for National Geographic News
About 1.5 million years ago, human ancestors walked upright with a spring in their steps just as modern humans do today, suggests an analysis of ancient footprints found in northern Kenya.
The prints are the oldest known to show modern foot anatomy.
The discovery also helps round out the picture of a cooling and drying episode in Africa that compelled tree-dwelling human ancestors to venture into the open landscape for food, said John Harris, a paleoanthropologist at Rutgers University in New Brunswick, New Jersey.
The ancient footprints indicate a rounded heel, pronounced arch, and a big toe parallel to the other toes just as modern humans have, Harris noted. The big toes of chimpanzees, by contrast, splay outward, which is useful for grasping branches.
"We've lost that, but what we've created is a platform from which we can step up on and balance ourselves on and push off on in bipedal locomotion," said Harris, who is a co-author of a paper describing the footprints in tomorrow's issue of the journal Science.
Embedded in Mud
The rare prints were found embedded in what was once muddy soil among tracks of ancient birds, lions, antelopes, and other critters. Harris said the print makers were likely walking to or from a watering hole.
The size and spacing of the footprints indicate they were made by people with bodies similar to modern humans. Given their age, the prints were most likely made by Homo erectus, the first human ancestor to sport long legs and short arms, Harris said.
At the time H. erectus emerged, about 1.5 to 1.7 million years ago, global climate was cooling and the African landscape was changing from tropical forest to open savanna. Food sources—nuts, fruits, vegetables, and animals—were becoming more dispersed.
"There was selection for creatures, including ourselves, that could walk over longer distances on the landscape between the patches of more productive food," Harris said.
Adapted for Running?
Daniel Lieberman is an anthropologist at Harvard University in Cambridge, Massachusetts, and an expert on the evolution of human locomotion. In an email exchange, he said the "prints unambiguously indicate that by 1.5 million years ago H. erectus had a human-like foot."
Other human ancestors such as the australopithecines may have also been efficient walkers, he said. But a more modern foot anatomy with spring-like arches and short toes is important for running, which may have contributed to the success of H. erectus.
"I would be surprised if this were not the case," Lieberman said. "Because how could H. erectus have hunted more than a million years before the invention of tipped spears—as we know it did—without the ability to run well?"
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