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When our human ancestors first walked tall

A 3.2-million-year-old fossil foot bone, discovered in Hadar, Ethiopia, is shedding new light on when our human ancestors first began to walk upright. 

This image shows the position of the fourth metatarsal Australopithecus afarensis (AL 333-160) recovered from Hadar, Ethiopia, in a foot skeleton. Credit: Carol Ward/University of Missouri

A fossil foot bone from an early human ancestor, 3.2 million years old, could profoundly change our understanding of human evolution. Discovered in Hadar, Ethiopia, it brings compelling evidence that this hominid, a species called Australopithecus afarensis, may have been the first human ancestor to walk upright. In a recently published paper in Science, a team of anthropologists from the United States and Ethiopia described the recently-found fossil as a fourth metatarsal, or mid-foot bone. It’s the only one ever found for Australopithecus afarensis, and it’s revealed that these ancient hominids had stiff, arched feet, similar to humans, that enabled them to walk like us.

Australopithecus afarensis fossils were first discovered in Ethiopia, in 1974. One of the best known representatives of this species, also found in Hadar, was Lucy. That was the nickname given to several hundred bone pieces that made up about forty per cent of one individual believed to be female. There was great controversy about whether Lucy and her relatives were strictly bipedal or if they had also been tree-climbers, or a bit of both. But the discovery of this mid-foot bone has likely put those questions to rest.

University of Missouri and Arizona State University researchers have found a bone that indicates human ancestors had arches in their feet, a major evolutionary shift for Lucy and her species. Credit: Elizabeth Harmon

One of the team members, Professor Carol Ward, said in a recent press release by the University of Missouri-Columbia,

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. 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 favor of life on the ground.

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. 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.

Lucy, Australopithecus afarensis, a cast of her skeleton. Credit: Wikipedia

Fossil evidence of a human ancestor that preceded Lucy’s species was Ardipithecus ramidus. This hominid, that lived about 4 million years ago, had powerful grasping legs which included a divergent mobile first toe, a feature seen in tree-dwelling primates that indicated they moved around on all four feet, occasionally walking upright. Previous fossil evidence of Lucy and her species, however, hinted that they were bi-pedal but some scientists thought they could have also been tree-dwellers. Now, with the discovery of this mid-foot bone, the only one known for Australopithecus afarensis, this new evidence strongly suggests that Lucy and her relatives stood and walked upright, perhaps the first human ancestor species to have this critical anatomical human trait.

We can only image what life must have been like for Lucy and her kind. They were small statured, perhaps covered in fur; males were just under five feet and weighed under 100 lbs, while females were shorter, about three and a half feet tall and 60 lbs. Their brains were smaller than ours, and they had powerful jaws that enabled them to eat leaves, seed, roots, fruit, nuts and insects. With the discovery of this fossil foot bone, we now know that they had arched feet, much like ours. They were likely the first, in the evolutionary path towards being human, that walked upright through ancient forests and open lands of Ethiopia, foraging for food.

Scientists discovered the 3.2 million-year-old fossil at this site in Hadar, Ethiopia. Photo Credit: Kimberly Congdon

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