After analyzing fossilized shoulder blades, researchers discovered that the last ancestor we shared with chimpanzees may have resembled a modern African ape. And our ancestors also stayed up in the trees much longer than we thought, according to findings published in Proceedings of the National Academy of Sciences this week. 

We split from other apes several million years ago, and apes have been evolving for just as long as we have. So, did the last common human-chimp ancestor resemble a non-human ape, and if so, which? We’re still not sure what the earliest members of the human lineage looked like since fossils from that time are so rare, and because of that, it’s been difficult to reconstruct the anatomical and behavioral changes that have occurred.

To test evolutionary hypotheses about the last common ancestor we shared with chimps, a team led by Nathan Young from the University of California at San Francisco compared 3D shape measurements of the shoulder blade, called the scapula. The researchers collected data on the scapulae of 31 humans, 56 chimpanzees, 36 bonobos, 72 gorillas, 48 orangutans, and various gibbons. "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 said in a statement.

The simplest explanation, they conclude, is that the human shoulder evolved gradually to its modern form from an African ape-like one. That means gorillas and chimpanzees have changed little in the past six million to seven million years – but that the human lineage has evolved considerably in that time. 

African ape shoulders consist of a trowel-shaped blade and a handle-like spine, and the joint with the arm points up towards the skull; this helps with climbing and swinging. In humans, however, this so-called scapular spine is pointed downwards. This change in shoulder shape during our evolution fits with our reduced reliance on tree-climbing arms and an increased need for high-speed throwing and the use of stone tools.

Furthermore, based on the fossil record, this shift to living on the ground occurred slowly: For the majority of human evolutionary history, our ancestors continued to search for food in the trees, and they also used them as a place to retreat from predators.

When fossils from extinct Australopithecus and Homo species were included in the analysis, they all seemed to fit along a roughly temporal gradient. In the image above, you can see a hypothesized model of shoulder shape evolution from African ape-like (top left) to modern human (bottom right), including predicted ancestral forms (in gray) and fossils of our extinct ancestors: Australopithecus afarensis (top right), Australopithecus sediba (middle left), Homo ergaster (middle right), and Homo neanderthalensis (bottom middle).