The rotating shoulders and extended elbows that allow humans to reach a high shelf or throw a ball may have originally evolved as a natural braking system for our primate ancestors, a study suggests.
According to the researchers, early humans needed the movements to slow their descent from trees so they could climb without dying.
When the first humans left the forests for the grassy savannah, researchers say, their flexible appendages were essential for gathering food and developing tools for hunting and defense.
Researchers from Dartmouth College in the US used sports analysis and statistics software to compare video and photo frames they took of chimpanzees and small monkeys called mangabeys climbing in the wild.
They found that the animals climb trees in a similar way, with the shoulders and elbows mostly bent close to the body.
However, when descending the chimpanzees reached their arms above their heads to hold on to branches like a person descending a ladder as their greater weight pulled them down first.
Luke Fannin, first author of the study and a graduate student in Dartmouth's Ecology, Evolution, Environment and Society program, said the findings are among the first to identify the importance of “descent” in the evolution of apes and early humans.
He added: “Our study analyzes the idea of descent as an underappreciated, but incredibly important factor in the divergent anatomical differences between apes and apes that would eventually manifest in humans.
“Descent represented such a significant physical challenge given the size of apes and early humans that their morphology would have responded through natural selection due to the risk of falls.”
Co-author Jeremy DeSilva, professor and chair of anthropology at Dartmouth, said: “For a long time our field has thought of apes as climbing trees—what has been virtually absent from the literature has been a focus on climbing out of a tree. We were ignoring the second half of this behavior.
“The first apes evolved 20 million years ago in these kinds of scattered forests where they would climb a tree to get their food and then come back to move on to the next tree.
“Getting out of a tree presents all kinds of new challenges. Great apes cannot afford to fall because it could kill or injure them badly.
“Natural selection would have favored those anatomies that allowed them to descend safely.”
The researchers suggest that flexible shoulders and elbows passed down from ancestral apes would have allowed early humans like Australopithecus to climb trees at night for safety and descend in daylight unscathed.
Once Homo erectus could use fire to protect itself from nocturnal predators, the human form took on broader shoulders capable of a 90-degree angle that made them excellent spear shots.
The researchers also studied the anatomical structure of chimpanzee and mangabey hands using skeletal collections at Harvard University and Ohio State University, respectively.
They found that, like humans, chimpanzees have a shallow ball-and-socket shoulder that—while more easily dislocated—allows for greater range of motion.
Also, like humans, they can fully extend their arms thanks to the reduced length of the bone just behind the elbow, known as the olecranon process.
Mangabeys and other apes are built more like quadrupedal animals like cats and dogs, with deep, pear-shaped shoulder sockets and elbows with prominent olecranon processes that make the joint look like the letter L.
Although these joints are more stable, they have much more limited flexibility and range of motion.
The research is published in the journal Royal Society Open Science.