Wild monkeys in West Africa can bend their feet in ways scientists thought only apes could, a discovery that throws a wrench into long-held assumptions about the last common ancestor of humans and chimpanzees. Researchers from Ohio State University watched sooty mangabey monkeys in an Ivory Coast forest shimmy up narrow vertical trunks to forage, hide, and sleep. The monkeys lack the highly flexible ankles that make chimpanzees such adept climbers, yet they managed the same vertical climbs anyway. The finding, published in Proceedings of the National Academy of Sciences, suggests fossil foot bones may be misleading scientists about how our ancient ancestors moved.
Feet That Do More Than Fossil Bones Suggest
The last common ancestor of humans and chimpanzees lived between 6 and 7 million years ago. Reconstructing its body and behavior is central to understanding how bipedalism emerged. For years, paleoanthropologists have used foot fossils to infer whether that ancestor climbed like a modern ape or moved more like a monkey. Chimpanzees have ankles that flex far beyond the human range, a trait tied to their ability to scale tree trunks. Monkeys, by contrast, were thought to rely mostly on their arms, with feet that were less specialized for vertical climbing. The new observations challenge that neat divide.
A Functional Equivalence in the Forest
Luke Fannin, assistant professor of anthropology at Ohio State and lead author of the study, and his team captured detailed video and measurements of sooty mangabeys in the wild. Despite lacking chimpanzee-like ankle joints, the monkeys bent parts of their feet enough to grip and push off narrow vertical trunks. The researchers describe this as a functional equivalence: the monkeys climb as well as apes, just with different anatomy. Fannin said the findings mean scientists cannot rule out vertical climbing in an ancestor simply because its fossilized foot does not look like a chimpanzee's.
What This Means for the Story of Bipedalism
The debate over the last common ancestor is not academic trivia. Understanding where and how it moved through forests helps researchers trace the evolutionary steps that led to habitual bipedalism, a form of locomotion unique to humans among living primates. Humans are apes, but the new evidence suggests an ancestor with monkey-like feet could still have climbed in ways comparable to modern apes. That widens the range of possible reconstructions of the ancestor and narrows the assumptions scientists can safely make from isolated fossil bones. The study adds a layer of caution to paleoanthropology: behavior does not always leave a clear signature in the skeleton.
