates, McNutt and DeSilva discovered that the size of one facet on the rear of the heel bone scales with the length of the Achilles’ tendon.
When they measured two A. afarensis heel bones, they could calculate that the species might have had an Achilles’ tendon that extended more than halfway up its calf, just as in modern humans. For comparison, chimps – who can’t run well on two legs – have an Achilles’ tendon that stretches barely higher than the ankle.
“Currently, I think A. afarensis is the earliest hominin for which we have good evidence for some of the key adaptions necessary for modern human-like running,” says McNutt. But we know there were earlier bipedal hominins, such as Ardipithecus. McNutt says as we learn more about these species, we might conclude that even earlier hominins were also born to run.
The Anatomical Record
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