Monkeys Just Challenged a Major Assumption About How Human Ancestors Climbed

New research on wild monkeys is complicating assumptions about how early human ancestors moved through trees. Researchers at The Ohio State University filmed wild sooty mangabey monkeys in Ivory Coast's Taï Forest and measured their foot movements as the animals climbed narrow, vertical tree trunks to hide, forage, or sleep.

Humans and chimpanzees diverged from a shared last common ancestor, or LCA, roughly 6 to 7 million years ago. Determining how that ancestor moved could help explain bipedalism, the habitual two-legged walking that sets humans apart from other primates. Chimpanzees are strong vertical climbers because their ankles can flex upward by about 45 degrees during a climb, compared with roughly 20 degrees in most human ankles. Monkeys lack that flexibility and have generally been thought to move through trees mainly with their arms.

The new measurements found that sooty mangabeys' midfoot bones, the middle section of the foot between the ankle and toes, flexed by 46 degrees while climbing vertically, matching the range chimpanzees reach with their ankles. "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."

Senior author W. Scott McGraw, professor and chair of anthropology at Ohio State, said the finding complicates efforts to reconstruct the LCA's foot shape, since an advanced ape-like foot is not required to perform vertical climbing, a biomechanically challenging behavior. Fannin said resolving whether the LCA was more monkey-like or ape-like will require additional fossils.

The study, "A kinematic convergence in ape and monkey vertical climbing informs debates on early hominin arborealism," was published August 17, 2026, in Proceedings of the National Academy of Sciences by Fannin, Carmen Pape, and McGraw.