The reason we can throw a football or reach the top shelf in our kitchen may have something to do with how our ape ancestors got down from trees.
That’s according to a new study from researchers at Dartmouth College, who set out to understand why apes – a group that includes gorillas, chimpanzees and humans – have more flexible elbows and shoulders than monkeys, our smaller primate cousins.
Humans and other apes “have the ability to raise their arms above their heads,” said Luke Fannin, a Dartmouth graduate student and the study’s lead author. “They have the ability to fully extend their elbows. And that's something a lot of monkeys don't do.”
Fannin said scholars have debated why apes evolved to have that kind of range of motion – perhaps to help them get up trees, or gather fruit from branches.
The study, published last week in the journal Royal Society Open Science, suggests a different answer: climbing down.
“Getting out of a tree is dangerous,” said Jeremy DeSilva, a professor of anthropology at Dartmouth and another of the authors. “The pull of gravity is going to want to sort of accelerate your body out of that tree, especially if you're big – and apes are big.”
For the study, the researchers compared footage of chimps and sooty mangabeys, a species of monkey. The two primates used similar movements to climb up trees. But coming down was a different story. When descending, the chimpanzees extended their elbow and shoulder joints to a much greater degree.
DeSilva said the chimps they observed use various techniques to get down from trees. Sometimes it was a slow, careful downclimb, going from handhold to handhold.
“Other times they would go down a tree almost like a fire pole,” he said. “There are times when an ape will almost crash through a free fall and just grab onto branches as they're falling – sort of almost a Spider-Man kind of move.”
Their greater range of motion helps chimpanzees execute all those moves, and get down from the trees safely, DeSilva said. That’s especially important for bigger animals like apes, who are more likely to seriously hurt themselves if they fall.
Using outstretched arms to drop from branch to branch – rather than descending in a more cramped position – could also allow apes to expend less energy and avoid muscle fatigue, said Mary Joy, a recent Dartmouth graduate who was one of the co-authors.
The idea came to her because she does a version of that herself. When she’s out running trails and has to descend a steep slope, she doesn’t clip her stride, but instead runs downhill in something more like a “controlled fall.”
“It helps you to not keep your quads under contraction for a ton of time, which fatigues you,” said Joy, who wrote up the results of the study for her undergraduate thesis. “And so I thought, ‘Well, you know what if they're doing this for the same reason?’ ”
That’s also a safety issue, she said. If an ape’s arms get too tired on the climb down, it’s more likely to make a mistake and fall.
Those flexible, apelike shoulders and elbows would go on to help early hominins in various ways, Fannin said. Early human ancestors would still have been going up into trees for safety, before the use of fire.
Later, humans would find new purposes for those same abilities – using tools, throwing spears and, eventually sports.
“An overhead throwing motion – that is an ape hallmark,” he said. “So having the ability to raise your arm above your head and then fully extend your elbow – I mean, that's what Tom Brady does when he throws a touchdown pass, right?”
Fannin – who was speaking from Borneo, where’s doing fieldwork with orangutans – said he thinks about that often when he watches sports.
“They're what make us humans, are our flexible forelimbs,” he said. “But [they] also tie us intimately to our evolutionary heritage – i.e. the fact that all of us are apes.”
