New research suggests that the last common ancestor of humans and chimpanzees may have been capable of vertical tree climbing, even if it had feet that differed from those of modern apes. A persistent debate in paleoanthropology focuses on whether this ancestor had ape-like feet, which are specialized for climbing vertical tree trunks, or more generalized monkey-like feet. In a new study, researchers from Dartmouth College and The Ohio State University found that sooty mangabeys (Cercocebus atys) can achieve climbing performance comparable to chimpanzees by using a different part of the foot.
Ardipithecus ramidus, a hominid that lived in Africa more than 4 million years ago. Illustration by Arturo Asensio, from Quo.es.
“People infer behavior from fossils, but some researchers argue that monkeys cannot climb vertically in the same way as great apes,” said Dr. Luke Fannin of The Ohio State University.
“We are talking about functional equivalence here. Just because monkeys do not have chimpanzee-like legs does not mean they are incapable of vertical climbing.”
Identifying the last common ancestor of humans and chimpanzees is considered essential for understanding the evolution of bipedalism, a form of habitual locomotion practiced only by humans among living primates.
“We know that humans are apes, but this study shows that even if our ancestors had a different type of foot morphology, they may still have been able to climb like apes,” Fannin said.
“Part of paleoanthropology is understanding how human evolution occurred. Researchers often examine movement because modern humans move very differently from our closest living primate relatives, chimpanzees. We are trying to understand why.”
Fannin and his colleagues filmed a sooty mangabey, a large African forest monkey, in Taï National Park in Côte d’Ivoire.
The researchers analyzed video recorded at 30 frames per second to measure how much the animals flexed their ankles and midfeet while climbing vertical tree trunks.
Chimpanzees and other African apes typically climb with their ankles hyperextended by more than 45 degrees. This movement brings the body’s center of gravity closer to the tree trunk, helping reduce the risk of falling backward. Monkeys generally lack the ankle anatomy needed to perform the same movement.
However, the study found that sooty mangabeys compensate by using a different joint. Rather than sharply bending the ankle, they bend the midfoot. The flexible region between the heel and toes is sometimes referred to as the “midfoot break.”
Sooty mangabeys flexed their midfeet by an average of nearly 46 degrees while climbing. This was comparable to the ankle flexion measured in climbing chimpanzees and slightly greater than the ankle flexion recorded in chimpanzees. Twa hunter-gatherers in Central Africa are also known for climbing trees to collect honey.
By contrast, the mangabeys’ ankles were relatively immobile, showing an average flexion angle of approximately 23 degrees—far less than that observed in chimpanzees and Twa tree climbers.
In other words, two different joints in two different types of feet produced surprisingly similar vertical-climbing performance.
Sooty mangabey (Cercocebus atys). Image credit: Luke Fannin, Dartmouth College.
The finding is important because it challenges a central assumption in the debate over the feet of the last common ancestor of humans and chimpanzees. Some researchers have argued that a more generalized, monkey-like foot would have made vertical climbing difficult or impossible for early members of the human lineage.
If a flexible midfoot can effectively substitute for a highly mobile ankle, as the mangabey data suggests, an ancestor with more generalized feet may still have been able to climb trees safely and regularly.
The researchers emphasize that their findings do not determine exactly what type of feet the last common ancestor possessed. Instead, the results show that both ape-like and more generalized foot anatomies may have supported frequent vertical tree climbing.
“What is exciting about this study is that it adds clarity while also making the overall picture more complicated,” said W. Scott McGraw, a professor at The Ohio State University.
“The assumption that vertical climbing requires advanced ape-like legs, and is therefore a particularly difficult behavior for monkeys, is not supported by these findings.”
“There are many living, photographable monkeys in the forests of West Africa. They are highly capable of performing biomechanically demanding behaviors.”
Arboreal behavior is fundamental to understanding primate evolution, including human evolution. Life in the trees has influenced the development of hands, feet, wrists, ankles and nails across many primate species.
“Regardless of where we begin, we do not yet know exactly what the last common ancestor looked like. But vertical climbing is widespread, and different anatomies can produce the same movement,” Fannin added.
“We need more fossil evidence to answer the question of whether our last common ancestor had more ape-like or generalized feet.”
The findings were published this month in Proceedings of the National Academy of Sciences.
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Luke D. Fannin et al. 2026. Kinematic convergence in vertical climbing of apes and monkeys informs debates about early arboreal life in humans. PNAS 123 (35): e2608183123; doi: 10.1073/pnas.2608183123
Source: www.sci.news