Early humans and apes likely evolved free-moving shoulders and flexible elbows to slow their descent from trees as gravity pulled on their heavier bodies, acording to new research. When early humans left forests for the grassy savanna, their versatile appendages were essential for gathering food and deploying tools for hunting and defense.
Sooty mangabeys (Cercocebus atys). Image credit: Luke Fannin, Dartmouth College.
“The forelimbs of hominoid primates (apes) are decidedly more flexible than those of monkeys, especially at the shoulder, elbow and wrist joints,” said Dartmouth College graduate student Luke Fannin and colleagues.
“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).”
In the study, the authors used sports-analysis and statistical software to compare videos and still-frames they took of chimpanzees and sooty mangabeys climbing in the wild.
They also studied the anatomical structure of chimp and mangabey arms using skeletal collections.
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 hold onto branches like a person going down a ladder as their greater weight pulled them downward rump-first.
“Our 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,” Fannin said.
“Existing research has observed chimps ascending and navigating trees — usually in experimental setups — but our videos from the wild allowed us to examine how the animals’ bodies adapted to climbing down.”
“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.”
“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,” said Dartmouth College Professor Jeremy DeSilva.
“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.”
“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.”
“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 — combined with free-moving shoulders and elbows — made our ancestors excellent shots with a spear (apes cannot throw accurately).”
“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.”
The study was published in the journal Royal Society Open Science.
_____
Luke D. Fannin et al. 2023. Downclimbing and the evolution of ape forelimb morphologies. R. Soc. open sci 10 (9): 230145; doi: 10.1098/rsos.230145
