Our Shoulders and Elbows Began as Brakes for Climbing Apes | Dartmouth
The first of these is a paper by Dartmouth researchers, published in The Royal Society Open Science, which shows that the human shoulder and elbow joints probably evolved in an arboreal ape ancestor on the evidence that chimpanzees have the same adaptations.
The adaptations evolved to act as brakes as the apes lowered themselves from trees, reducing their chance of falling or injuring these joints because of their relatively large bodies.
The research, and its significance for understanding human evolution, is explained in a Dartmouth news release:
The rotating shoulders and extending elbows that allow humans to reach for a high shelf or throw a football with friends may have first evolved as a natural braking system for our primate ancestors who simply needed to get out of trees without dying.The authors explain more in the abstract to their paper in Royal Society Open Science:
Dartmouth researchers report in the journal Royal Society Open Science that apes and early humans likely evolved free-moving shoulders and flexible elbows to slow their descent from trees as gravity pulled on their heavier bodies. When early humans left forests for the grassy savanna, the researchers say, their versatile appendages were essential for gathering food and deploying tools for hunting and defense.
The researchers used sports-analysis and statistical software to compare videos and still-frames they took of chimpanzees and small monkeys called mangabeys climbing in the wild. They found that chimps and mangabeys scaled trees similarly, with shoulders and elbows mostly bent close to the body. When climbing down, however, chimpanzees extended their arms above their heads to support their weight, holding onto branches like a person going down a ladder.
Luke Fannin, first author of the study and a PhD candidate in the Ecology, Evolution, Environment and Society program in the Guarini School of Graduate and Advanced Studies, says the findings are among the first to identify the significance of “downclimbing” in the evolution of apes and early humans, which are more genetically related to each other than to monkeys. Existing research has observed chimps ascending and navigating trees—usually in experimental setups—but the researchers’ extensive video from the wild allowed them to examine how the animals’ bodies adapted to climbing down, Fannin says.
Luke Fannin, Guarini School of Graduate and Advanced Studies, said the findings are among the first to identify the role of “downclimbing” in the evolution of apes and early humans.
Photo: Luke Fannin.Our study broaches the idea of downclimbing as an undervalued, yet incredibly important factor in the diverging anatomical differences between monkeys and apes that would eventually manifest in humans. Downclimbing represented such a significant physical challenge given the size of apes and early humans that their morphology would have responded through natural selection because of the risk of falls.
Luke Fanin, Lead author
Department of Anthropology
Dartmouth College, Hanover, NH, USA.Flexible shoulders and elbows passed on from ancestral apes would have allowed early humans such as Australopithecus to climb trees at night for safety and come down in the daylight unscathed, DeSilva says. Once Homo erectus could use fire to protect itself from nocturnal predators, he says, the human form took on broader shoulders capable of a 90-degree angle that—combined with free-moving shoulders and elbows—made our ancestors excellent shots with a spear (apes cannot throw accurately).Our field has thought about apes climbing up trees for a long time—what was essentially absent from the literature was any focus on them getting out of a tree. We’ve been ignoring the second half of this behavior. The first apes evolved 20 million years ago in the kind of dispersed forests where they would go up a tree to get their food, then come back down to move on to the next tree. Getting out of a tree presents all kinds of new challenges. Big apes can’t afford to fall because it could kill or badly injure them. Natural selection would have favored those anatomies that allowed them to descend safely.
It’s that same early-ape anatomy with a couple of tweaks. Now you have something that can throw a spear or rocks to protect itself from being eaten or to kill things to eat for itself. That’s what evolution does—it’s a great tinkerer.
Climbing down out of a tree set the anatomical stage for something that evolved millions of years later,” he says. “When an NFL quarterback throws a football, that movement is all thanks to our ape ancestors.
Professor Jeremy M. DeSilva, co-author
Department of Anthropology
Dartmouth College, Hanover, USA.
Despite chimps’ lack of grace, Fannin says, their arms have adapted to ensure the animals reach the ground safely—and their limbs are remarkably similar to those of modern humans.
The researchers also studied the anatomical structure of chimp and mangabey arms using skeletal collections at Harvard University and The Ohio State University, respectively. Like people, chimps have a shallow ball-and-socket shoulder that—while more easily dislocated—allows for a greater range of movement, Fannin says. And like humans, chimps can fully extend their arms thanks to the reduced length of the bone just behind the elbow known as the olecranon process.It’s the template that we came from—going down was probably far more of a challenge for our early ancestors, too. Even once humans became upright, the ability to ascend, then descend, a tree would’ve been incredibly useful for safety and nourishment, which is the name of the game when it comes to survival. We’re modified, but the hallmarks of our ape ancestry remain in our modern skeletons.
Luke Fanin.
Mangabeys and other monkeys are built more like quadrupedal animals such as cats and dogs, with deep pear-shaped shoulder sockets and elbows with a protruding olecranon process that make the joint resemble the letter L. While these joints are more stable, they have a much more limited flexibility and range of movement.
The researchers’ analysis showed that the angle of a chimp’s shoulders was 14 degrees greater during descent than when climbing up. And their arm extended outward at the elbow 34 degrees more when coming down from a tree than going up. The angles at which mangabeys positioned their shoulders and elbows were only marginally different—4 degrees or less—when they were ascending a tree versus downclimbing.
Mary Joy ’21, who co-led the study as an undergraduate, used her experience as a trail runner to think about why chimps descend trees differently.
Photo: Luke Dewees.Co-author Mary Joy ’21, who led the study with Fannin for her undergraduate thesis, was reviewing videos of chimps that DeSilva had filmed when she noticed the difference in how the animals descended trees than how they went up them.If cats could talk, they would tell you that climbing down is trickier than climbing up and many human rock climbers would agree. But the question is why is it so hard. The reason is that you’re not only resisting the pull of gravity, but you also have to decelerate. Our study is important for tackling a theoretical problem with formal measurements of how wild primates climb up and down. We found important differences between monkeys and chimpanzees that may explain why the shoulders and elbows of apes evolved greater flexibility.
Professor Nathaniel Dominy, Supervising author
Department of Anthropology
Dartmouth College, Hanover, NH USA.
It was very erratic, just crashing down, everything’s flying. It’s very much a controlled fall. In the end, we concluded that the way chimps descend a tree is likely related to weight. Greater momentum potentially expends less energy and they’re much more likely to reach the ground safely than by making small, restricted movements.
Mary S. Joy, co-author
Department of Anthropology
Dartmouth College, Hanover, NH, USA.Joy, a trail runner who was president of the Dartmouth Outing Club when she worked on the study, knew the pained feeling of inching down an incline in short clips instead of just hurtling down the path with the pull of gravity, her legs extended forward to catch her at the end of each stride.When I’m moving downhill, the slower I’m going and restricting my movement, the more I’m fatiguing. It catches up to me very quickly. No one would think the speed and abandon with which chimps climb down from trees would be the preferred method for a heavier primate, but my experience tells me it’s more energy efficient.
Movement in humans is a masterpiece of evolutionary compromises. This increased range of motion that began in apes ended up being pretty good for us. What would the advantage of losing that be? If evolution selected for people with less range of motion, what advantages would that confer? I can’t see any advantage to losing that.
Mary S. Joy
AbstractJust another incidental rebuttal of creationist claims and a confirmation of common ancestry of the great apes, including humans. And, of course, the scientists are not in the slightest doubt that evolution by natural selection is the only rational explanation of the observable facts.
The forelimbs of hominoid primates (apes) are decidedly more flexible than those of monkeys, especially at the shoulder, elbow and wrist joints. It is tempting to link the greater mobility of these joints to the functional demands of vertical climbing and below-branch suspension, but field-based kinematic studies have found few differences between chimpanzees and monkeys when comparing forelimb excursion angles during vertical ascent (upclimbing). There is, however, a strong theoretical argument for focusing instead on vertical descent (downclimbing), which motivated us to quantify the effects of climbing directionality on the forelimb kinematics of wild chimpanzees (Pan troglodytes) and sooty mangabeys (Cercocebus atys). We found that the shoulders and elbows of chimpanzees and sooty mangabeys subtended larger joint angles during bouts of downclimbing, and that the magnitude of this difference was greatest among chimpanzees. Our results cast new light on the functional importance of downclimbing, while also burnishing functional hypotheses that emphasize the role of vertical climbing during the evolution of apes, including the human lineage.
Figure 1. Bouts of vertical climbing in chimpanzees (a,b) and sooty mangabeys (c,d). Maximum angles of shoulder flexion and elbow extension were greater during downclimbs (b,d) compared to upclimbs (a,c), and the magnitudes of these differences were greatest among chimpanzees. Panels (a) and (b) taken by J.M.D and panels (c) and (d) taken by L.D.F.Luke D. Fannin; Mary S. Joy; Nathaniel J. Dominy; W. Scott McGraw and Jeremy M. DeSilva
Downclimbing and the evolution of ape forelimb morphologies
Royal Society Open Science 10(9), 230145; DOI: 10.1098/rsos.230145
Copyright: © 2023 The authors.
Published by [publisher]. Open access.
Reprinted under a Creative Commons Attribution 4.0 International license (CC BY 4.0)
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