Looking for the earliest common ancestor of apes and humans, before early humans came down from the trees for the savannahs of Africa, has led researchers to consider bones — shoulder bones.
A study of bones by researchers from the University of California suggests that last common ancestor more likely resembled, in looks and behavior, modern chimpanzees much more than it did modern humans.
Exactly what that ancestor looked like has long been unclear, they note.
"Humans are unique in many ways," says lead study author Nathan Young. "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."
The easiest explanation — that the ancestor resembled a chimp or a gorilla — seems the best one, at least if we look at the shoulders, he says.
Changes to shoulder shape in early humans are evidence of new behaviors, including reduced tree climbing and the first developments of tool use, he explains.
In modern African apes, the shoulder consists of a trowel-shaped blade and a handle-like spine that eases movement of the arm upward toward the skull, an advantage when climbing trees or swinging through their branches.
In contrast, the shoulder joint is pointed generally more downwards in modern monkeys and even more so in humans, evidence of behaviors including stone tool-making and throwing objects like rocks or spears at high speed, the researchers point out.
The human lineage diverged from that of chimpanzees, our closest living relative, around six million or seven million years ago.
With fossils of possible earliest common ancestors hard to come by, the researchers used 3D models of shoulders taken from museum examples of early hominids, modern humans, chimps, gorillas, gibbons, monkeys and orangutans.
They compared them with two early Australopithecus hominid species, the primitive A. afarensis and the younger A. sediba.
The found the A. afarensis shoulder more resembled that of an African ape than of a human, while A. sediba was closer in form to a human's shoulder than to an ape's.
"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," says Zeray Alemseged, senior anthropology curator at the California Academy of Sciences. "This is a primate clearly on its way to becoming human."
Young says he agrees with that assessment.
"We found australopithecines were perfect intermediate forms between African apes and modern humans," he says.
The study strongly suggests a long, gradual shift down from trees and an increasing reliance on tools drove the evolution of the human lineage, he says.