Rosa Rubicondior: Refuting Creationism - Refining The Details - How Did The Common Ancestor of Hominins And Chimps Climb Trees?

Researchers observed and confirmed with measurements that wild sooty mangabey monkeys in an Ivory Coast forest display foot bone flexibility that facilitates vertical climbing up skinny tree trunks to hide, forage or sleep.

Photo: W. Scott McGraw.

Last common ancestor, no matter its feet, could have been a vertical climber

One of the standard creationist tactics is to portray disagreement between scientists as evidence that the science itself is unreliable. Yet disagreement over the details of an evolutionary history is not disagreement over whether evolution occurred. On the contrary, it is often the means by which science advances: competing explanations are identified, their testable implications are examined, and evidence is gathered to determine which assumptions remain tenable.

A long-running example concerns the last common ancestor of humans and chimpanzees, whose descendant lineages diverged some six to seven million years ago. Palaeoanthropologists agree that such an ancestor existed; the disagreement concerns what it looked like and how it moved. In particular, was its foot more like that of a modern African ape, with an ankle capable of substantial upward flexion during vertical climbing, or more like that of a monkey adapted principally to moving along branches?

That question matters because the evolution of habitual bipedalism cannot be understood without reconstructing the locomotion from which it developed. Fossilised foot and ankle bones preserve anatomical structure but not behaviour, so palaeoanthropologists must infer what their owners could do by comparing them with living primates. One influential assumption was that a primate needed a chimpanzee-like ankle to climb vertical tree trunks proficiently. A fossil with a more monkey-like foot might therefore be interpreted as evidence against frequent vertical climbing.

Luke D. Fannin, Carmen Pape and W. Scott McGraw have now tested that assumption with evidence from living animals. For their study published in Proceedings of the National Academy of Sciences, they recorded wild sooty mangabeys, Cercocebus atys, climbing trees in Côte d’Ivoire’s Taï Forest and measured the movements of their feet. Mangabeys lack the highly flexible ankle of a chimpanzee, yet they achieved an average of about 46 degrees of upward flexion through the middle of the foot—virtually the same range that chimpanzees achieve at the ankle.

In other words, two differently constructed feet can solve the same biomechanical problem in different ways. Chimpanzees flex chiefly at the ankle; mangabeys obtain comparable movement from the midfoot. This is a striking example of convergent evolution: similar selection pressures have produced functionally equivalent performances by modifying different parts of an inherited anatomical system.

The result does not tell us whether the human–chimpanzee last common ancestor had an ape-like or monkey-like foot, nor does it identify that ancestor. What it does is remove an unjustified restriction from the debate. A monkey-like foot can no longer be treated as evidence that its owner was incapable of proficient vertical climbing. As the researchers acknowledge, resolving the ancestor’s actual anatomy will require more fossils, but one disputed inference about its possible behaviour has now been tested against observations and found wanting.

This is science working as it should. Researchers did not settle the matter by appealing to authority, intuition or an ancient text. They took an assumption underlying competing evolutionary reconstructions into a West African forest and tested it by measuring what living primates actually do. The result narrowed one area of disagreement while revealing that the relationship between anatomy and behaviour is more flexible than had been supposed.

The theory of evolution and common ancestry were fundamental to every stage of that reasoning. They supplied the family relationships being investigated, explained why comparisons between humans, chimpanzees, monkeys and fossil hominins are informative, and allowed the researchers to distinguish inherited similarities from independently evolved functional convergence. Evolution was not an interpretation added to the results afterwards; it was the explanatory framework that generated the question, guided the observations and made the findings intelligible.

Creationism contributes nothing comparable. Separate creation predicts neither the distribution of anatomical similarities among primates nor the different ways in which related feet can be modified to perform the same task. It offers no method for reconstructing the human–chimpanzee ancestor, no test for choosing between competing locomotor models and no explanation for why the evidence falls into a branching pattern of inheritance, modification and convergence.

From Fossil Feet to Ancestral Behaviour. What was the human–chimpanzee last common ancestor?

Humans did not evolve from modern chimpanzees. Humans and chimpanzees are the surviving descendants of two evolutionary lineages that separated from a shared ancestral population approximately six to seven million years ago. This extinct population is known as the human–chimpanzee last common ancestor, or LCA.

We do not yet know exactly what the LCA looked like or how it moved. Fossils including Sahelanthropus tchadensis, Orrorin tugenensis and Ardipithecus kadabba come from close to the relevant period, but none can confidently be identified as the LCA itself. Each preserves only part of the anatomical and evolutionary evidence.

The LCA was not necessarily identical to either a modern chimpanzee or a monkey. Chimpanzees have continued evolving for as long as humans have, so their anatomy cannot simply be treated as an unchanged model of the ancestral condition. Scientists must reconstruct the LCA by combining evidence from fossils, genetics, evolutionary relationships and comparisons among living primates.

How can bones reveal behaviour?

Bones do not preserve an animal’s movements directly, but their shape can reveal the mechanical demands placed upon them. Palaeoanthropologists examine such features as:

the shapes and orientations of joint surfaces;

the possible range and direction of joint movement;

the proportions and curvature of limb bones;

the attachment sites of muscles and ligaments;

patterns of wear and internal bone reinforcement; and

comparisons with living animals whose movements can be observed and measured.

This comparative method allows researchers to test whether a proposed anatomy could have supported walking, climbing, suspension or some combination of behaviours. Nevertheless, the connection between anatomy and behaviour is not always one-to-one. Soft tissues can provide flexibility that is invisible in fossils, while different anatomical arrangements can sometimes produce similar movements.

The sooty mangabey demonstrates precisely this limitation. Its ankle lacks the extreme flexibility seen in chimpanzees, but flexibility in the middle of its foot allows it to achieve much the same effective movement during vertical climbing. Consequently, the absence of a chimpanzee-like ankle in a fossil does not, by itself, demonstrate an inability to climb vertical tree trunks.

Homology and convergence

The feet of humans, chimpanzees and mangabeys are homologous: their bones are variations on a fundamental primate structure inherited from common ancestors. Their similarities are therefore not independent creations but products of descent with modification.

Within that inherited structure, however, natural selection can produce different solutions to the same mechanical problem. Chimpanzees obtain much of the flexibility needed for vertical climbing at the ankle, whereas mangabeys obtain comparable flexibility through the midfoot. This is kinematic convergence—similar movement and functional performance reached through different anatomical routes.

Distinguishing homology from convergence is one of the central tasks of evolutionary biology. Homology reveals shared ancestry; convergence shows how similar environmental and mechanical demands can independently shape inherited structures. Both concepts depend upon the theory of evolution and allow scientists to make testable predictions about extinct organisms.

The mangabey evidence does not tell us which kind of foot the human–chimpanzee LCA possessed. Instead, it shows that both an ape-like and a more monkey-like foot could have supported proficient vertical climbing. It therefore removes an unsupported assumption from the debate and leaves the remaining alternatives to be tested against further fossil evidence.

The Ohio State University news report describes how evolutionary science has resolved one part of a genuine scientific disagreement—not by weakening the evidence for human evolution, but by applying that evidence more carefully.

Last common ancestor, no matter its feet, could have been a vertical climber
Study shows monkeys’ feet make them as adept as chimps at climbing trees
New research suggests that no matter what we eventually determine the last common ancestor of humans and chimpanzees to be, we’ll learn it was a primate that climbed trees.

Humans and chimpanzees began their separate evolutionary paths between 6 and 7 million years ago when they split from the last common ancestor (LCA) they shared. Debate about the LCA hinges in part on the tree-dwelling environment from which humans emerged – which involved living among the high-up horizontal branches or being able to get there from the ground, or, more likely, both.

Chimpanzees have ankle flexion that makes them skilled tree trunk climbers, but monkeys have been thought to navigate trees with their arms – which would influence what kind of ancestor we’re looking for, one that climbed up trunks or lived in treetops full time.

Video evidence from West Africa suggests there may be no such distinction, at least based on foot structure: Researchers from The Ohio State University observed and confirmed with measurements that wild sooty mangabey monkeys, known to have feet that lack chimps’ versatile ankles, display foot bone flexibility that facilitates vertical climbing up skinny tree trunks to hide, forage or sleep.

People are making behavioral inferences from fossils, and some say a monkey can’t vertically climb as well as an ape can. 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.

Assistant Professor Luke D. Fannin, lead author.
Department of Anthropology
Dartmouth College, Hanover, NH, USA.

The research appears this week in Proceedings of the National Academy of Sciences.

Identifying the last common ancestor is considered key to helping us understand what led to bipedalism, which is done habitually only by humans. We know we’re apes, but this study makes the point that even if the ancestor had a more monkey-like foot morphology, it could still climb like an ape.

Part of paleoanthropology is understanding how our evolution happened, and it’s often through the hallmark of how we move because modern humans move quite differently than our cousins, the chimpanzees, do, and they’re the living primate we’re most closely related to, and we’re trying to understand why that is.

Assistant Professor Luke D. Fannin.

Fannin recorded the videos of mangabeys in the wild at the Taï Forest Monkey Project field station in Ivory Coast that is co-directed by W. Scott McGraw, professor and chair of anthropology at Ohio State and senior author of the paper.

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. So we can actually say how the joint is loaded. Before, we had hunches, but in this way, it’s far more precise.

Professor W. Scott McGraw, senior author.
Department of Anthropology
Dartmouth College, Hanover, NH, USA.

The chimpanzee ankle can flex upward at a 45-degree angle during a tree climb, compared to most human ankles’ capacity to flex about 20 degrees. Fannin found that the mangabey foot achieves a 46-degree flex in its midfoot during a vertical climb.

This poses a challenge to differing schools of thought about how the LCA moved and the kind of foot it needed to get around a forest environment – theories differ on whether the foot structure was required to be more like a chimp’s or a monkey’s.

A sooty mangabey rests in a tree in the Taï Forest.

Photo: W. Scott McGraw

What’s neat about this paper is it adds clarity, but it also makes the broader picture more fuzzy. Because this notion that you have to have a foot like an advanced ape to vertically climb, which is a difficult task, 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.

The videos that Luke made in Taï are particularly important because they provide some of the first kinematic documentation of a locomotor behavior in a monkey that has largely been unrecognized or underappreciated.

Professor W. Scott McGraw.

There are modern hunter-gatherers and human foragers who are competent vertical climbers as well, a skill believed to be associated with flexibility in their ligaments, tendons and muscles rather than their foot bones.

But Fannin and McGraw note that though modern humans may be defined by walking on two feet, we are focused on ascent, on reaching new heights in a literal sense – a sign that our arboreality, that ancestral connection to trees, is not just anatomical but also a behavioral hallmark.

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..

Professor W. Scott McGraw.

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. So I think that to get at the question of a monkey-like or ape-like last common ancestor, we need more fossils.

Assistant Professor Luke D. Fannin.

This 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.

Former Ohio State undergraduate Carmen Pape was also a co-author.

Publication:
L.D. Fannin,C. Pape, & W.S. McGraw
A kinematic convergence in ape and monkey vertical climbing informs debates on early hominin arborealism
Proc. Natl. Acad. Sci. U.S.A. 123 (35) e2608183123, https://doi.org/10.1073/pnas.2608183123 (2026).

Abstract
Recent debates on early hominin arborealism center around interpretations of fossilized foot bones as being either “African ape”- or “monkey”- like in their morphology. These dueling interpretations fuel competing locomotor reconstructions of the last common ancestor of chimpanzees and humans (LCA), including disagreements about the prevalence of vertical climbing behaviors. Yet it remains unclear if possessing an ape- or monkey-like foot impacts the safety and efficiency of climbing, in part because few kinematic data exist on cercopithecoid monkeys moving in their natural habitats. Here, we present detailed kinematic data on the feet of a prolific vertically-climbing cercopithecoid monkey (Cercocebus atys; the sooty mangabey) in the wild. We show that sooty mangabeys exhibit an extraordinary degree of midfoot dorsiflexion (~46°) during vertical climbs, a value nearly indistinguishable in magnitude to the ankle dorsiflexion of chimpanzees and slightly larger than the ankle dorsiflexion of climbing rainforest hunter-gatherers. This result suggests that, regardless of overall pedal morphology, the fitness challenges posed by vertical movements produced a kinematic convergence in primate foot biomechanics to improve climbing safety. Even if the LCA retained a more monkey-like foot compared to later hominins, our findings suggest that vertical climbing would still be a plausible component of its locomotor repertoire.

L.D. Fannin,C. Pape, & W.S. McGraw
A kinematic convergence in ape and monkey vertical climbing informs debates on early hominin arborealism
Proc. Natl. Acad. Sci. U.S.A. 123 (35) e2608183123, https://doi.org/10.1073/pnas.2608183123 (2026).

Copyright: © 2026 The authors.
Published by National Academy of Science of the USA. Open access.
Reprinted under a Creative Commons Attribution 4.0 International license (CC BY 4.0)

This study does not identify the last common ancestor of humans and chimpanzees, nor does it establish whether that ancestor possessed a more ape-like or monkey-like foot. Its achievement is more precise: it tests an assumption used in competing evolutionary reconstructions and shows that the assumption was unwarranted. A primate does not need a chimpanzee-like ankle to be an accomplished vertical climber because comparable movement can be produced through flexibility elsewhere in the foot.

That is how scientific disagreements are resolved. Alternative interpretations are not suppressed or settled by declarations of faith; their underlying assumptions are exposed to empirical testing. In this case, researchers went into a West African forest, recorded the movements of living primates and obtained measurements that changed what can reasonably be inferred from fossil anatomy. The result makes the reconstruction of our ancestry more accurate, even though it leaves the identity and precise anatomy of the common ancestor unresolved.

Nothing about this disagreement casts doubt upon human evolution. Both sides of the debate begin with the evidence that humans and chimpanzees share an extinct ancestral population. The questions concern the anatomy, behaviour and environment of that ancestor—not whether it existed. Common ancestry provides the reason for comparing fossil hominins with living humans, apes and monkeys, while evolutionary theory explains both their inherited anatomical similarities and the convergent ways in which natural selection can modify those structures to perform similar functions.

The different climbing mechanisms of chimpanzees and mangabeys also illustrate the opportunistic character of evolution. Natural selection does not work towards a predetermined ideal or redesign organisms from first principles. It modifies what each lineage has inherited, producing a flexible ape ankle in one lineage and a flexible monkey midfoot in another. Different anatomical routes can therefore lead to much the same functional outcome without planning, foresight or an intended destination.

Creationism offers no competing research programme capable of producing these findings. Separate creation predicts no last common ancestor, no branching pattern of inherited anatomy and no measurable distinction between homology and convergence. Nor does an appeal to supernatural design tell researchers which joint should flex, how much it should move or what a fossil foot could have enabled its owner to do. Those questions become scientifically tractable only within the framework of evolution.

More fossils will be needed before the human–chimpanzee last common ancestor can be reconstructed with confidence. Science makes no embarrassment of that fact: acknowledged uncertainty identifies where further evidence is required. Evolutionary science progresses by testing, correcting and refining its explanations; creationism merely repeats an ancient conclusion for which it has never supplied a test.

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