Humans’ advanced ability to throw things may have started with having evolved shoulders and elbows that stopped our ancestors from falling as they climbed down trees.
Bulkier and heavier than agile monkeys, humans and other great apes can be proficient upward climbers, but struggle to climb back down safely. To counteract the pull of gravity as primates’ body shapes evolved, arm joints became more flexible, allowing apes to stretch away from trees while climbing down so that their feet could brake more efficiently, says Nathaniel Dominy at Dartmouth College in New Hampshire.
“Going downward is much more challenging than going upward because you have to carefully dissipate that potential energy that would cause you to fall,” he says. “Animals that are heavier need to be much more careful, not only in the way that they move across the [forest] canopy, but also in descending down tall trees.”
Compared with monkeys, humans and other apes have greater range of motion in their shoulders, elbows and wrists. Scientists originally suspected that these differences evolved to help prevent falls in the heavier primates as they climbed up trees. But that idea has been hard to prove, since wild chimpanzees use their arms much like monkeys do, with similar shoulder and elbow angles, as they climb up.
Luke Fannin and Mary Joy, also at Dartmouth College, started wondering if it might have more to do with climbing downwards. Watching videos previously filmed by their research team, they realised that chimpanzees’ upward climbing style was markedly different from their downward style.
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To investigate further, Fannin, Joy and their colleagues filmed wild chimpanzees (Pan troglodytes) in Uganda for six weeks and wild sooty mangabeys (Cercocebus atys), a kind of monkey, in Ivory Coast for four months. Using sports analysis and statistical software on their video stills, the researchers measured the angles of the animals’ shoulders and elbows while they climbed up and down. They also examined the arm anatomies of both species from preserved skeletons.
The group found that, when climbing up, both monkeys and apes had similar shoulder and elbow angles. But when they were climbing down, chimpanzees flexed their shoulders 21 degrees more and their elbows 33 degrees more than the mangabeys did. Anatomical analyses confirmed that the apes’ shoulders and elbows allowed for greater rotation and flexibility.
Wider joint angles allow apes to hold their bodies further away from the tree, thereby pressing their weight against the trunk with their feet rather than slipping against the trunk towards the ground, says Dominy.
“It’s sort of like finding the piece to a puzzle that you’ve been looking for and then realising it was right in front of you all along,” says Susan Larson at Stony Brook University in New York, who wasn’t involved in the study.
The findings prompt a “chicken or egg” question, she says. “Did this range of motion of the elbow and shoulder allow apes to become larger-bodied, or were they large-bodied and then this gradually developed?” Certain lighter-weight apes, like gibbons, also have a wide range of motion in their shoulders, says Larson.
While the increased range of motion in the arms would have enabled good braking with the feet pushed against the tree, those rotation and extension abilities would then lead to additional arm skills, says Dominy. In particular, humans would later be able to use their arms for gathering fruit, throwing spears, wielding defence weapons and eventually tossing balls and using ladders. The price we pay is that our flexible joints mean our shoulders are more prone to dislocation, he says.
Royal Society Open Science DOI: 10.1098/rsos.230145