A new analysis reveals clues about when primates started walking on two feet

Walking around upright on two feet is something no other primate does routinely. It seems to be one of the earliest major shifts in the evolutionary path that eventually led to us, modern humans. But exactly when that shift occurred is a mystery. NPR's Nell Greenfieldboyce reports on some new clues from bones that are seven million years old.

NELL GREENFIELDBOYCE, BYLINE: Walking on two feet - aka bipedalism - is something chimps can do a little bit. For that matter, so can bears and even dogs.

SCOTT WILLIAMS: But those animals are not adapted to bipedalism, right? You can look at their bones, and there are no adaptations to bipedalism.

GREENFIELDBOYCE: Scott Williams is a paleoanthropologist at New York University. He says way back in the human family tree, millions of years ago, there were ape-like creatures that could stride around on two legs. They had small brains, similar to a chimpanzee's.

WILLIAMS: These are animals that I don't think any person today, if they were to observe them, would call them humans.

GREENFIELDBOYCE: But their ability to walk upright was really important. It meant they could leave the trees and spread into new kinds of environments. And it freed up their hands so they could manipulate objects and start making stone tools.

WILLIAMS: Bipedalism allowed for the eventual evolution of large brains.

GREENFIELDBOYCE: As far back as four million years ago, a group of human relatives called australopiths were walking around on two feet. Williams says pretty much all scientists would agree on that.

WILLIAMS: Anything before that, it starts to get controversial.

GREENFIELDBOYCE: Take a species called Sahelanthropus tchadensis. It lived in Africa about seven million years ago. Scientists have just a few of its bones, which were found at one spot in the desert in Chad. There's a crushed cranium, a couple of forearm bones and a part of a big leg bone - a femur. People have been arguing for years over whether these bones have any features associated with upright walking. Williams used some new methods to investigate casts of the fossils held by Harvard's Peabody Museum.

WILLIAMS: So this is basically taking 3D models of the bones and adding landmarks on them in a large comparative data set with other fossils and humans and living apes.

GREENFIELDBOYCE: In the journal Science Advances, he and some colleagues lay out evidence that they say shows human-like adaptations for walking upright, like an inward curve in the shaft of the femur bone and a bony structure on the femur where a powerful ligament attaches.

WILLIAMS: This animal, that otherwise looks very chimpanzee-like in its overall shape, has these bipedal adaptations hafted on top. And therefore, we're looking at a very early biped, a very early member of our lineage.

GREENFIELDBOYCE: He says it's not clear how much it walked. It may have had a grasping big toe and other features associated with life in the trees.

Carol Ward is a paleoanthropologist at the University of Missouri, who wasn't part of the research team. She thinks this new analysis captures interesting features of the bones, but it's unlikely to end the debate over this species.

CAROL WARD: Because there's just not the anatomy preserved that we really need. The conclusive pieces just haven't been found yet.

GREENFIELDBOYCE: She says telltale signs could come from, say, a knee joint or a pelvis.

WARD: We do not have any of those bones.

GREENFIELDBOYCE: Still, she's optimistic scientists will eventually figure out how the transition to upright walking happened. She says more and more fossils are being discovered all the time.

Nell Greenfieldboyce, NPR News.

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