A Forest Monkey Just Complicated A Major Theory About Human Evolution

Scientists Find a Surprising Climbing Trick Shared by Monkeys and Apes

A long-running argument about how ancient human ancestors moved through the trees may have just gotten an important new piece of evidence from a forest monkey in West Africa. A study published in the Proceedings of the National Academy of Sciences found that wild sooty mangabeys in Côte d’Ivoire bend their feet to nearly the same extreme angle as chimpanzees do when scaling vertical tree trunks. Monkeys and apes have very different foot structures, yet they appear to have arrived at the same solution for climbing safely. For researchers trying to reconstruct how human ancestors moved millions of years ago, that discovery changes the calculus considerably.

At the heart of the debate is a deceptively simple question: what did the feet of the last common ancestor of chimpanzees and humans actually look like, and how did that creature get around? Fossil foot bones from early human relatives have split scientists into two camps. One side argues the ancestor had feet shaped more like a modern African ape, built for both ground travel and tree climbing. The other contends the ancestor had feet more like a generalized monkey, suited primarily for moving along the tops of branches rather than hauling up vertical trunks. That second view has long carried an implied assumption: a monkey-like foot would make serious tree climbing unlikely or even dangerous. New data from the field suggest that assumption may be wrong.

Mangabey Midfoot Bend Matches Chimpanzee Ankle Bend Almost Exactly

Chimpanzees are known for bending their ankles to extreme angles, often greater than 45 degrees, when climbing vertical surfaces. That deep bend pulls the chimp’s body closer to the tree trunk, reducing the backward pull of gravity that could otherwise send the animal tumbling. Ape ankle joints are built wide enough to handle that stress routinely.

Monkeys like the sooty mangabey don’t have that kind of ankle. Their joints are narrower and less suited to that extreme bend. For a long time, that difference led scientists to assume monkeys, and any ancient human relative with a similar foot, couldn’t climb vertical surfaces as safely as apes.

But sooty mangabeys found a workaround. Instead of bending deeply at the ankle, they bend at the middle of the foot, almost folding it in half. Researchers measured that midfoot bend during climbing and found an average maximum of about 46 degrees, statistically the same as the roughly 45.5 degrees chimpanzees bend at the ankle, and a bit more than the 40.7 degrees reached by Twa hunter-gatherers from Central Africa, who are also known for their tree-climbing skill.

Two completely different joints, in two very differently shaped feet, ended up bending to almost the same degree. Scientists call that a convergence: nature solving the same problem in two different ways. Researchers also checked how close each animal kept its body to the tree trunk while climbing, since staying close reduces the risk of tipping backward. Once body size was accounted for, mangabeys and chimpanzees came out about even there too.

Fossil Evidence Ties Flexible Midfeet to Early Human Relatives

Sooty mangabeys are large, forest-dwelling African monkeys that forage on the ground but also climb vertical tree trunks with ease. To get the numbers above, researchers filmed habituated individuals, animals accustomed to human presence, at the Taï Monkey Project research site in Taï National Park, using a tripod-mounted video camera to capture 21 climbing bouts and measure the angles reached at the ankle and midfoot. Those numbers were then checked against previously published measurements of chimpanzees, Twa hunter-gatherers, and how the human and chimp foot moves during ordinary walking rather than climbing.

Several early human relatives, including species of Australopithecus, left behind foot bones that suggest a more flexible midfoot than modern humans have, based on the shape of specific bones near the outside of the foot. Earlier researchers had guessed that flexibility might have helped with climbing, but nobody had ever measured how a flexible, monkey-like midfoot actually performs on a real vertical climb. Now there is movement data to back that guess up.

Researchers behind the study are careful not to pick a side in the larger fossil debate. Their results don’t prove the last common ancestor had a monkey-like foot or an ape-like foot. What they do show is that ruling out climbing ability in a monkey-footed ancestor never had strong evidence behind it in the first place. Whether that ancestor’s foot looked more like a chimp’s or more like a mangabey’s, climbing a tree safely was well within reach either way.

Ancient Feet, Reconsidered

What emerges from these findings is a more flexible view of primate evolution. Movement strategies, it turns out, are not rigidly locked to foot shape. A primate can get up a tree safely by bending at the ankle or by bending in the middle of the foot. That functional flexibility may have given early human relatives more behavioral options than scientists previously credited. The argument that a monkey-like foot rules out serious tree climbing no longer has the legs it once did.

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