For the past year, Jeremy DeSilva has been getting quizzical looks from students as he walks around the Boston University campus with an odd, wobbly gait: landing on the outer edge of his foot, then rolling his weight inward with each step.
But this is not the mincing waddle of an absent-minded professor. It is an informal experiment.
“I feel like I can understand the forces acting on her body better if I try to emulate the way she walked,” DeSilva said, gesturing toward a skeleton of an early human ancestor laid out on a table in his basement laboratory.
A thorough study of this unusually complete skeleton of a nearly 2-million-year-old female helped DeSilva piece together a puzzle about how this species — with a confusing mosaic of human- and chimpanzee-like features — got around.
His presumption-shattering finding: The popular notion of how humans evolved is flawed.
Most people have seen the tidy cartoon often used to illustrate evolution, as a clear progression from stooped-over, chimp-like prehumans to modern people who stride around on two legs. But DeSilva’s research, published Thursday in the journal Science along with five other papers, muddies the picture, suggesting that evolution wasn’t a straight line toward upright walking, but could have led to a number of prehuman species that got around in different ways.
He concludes that this creature, Australopithecus sediba , had a body built to walk upright, but with a distinctive foot that also allowed it to climb trees efficiently.
“The important point is that if [species of] Australopithecus are not all the same, there wasn’t one way to walk,” said Daniel Lieberman, a professor of human evolutionary biology at Harvard University who was not involved in the research. “We live in a world today where there’s only one species of humans around, and I think that has biased us. We think all humans, the hominins, must be like us.”
But, he added, look at the ways that birds are different from one another, and apply that model to prehumans: “You look out in the backyard, and you see quite a few species of songbirds with different wings, different feet, different songs — and hominins were no different 2.5 million years ago.”
Nathaniel Dominy, associate professor of anthropology at Dartmouth College, said the finding suggests that sediba “were probably living in trees [as part of their lifestyle] — but simultaneously, you might have had other hominins living on the ground . . . and that becomes really cool to think about.
“We’re talking about multiple species [of human ancestors] living on the same landscape, competing for the same resources,” he said.
That epiphany wasn’t sudden. In 2010, DeSilva was a new professor at BU. He had recently finished a years-long study of the ankles and feet of numerous early human ancestors, ranging from the Australopithecus afarensis, best known as Lucy, to a more obscure species that DeSilva playfully compares to “upright walking cows” because they had gigantic teeth, ideal for prolonged chewing.
He had met Lee Berger, the paleoanthropologist who in 2008 discovered the Australopithecus sediba fossils, so when DeSilva was offered the opportunity to join the team analyzing the find, he was excited. He was also confident it would be a relatively trivial task to understand how the new foot and ankle fit into the spectrum of diversity. Australopithecus species generally had human-like feet, including a broad, flat heel that suggested they walked on two legs and had come down out of the trees.
“I had this amazing arrogance and thought I knew something about this,” DeSilva said. So it seemed like a prank when he opened a small cardboard box in his office and pulled out a cast of a small chunk of bone that tapered to a point, with an almost hook-like beak.
“I thought someone had sent me a chimp heel, just to mess with me,” DeSilva recalled.
DeSilva and colleagues worked to describe the creature’s odd foot and ankle. He traveled to South Africa to look at the original fossils and examine the lower limbs, the spine, and the pelvis. Some features are distinctly human-like: its hand, its pelvis, its teeth. But it also seemed to have longer arms, a primitive shoulder, and a foot with strangely ape-like features. In 2011, the scientists fully described the bizarre foot, but they were no closer to understanding why it had such a strange amalgam of features.
“The foot is odd, but what does that mean? Frankly, I was lost,” DeSilva said.
After DeSilva gave a talk to a group of physical and occupational therapists at BU, one of the audience members, Kenneth Holt, approached him with very specific questions about other body parts that suggested he had an intuition about how the creature walked.
DeSilva was surprised. Holt — a physical therapist and a specialist on the present-day problems of human gait, posture, and movement — began to guess accurately what ancient anatomy looked like, including body parts DeSilva had not described in his talk.
The two scientists examined the cast of the fossil and came up with a hypothesis about how the creature walked: a gait that in modern humans is called hyperpronation. They realized that this creature’s body was perfectly adapted to this way of walking, which in humans would cause all kinds of knee, hip, and back problems.
Instead of landing on its heel, the anatomy suggested sediba probably landed on the outer edge of its foot and rolled the foot inwards. This would exert torque on its legs, and evidence from the skeleton suggested that the creature had been biologically tuned to walk that way. A high ridge on the knee, for example, seemed to assure that its kneecap would stay in place.
DeSilva walks with the strange gait himself to understand it better, but he is also rigorously studying how people walk today, in the hopes that people who hyperpronate might provide some insight into the form of locomotion of sediba. He is also looking for more clues from fossils — a big toe, for example. And he is hoping fossil fragments of younger individuals could provide evidence about how the bones develop.
Determining which of the Australopithecus species could have given rise to later species of humans is contentious. No one knows whether sediba is that species, or an extinct side branch, but the fossil record paints a very different picture of life on Earth.