Wild monkeys in Ivory Coast can bend the middle of their feet nearly as much as chimpanzees flex their ankles while climbing — a finding that complicates a longstanding assumption paleoanthropologists have used to infer how early human ancestors moved, based on fossilized foot bones that can't preserve soft-tissue flexibility directly. [1]

Researchers from Ohio State University studying wild sooty mangabey monkeys at the Taï Forest Monkey Project field station recorded the animals bending their midfoot by as much as 46 degrees while scaling narrow vertical tree trunks to forage, hide, or find sleeping spots. That figure is close to the roughly 45 degrees of ankle flexion chimpanzees achieve during the same kind of climb — a capability human ankles, which flex only about 20 degrees, do not share. [1] Mangabeys lack the flexible, ape-like ankle joint long assumed necessary for efficient vertical climbing. The new video evidence, published in the Proceedings of the National Academy of Sciences, shows they achieve a functionally similar result through an entirely different joint. [1]

"People are making behavioral inferences from fossils, and some say a monkey can't vertically climb as well as an ape can," said Luke Fannin, the study's first author and an assistant professor of anthropology at Ohio State. "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." [1] The finding matters because reconstructing the locomotion of the last common ancestor of humans and chimpanzees — a species that lived six to seven million years ago, before the two lineages diverged — depends heavily on inferring soft-tissue capability from bone shape alone. If a monkey-like foot can support the same climbing behavior as an ape-like one, then fossil foot bones alone cannot settle which body plan the shared ancestor actually had.

Senior author W. Scott McGraw, who co-directs the Taï Forest project and chairs Ohio State's anthropology department, was candid that the discovery cuts against tidy conclusions rather than toward them. "What's neat about this paper is it adds clarity, but it also makes the broader picture more fuzzy," he said. "This notion that you have to have a foot like an advanced ape to vertically climb, which is a difficult task, is not true. You've got a bunch of monkeys that aren't extinct — and which can be filmed — that are very competent at performing a biomechanically challenging behavior right now in a forest in West Africa." [1] The video documentation itself is part of the contribution: earlier assumptions about monkey climbing limitations rested on comparatively little direct observation, McGraw noted, calling the new footage "some of the first kinematic documentation of a locomotor behavior in a monkey that has largely been unrecognized or underappreciated." [1]

Vertical climbing persists in modern humans too, the researchers note, just through a different mechanism than bone structure: some human foragers remain highly capable climbers, an ability attributed more to flexible ligaments and tendons than to any distinctive foot-bone shape. "To get at the question of a monkey-like or ape-like last common ancestor, we need more fossils," Fannin said. [1]

That admission is the honest center of the study. It does not resolve what the human-chimpanzee ancestor's foot looked like. It removes an assumption scientists had used to rule possibilities out — which, in a field built on scarce fossils, counts as real progress.

-- KENJI NAKAMURA, Tokyo