Fossil foot bone shows our ancestor had two feet firmly on the ground

The discovery that Australopithecus had arched feet suggests this human ancestor had already abandoned life in the trees
Foot bone from Australopithecus afarensis and human foot
The 3.2m-year-old foot bone resembles its equivalent in humans and shows Australopithecus afarensis had arched feet. Photograph: Kimberly A. Congdon/Carol Ward/Elizabeth Harman/Science

The ancestors of humans were walking upright more than 3m years ago, according to an analysis of a fossilised foot bone found in Ethiopia. The fossil, the fourth metatarsal bone from the species Australopithecus afarensis, shows that this forerunner of early humans had a permanently arched foot like modern humans, a key requirement for an upright gait.

Arches in human feet put a spring in our step: they are stiff enough to propel us forward but flexible enough to absorb the shock at the end of each stride.

Scientists already knew that A. afarensis could walk on two feet but were unsure whether the creatures climbed and grasped tree branches as well, much like their own ancestor species and modern nonhuman apes. The fourth metatarsal, described on Thursday in Science, shows that A. afarensis moved around more like modern humans.

"Now that we know Lucy and her relatives had arches in their feet, this affects much of what we know about them, from where they lived to what they ate and how they avoided predators," said Carol Ward, a professor of integrative anatomy at the University of Missouri-Columbia who led the analysis of the fossil.

"The development of arched feet was a fundamental shift toward the human condition, because it meant giving up the ability to use the big toe for grasping branches, signaling that our ancestors had finally abandoned life in the trees in favour of life on the ground."

The best-known example of A. afarensis is "Lucy", who lived in eastern Africa more than 3m years ago. Before that, more than 4.4m years ago, Ethiopia was populated by Ardipithecus ramidus, which seems to have been a part-time terrestrial biped, though its foot had many of the features of tree-dwelling primates, including a highly mobile big toe.

Unlike other primates, human feet have two arches, which stretch along the length of the foot and across it. Ape feet do not have these arches and are far more flexible, with a mobile large toe that is useful for climbing trees and holding onto branches.

These ape-like features are not present in the foot of A. afarensis, however. Given that its foot was more like that of modern humans, scientists think that A. afarensis no longer depended on the trees for refuge or resources 3m years ago.

"Arches in the feet are a key component of human-like walking because they absorb shock and also provide a stiff platform so that we can push off from our feet and move forward," said Ward. "People today with 'flat feet' who lack arches have a host of joint problems throughout their skeletons. Understanding that the arch appeared very early in our evolution shows that the unique structure of our feet is fundamental to human locomotion.

"If we can understand what we were designed to do and the natural selection that shaped the human skeleton, we can gain insight into how our skeletons work today. Arches in our feet were just as important for our ancestors as they are for us."

Isabelle De Groote, a palaeontologist at the Natural History Museum in London, said: "These findings confirm that our human ancestors were walking on two legs by about 3.2m years ago.

"Bipedal locomotion or two-legged walking is one of the hallmarks of the human species. Older human fossils still show adaptations to spending some of their time in the trees ... for feeding or nesting, but the evidence here suggests that by 3.2m years ago one of our ancestors, Australopithecus afarensis, was fully committed to bipedal walking."

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