Chimps \'r\' not us: shoulder development (Introduction)

by David Turell , Thursday, September 07, 2023, 21:48 (444 days ago) @ David Turell

Our shoulders appear in Chimp's evolution:

https://www.sciencedaily.com/releases/2023/09/230905202500.htm

"Researchers report that the flexible shoulders and elbows that allow us to throw a football or reach a high shelf may have evolved as a natural braking system that let our primate ancestors get out of trees without dying. The researchers used sports-analysis software to compare the climbing movements of chimpanzees and small monkeys called mangabeys. While the animals climb up trees similarly, the researchers found that the shallow, rounded shoulder joints and shortened elbow bones that chimps have -- similar to humans -- allow them to fully extend their arms above their heads when climbing down, holding onto branches like a person going down a ladder to support their greater weight. When early humans left forests for the grassy savanna, these versatile appendages would have been essential for gathering food and using tools for hunting and defense. The findings are among the first to identify the significance of 'downclimbing' in the evolution of apes and early humans.

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"The researchers used sports-analysis and statistical software to compare videos and still-frames they took of chimpanzees and small monkeys called mangabeys climbing in the wild. 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.

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"'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, DeSilva said. 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).

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"Mangabeys and other monkeys are built more like quadrupedal animals such as cats and dogs, with deep pear-shaped shoulder sockets and elbows with a protruding olecranon process that make the joint resemble the letter L. While these joints are more stable, they have a much more limited flexibility and range of movement.

"The researchers' analysis showed that the angle of a chimp's shoulders was 14 degrees greater during descent than when climbing up. And their arm extended outward at the elbow 34 degrees more when coming down from a tree than going up. The angles at which mangabeys positioned their shoulders and elbows were only marginally different -- 4 degrees or less -- when they were 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," said study co-author Nathaniel Dominy, the Charles Hansen Professor of Anthropology and Fannin's adviser.

"'The reason is that you're not only resisting the pull of gravity, but you also have to decelerate," Dominy said. "Our study is important for tackling a theoretical problem with formal measurements of how wild primates climb up and down. We found important differences between monkeys and chimpanzees that may explain why the shoulders and elbows of apes evolved greater flexibility.'"

Comment: written in Darwin-speak this study could easily be seen as a designer preparing for the amazing agility of humans.