The mechanics of how athletes like New York Giants quarterback Daniel Jones’ are able to throw a perfect spiral or how wide receiver Darius Slayton may extend his elbow to reach for the catch may have ancient roots. These skills may have first evolved as a natural braking system for our primate ancestors who simply needed a safe way to get out of trees.
In a study published September 6 in the journal Royal Society Open Science, a team from Dartmouth found that apes and early human ancestors likely evolved free-moving shoulders and flexible elbows as a way to slow their descent from trees while gravity pulled down on their bodies. Versatile appendages that could throw spears for hunting and defense, climb trees, and gather food were essential for survival—especially as early humans left forests for grassy savannas.
“There’s a lot we still don’t understand about the origin of apes,” study co-author and Dartmouth University paleoanthropologist Jeremy DeSilva tells PopSci. “There was a common ancestor to monkeys and apes that lived about 25 to 30 million years ago and then there was a divergence and now we have these two different kinds of primates. But why the convergence?”
One of the possibilities is different ecological, physical, and behavioral niches related to primate size. The first apes evolved about 20 million years ago and are bigger than other early primates. Getting out of a tree presented a new set of challenges for these bigger primates, since typically the bigger the animal, the greater the risk of injury from a fall. Natural selection would have eventually favored anatomies that allowed early apes to safely descend from the trees.
In the study, the team used sports-analysis and statistical software to compare videos and still-frames of chimpanzees and small monkeys called mangabeys climbing in the wild. They saw that mangabeys and chimps climbed up the trees similarly, with their shoulders and elbows mostly bent close to the body.
However, when it was time to climb down, chimpanzees extended their arms above their heads to hold onto branches, similar to how a person going down a ladder, as their weight pulls them down. This process called “downcliming” appears to be significant in the evolution of apes and early humans.
“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,” study co-author and Dartmouth graduate student Luke Fannin said in a statement.
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These flexible shoulders and elbows passed on from ancestral apes would have allowed early humans such as Australopithecus to climb into trees at night for safety and then come down in the daylight unscathed. Once Homo erectus could use fire to protect itself at night, the human form took on the broader shoulders capable of a 90-degree twist that worked with free moving shoulders and elbows to make human ancestors excellent shots with a spear for hunting.
“The idea that downclimbing could be such a strong evolutionary force as to change the nature of how our bones and range of motion evolved was very fascinating,” study co-author Mary Joy tells PopSci. “Not a lot of the field really thinks about downclimbing as its own motion with implications on natural selection.” Joy brought her experience as a trail runner and athlete to the study to bring in a different perspective to looking at biological sciences and evolution.
The team also used skeletal collections from Harvard University to study the anatomical structure of chimpanzee arm alongside remains in The Ohio State University’s collections to study mangabey arms. Chimpanzees are more like humans than mangabeys and have a shallow ball-and-socket shoulder that allows for a greater range of movement. Chimps can also fully extend their arms due to a reduced length of bone located just behind the elbow called the olecranon process.
Mangabeys and other monkeys are built more like four-legged animals like cats and dogs, with deep pear-shaped shoulder sockets and elbows that have a protruding olecranon process, which makes the joint look like the letter L. These joints are more stable, but they have a more limited range of movement and flexibility.
The analysis showed that the angle of a chimp’s shoulders was 14 degrees greater during their descent than when scaling a tree. The arm also extended outward at the elbow 34 degrees more when climbing down a tree than climbing up. The angles at which the mangabeys positioned their shoulders and elbows were only about four degrees or less when ascending a tree versus downclimbing.
“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,” study co-author and
anthropologist and evolutionary biologist Nathaniel Dominy said in a statement. “The reason is that you’re not only resisting the pull of gravity, but you also have to decelerate.
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According to DeSilva, the question of “how did we not see this before” in regards to downclimbing was one of the most surprising parts of the study. The fresh eyes of both Joy and graduate student Fannin were crucial in uncovering one of evolution’s hidden wonders.
“Our evolutionary ancestry is this wonderful example of how evolution just sort of tinkers and tweaks pre-existing forms,” says DeSilva. “Our bodies are bodies that have been just tweaked and modified through natural selection over millions of years, to give us the bodies we have now, but there are all these wonderful echoes of our ancestry in our bodies today.”