Evolution and humans; scapula and shoulder shapes (Evolution)

by David Turell @, Tuesday, September 08, 2015, 20:09 (1195 days ago) @ David Turell

Still trying to decide on the last common ape/human ancestor, the shoulder anatomy gives clues:

http://phys.org/news/2015-09-shouldering-burden-evolution.html

"'Humans are unique in many ways. We have features that clearly link us with African apes, but we also have features that appear more primitive, leading to uncertainty about what our common ancestor looked like," said Nathan Young, PhD, assistant professor at UC San Francisco School of Medicine and lead author of the study. "Our study suggests that the simplest explanation, that the ancestor looked a lot like a chimp or gorilla, is the right one, at least in the shoulder."

"It appears, he said, that shoulder shape tracks changes in early human behavior such as reduced climbing and increased tool use. The paper, titled 'Fossil Hominin Shoulders Support an African Ape-like Last Common Ancestor of Chimpanzees and Humans,' published online Sept. 6, in the journal PNAS.

"The shoulders of African apes consist of a trowel-shaped blade and a handle-like spine that points the joint with the arm up toward the skull, giving an advantage to the arms when climbing or swinging through the branches. In contrast, the scapular spine of monkeys is pointed more downwards. In humans this trait is even more pronounced, indicating behaviors such as stone tool making and high-speed throwing. The prevailing question was whether humans evolved this configuration from a more primitive ape, or from a modern African ape-like creature, but later reverted back to the downward angle.

***

"The results showed that australopiths were intermediate between African apes and humans: the A. afarensis shoulder was more like an African ape than a human, and A. sediba closer to human's than to an ape's. This positioning is consistent with evidence for increasingly sophisticated tool use in Australopithecus.

"'The mix of ape and human features observed in A. afarensis' shoulder support the notion that, while bipedal, the species engaged in tree climbing and wielded stone tools. This is a primate clearly on its way to becoming human," Alemseged said.

"These shifts in the shoulder also enabled the evolution of another critical behavior - human's ability to throw objects with speed and accuracy, said Neil T. Roach, PhD, a fellow of human evolutionary biology at Harvard University. A laterally facing shoulder blade allows humans to store energy in their shoulders, much like a slingshot, facilitating high-speed throwing, an important and uniquely human behavior.

"'These changes in the shoulder, which were probably initially driven by the use of tools well back into human evolution, also made us great throwers," Roach said. "Our unique throwing ability likely helped our ancestors hunt and protect themselves, turning our species into the most dominant predators on earth.'"

Comment: this represents an enormous anatomical change in one joint, allowed because humans are not knuckle draggers.


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