COLUMBUS, Ohio—The last common ancestor shared by humans and chimpanzees may have looked less like a modern ape than many scientists have assumed, but it almost certainly knew how to climb. New research on wild sooty mangabey monkeys shows that a primate does not need the highly flexible ankle of a chimpanzee to scale a tree trunk efficiently. Instead, the monkey can generate comparable movement through the middle of its foot, revealing an unexpected form of biomechanical convergence that could reshape ideas about how the earliest members of the human lineage moved through forests.
Humans and chimpanzees began following separate evolutionary paths roughly 6 million to 7 million years ago, after diverging from a last common ancestor whose anatomy remains unknown. Fossils from this critical period are rare and often fragmentary, leaving researchers to infer behavior from isolated bones. One of the most important questions is whether that ancestor lived primarily in the treetops, climbed vertical trunks from the forest floor, or combined both strategies. The answer could help explain how later hominins gradually evolved the distinctive combination of climbing ability and habitual walking on two legs that defines humans.
For decades, scientists have often contrasted the feet of apes and monkeys when reconstructing this ancient lifestyle. Chimpanzees possess an ankle joint capable of substantial upward flexion, allowing the foot to press against vertical trunks as the animal ascends. Most monkeys lack the same ankle configuration and have therefore sometimes been viewed as less capable trunk climbers, relying more heavily on their arms or moving along horizontal branches. That distinction has influenced interpretations of fossil feet: a specimen with a monkey-like structure might be considered evidence of an animal adapted to the treetops but not to demanding vertical climbs.
Observations from West Africa now challenge that assumption. Researchers from The Ohio State University filmed wild sooty mangabeys climbing narrow, nearly vertical tree trunks in the Taï Forest of Ivory Coast. The monkeys used this behavior when escaping danger, searching for food and reaching elevated locations for rest or sleep. Their feet did not resemble chimpanzee feet, yet the animals repeatedly performed a movement that is mechanically similar to chimpanzee vertical climbing. The finding suggests that the same functional outcome can arise from different anatomical designs—a phenomenon known as kinematic or functional convergence.
The key discovery involved the mangabeys’ midfoot. During a climb, the monkeys flexed this region by approximately 46 degrees, a range comparable to the roughly 45-degree upward ankle flexion measured in chimpanzees during trunk ascent. By comparison, a typical human ankle flexes upward by about 20 degrees. Although the location of the movement differs, the result is similar: the foot can conform to a steep surface and maintain contact while the animal pushes its body upward. Rather than depending on a single highly mobile ankle joint, the mangabey distributes the motion across the foot, producing a flexible platform capable of gripping slender trunks.
This distinction matters because fossils preserve bones, not the complete network of ligaments, tendons, muscles and soft tissues that control movement. A fossil foot may therefore appear poorly suited for vertical climbing if researchers judge it only by resemblance to a living chimpanzee. The mangabey evidence demonstrates that bone shape alone can provide an incomplete picture of locomotor performance. Two species may use different joints, loading patterns and muscle actions to accomplish the same task. As a result, scientists investigating early hominin evolution may need to consider multiple biomechanical pathways rather than searching for a single “ape-like” anatomical signature.
The study also highlights the value of recording animals in their natural environment. Laboratory experiments can measure joint motion precisely, but they may not capture the complex decisions animals make in real forests, where trunks vary in diameter, texture, angle and flexibility. The high-resolution videos obtained by Luke Fannin allowed the researchers to examine how the mangabeys positioned their feet and how the joints were loaded during actual climbs. Those observations converted a long-standing behavioral assumption into measurable kinematic data, showing that a monkey traditionally considered less specialized for vertical ascent can perform the behavior repeatedly and effectively.
The implications reach beyond the question of monkeys versus apes. If a monkey-like foot could support vertical climbing, then the last common ancestor of humans and chimpanzees may have possessed anatomy that looked unlike either modern group while still being highly competent in trees. The ancestor may have climbed trunks, moved along branches and descended to the ground using a combination of traits that has since been reshaped independently in monkeys, apes and humans. This possibility makes the evolutionary story more complicated, but it also makes it more realistic: natural selection often modifies existing structures in different ways to solve similar mechanical problems.
Vertical climbing may also have played a more persistent role in human evolution than the fossil record alone suggests. Modern human hunter-gatherers and forest-dwelling foragers can climb efficiently, often relying on the coordinated flexibility of muscles, tendons and ligaments rather than on an especially specialized foot skeleton. Humans are defined by habitual bipedal walking, yet the ability to ascend remains a powerful reminder of our arboreal inheritance. Hands, feet, wrists, ankles and nail-bearing digits all retain clues to a deep history shaped by life in trees. The new findings suggest that this history cannot be reconstructed by anatomy in isolation from behavior.
The researchers argue that more fossils will be needed to identify the physical form of the last common ancestor, but the search should now be guided by a broader understanding of locomotion. A foot that lacks chimpanzee-like ankle flexibility cannot automatically be ruled out as the foot of a capable vertical climber. By revealing that monkeys and apes can reach similar climbing performance through different joint mechanics, the study turns a seemingly simple question into a more challenging one: not whether an ancient primate could climb, but how its particular anatomy made climbing possible. That shift could influence interpretations of early hominin fossils and bring scientists closer to understanding the movements that preceded walking upright.
Subject of Research: Vertical climbing biomechanics in wild sooty mangabey monkeys and implications for the locomotion of the last common ancestor of humans and chimpanzees.
Article Title: A kinematic convergence in ape and monkey vertical climbing informs debates on early hominin arborealism
News Publication Date: 17 August 2026
Web References: Ohio State University anthropology; Taï Forest Monkey Project; Proceedings of the National Academy of Sciences DOI: https://doi.org/10.1073/pnas.2608183123
References: Fannin, L., McGraw, W. S., Pape, C., et al. “A kinematic convergence in ape and monkey vertical climbing informs debates on early hominin arborealism.” Proceedings of the National Academy of Sciences. DOI: 10.1073/pnas.2608183123.
Keywords: human evolution, chimpanzees, last common ancestor, primate locomotion, vertical climbing, sooty mangabey, biomechanics, arborealism, hominin evolution, foot flexibility, ankle movement, fossil interpretation
