Fossil feet may be misleading scientists about human evolution

Humans and chimpanzees diverged from a shared last common ancestor (LCA) between 6 and 7 million years ago. Reconstructing how that ancestor moved through forests -- along high branches, up vertical trunks, or both -- is central to understanding human evolution, including the emergence of bipedalism, walking regularly on two legs.

Chimpanzees are strong trunk climbers, helped by ankles that flex upward by about 45 degrees, versus roughly 20 degrees in most human ankles. Monkeys have generally been seen as relying more on their arms in trees, shaping ideas about whether the LCA had ape-like or monkey-like feet.

New research from Ohio State University challenges that view. Lead author Luke Fannin, an assistant professor of anthropology, filmed wild sooty mangabey monkeys -- which lack chimpanzee-style flexible ankles -- at the Tai Forest Monkey Project field station in Ivory Coast. Measurements showed the mangabeys flex their midfoot by as much as 46 degrees, enough to climb narrow vertical trunks while seeking food, hiding or finding sleeping spots. "Some say a monkey can't vertically climb as well as an ape can," Fannin said. "We're saying there's a functional equivalence here. So you can't rule out vertical climbing just because something doesn't have a chimpanzee-like foot."

The study, published this week in Proceedings of the National Academy of Sciences (PNAS), was senior-authored by W. Scott McGraw, an anthropology professor at Ohio State who co-directs the Tai project. "This notion that you have to have a foot like an advanced ape to vertically climb ... is not true," McGraw said. The findings complicate competing ideas about the LCA's feet: some theories held an ape-like foot was necessary, others already allowed for a monkey-like one.

Fannin and McGraw also noted that vertical climbing is not limited to nonhuman primates: some human foragers are capable climbers too, an ability thought to rely more on flexible ligaments, tendons and muscles than on foot-bone shape.