"People are making behavioral inferences from fossils, and some say a monkey can't vertically climb as well as an ape can," said first author Luke Fannin, 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."

What did the field work involve?

Fannin filmed wild mangabeys in the Taï forest and analyzed the footage frame by frame. Senior author W. Scott McGraw, professor and chair of anthropology at Ohio State and co-director of the field site, said the high-resolution video made the difference.

"The great thing about having the video is that it allows us to capture exactly what the monkey is doing in a high-resolution manner," McGraw said. "So we can actually say how the joint is loaded. Before, we had hunches, but in this way, it's far more precise."

Former Ohio State undergraduate Carmen Pape also co-authored the study.

Does this settle the debate?

McGraw is blunt that the picture got less clear, not more.

"What's neat about this paper is it adds clarity, but it also makes the broader picture more fuzzy," he said. The notion that an early primate needed an advanced ape-like foot to climb vertically "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."

McGraw called the Taï videos "some of the first kinematic documentation of a locomotor behavior in a monkey that has largely been unrecognized or underappreciated."

What does this mean for human evolution?

The authors stress that tree climbing did not end with bipedalism. Some modern hunter-gatherers climb vertical trunks with ease, relying on flexible ligaments, tendons, and muscles rather than specialized foot bones. Humans still carry a behavioral pull toward heights that may trace back to arboreal ancestors.

"Arboreality is fundamental to understanding primates, including ourselves, because it has shaped our body to a large degree: hands and feet, wrists and ankles, nails instead of claws, etc.," McGraw said.

Fannin argues the next step is fossil-based. "Regardless of where you're starting from, which we don't know yet, vertical climbing is universal, and the anatomy is going to perform that behavior," he said. "So I think that to get at the question of a monkey-like or ape-like last common ancestor, we need more fossils."

What are the limits of the study?

The measurements come from a single monkey species at a single site, so the team cannot yet say how widespread midfoot-driven climbing is across other monkeys. Foot bone structure can still constrain motion in ways soft tissue cannot overcome, and the paper does not show that any monkey foot, regardless of shape, can match ape performance. The researchers also caution that their kinematic data describe what the joints do, not the long-term evolutionary pressures that shaped them.

The work was supported by Dartmouth College, the Explorer's Club, the U.S. National Science Foundation, the Emory National Primate Research Center, the Primate Society of Great Britain, and a Schmidt Sciences Postdoctoral Fellowship.