New York: Examining the evolution of the human shoulder, researchers have determined that our ancestors looked more like a chimpanzee or gorilla than an orangutan or even a monkey.

Humans split from our closest African ape relatives in the genus Pan - including chimpanzees and bonobos - around six to seven million years ago.

Yet certain human traits resemble the more distantly related orangutan or even monkeys.

This combination of characteristics calls into question whether the last common ancestor of modern humans and African apes looked more like modern day chimps and gorillas or an ancient ape unlike any living group.

"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," said lead author of the study Nathan Young, assistant professor at University of California San Francisco School of Medicine in the US.

"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," Young explained.

For the study, the researchers tested the competing theories by comparing 3-D 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.

The study was published in the journal PNAS.