Ape to Human Evolution: More Clues in Shoulders

As humans evolved to carrying tools and throwing instead of swinging, their shoulder shape changed too. In a recent study, researchers examined shoulder blades of two early human Australopithecus species, shedding light on the common ancestor of humans and apes. 

"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," Nathan Young, lead author and assistant professor at the University of California-San Francisco School of Medicine, said in a news release. "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."

The researchers explained that African apes have a trowel-shaped shoulder blade and a handle-like spine, which enables them to climb and swing from branch to branch. On the other hand, monkeys have a scapular spine that is pointed more downwards. This is similar to humans, who have a more pronounced scapular spine that enables them to make stone tools and throw at high speeds, according to the release.

"Human shoulder blades are odd, separated from all the apes. Primitive in some ways, derived in other ways, and different from all of them," Young explained in the release. "How did the human lineage evolve and where did the common ancestor to modern humans evolve a shoulder like ours?"

To answer this question, the researchers used 3D measurements of fossil shoulder blades. They examined those of early hominins and modern humans, and compared them to African apes, orangutan, gibbons and large, tree-dwelling monkeys. They found that modern human shoulders most similarly resemble the orangutans' lateral orientation and the African apes' blade shape.

In order to distinguish exactly where the modern human shoulder blade lay in the spectrum, the researchers further compared their findings to early human Australopithecus species. This included A. afarensis and A. sediba, as well as H. ergaster and Neandertals.

While the A. afarensis shoulder was most similar to the African ape and the A. sediba shoulder was closer to humans', this positioning confirms that Australopithecus species were able to use sophisticated tools. The researchers noted that this signifies the species was on its way to becoming a human.

"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," Neil T. Roach, a fellow of human evolutionary biology at Harvard University, said in a statement. "Our unique throwing ability likely helped our ancestors hunt and protect themselves, turning our species into the most dominant predators on earth."

However, with this evolutionary trait, humans today are prone to shoulder injuries. If scientists are able to better understand the varying shoulder shape among modern humans, and the genetic sequences that cause the difference, they can distinguish those that are more likely to face a shoulder-related injury. 

Their study was recently published in the journal Proceedings of the National Academy of Sciences.  

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