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 Achillesā tendon just like modern humans do ā a feature that helps us to run more efficiently.
Conventional thinking is that early hominins like A. afarensis ā the species to which the famous Lucy fossil belonged ā 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 theĀ origin of bipedal walking more than 10 million years ago. But many researchers think it was only with the appearance of the human genusĀ Homo, between 2 and 3 million years ago,Ā 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 speciesĀ appeared about 3.9 million years agoĀ 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 Achillesā tendon, a band of tissue connecting the calf muscles to the heel.
Modern humans have a long Achillesā 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. āA long Achillesā tendon is helpful for efficient walking, but it is especially critical for efficient running,ā says McNutt.
TendonsĀ rarely fossilise. But by studying the shape of the heel bone in humans and 11 other livingĀ primates, McNutt and DeSilva discovered that the size of one facetĀ on the rear of the heel bone scales with the length of the Achillesā tendon.
When they measured twoĀ A. afarensis heel bones, they could calculate that the species might have had an Achillesā tendon that extended more than halfway up its calf, just as in modern humans. For comparison, chimps ā who canāt run well on two legs ā have an Achillesā tendon that stretches barely higher than the ankle.
āCurrently, I thinkĀ A. afarensisĀ is the earliest hominin for which we have good evidence for some of the key adaptions necessary for modern human-like running,ā 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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