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

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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," said first study author Luke Fannin, assistant professor of anthropology at Ohio State. "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."

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," Fannin said. "And we're trying to understand why that is."

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," McGraw said.

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.

"What's neat about this paper is it adds clarity, but it also makes the broader picture more fuzzy," McGraw said. "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."

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.," McGraw said.

Added Fannin, "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."

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.

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