Washington: The last common ancestor of humans and chimpanzees had shoulders similar to those of modern African apes, scientists have found.
Humans split from their closest African ape relatives in the genus Pan - including chimpanzees and bonobos - 6 to 7 million years ago. Yet certain human traits resemble the more distantly related orangutan or even monkeys.
"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 lead author Nathan Young, assistant professor at University of California, San Francisco School of Medicine in US.
"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," Young said.
It appears, he said, that shoulder shape tracks changes in early human behaviour such as reduced climbing and increased tool use.
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 behaviours 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 researchers 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.
They found that the modern human's shoulder shape is unique in that it shares the lateral orientation with orangutans and the scapular blade shape with African apes; a primate in the middle.
Young and his team then analysed two early human Australopithecus species, the primitive A afarensis and younger A sediba, as well as H ergaster and Neandertals, to see where they fit on the shoulder spectrum.
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," said Zeray Alemseged, senior curator of Anthropology at the California Academy of Sciences.
The study was published in the journal PNAS.