Humans split from our closest African ape relatives in the genus Pan around six to seven million years ago. We have features that clearly link us with African apes, but we also have features that appear more primitive. This combination calls into question whether the Homo-Pan last common ancestor looked more like modern day chimpanzees and gorillas or an ancient ape unlike any living group. A new study, published online in the Proceedings of the National Academy of Sciences, suggests that the simplest explanation – that the ancestor looked a lot like a chimpanzee or gorilla – is the right one, at least in the shoulder.

“It appears that shoulder shape tracks changes in early human behavior such as reduced climbing and increased tool use,” said the study’s lead author Dr Nathan Young of the University of California, San Francisco.

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.

Dr Young and his colleagues from Harvard University, American Museum of Natural History, and California Academy of Sciences, tested these competing theories by comparing 3D measurements of fossil shoulder blades of early hominins and modern humans against African apes, orangutan, gibbons and large, tree-dwelling monkeys.

The scientists found that the shoulder shape of anatomically modern Homo sapiens is unique in that it shares the lateral orientation with orangutans and the scapular blade shape with African apes; a primate in the middle.

“Human shoulder blades are odd, separated from all the apes. Primitive in some ways, derived in other ways, and different from all of them,” Dr Young said.

“How did the human lineage evolve and where did the common ancestor to modern humans evolve a shoulder like ours?”

To find out, the researchers analyzed two early human ancestors – Australopithecus afarensis and A. sediba – as well as Homo ergaster and Neanderthals, to see where they fit on the shoulder spectrum.

The results showed that australopiths were intermediate between African apes and humans.

The shoulder of Australopithecus afarensis was more like an African ape than a human, and Australopithecus 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 Australopithecus 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,” explained co-author Dr Zeray Alemseged from the California Academy of Sciences.

These shifts in the shoulder also enabled the evolution of another critical behavior – human’s ability to throw objects with speed and accuracy.

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,” said co-author Dr Neil Roach of Harvard University.

“Our unique throwing ability likely helped our ancestors hunt and protect themselves, turning our species into the most dominant predators on Earth.”

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Nathan M. Young et al. Fossil hominin shoulders support an African ape-like last common ancestor of humans and chimpanzees. PNAS, published online September 8, 2015; doi: 10.1073/pnas.1511220112