The shoulders of the most complete fossil of a human ancestor reveal our foreparents were still living a partially arboreal lifestyle three million years ago, even though their lower bodies had adapted to walking upright by then.

The question of when humans diverged from the great apes and shifted to life on the ground is one of the biggest unanswered questions about our evolution. Knowing how the timing of this crucial event compared with many other changes our lineage went through, as well as environmental shifts taking place in Africa, will help us understand many things about our deep past.

Unfortunately, we have found only a tiny number of fossils from this crucial time, most of which are just scraps. All of this makes the fossil known as Little Foot one of the most valuable specimens ever found.

"Little Foot is the Rosetta stone for early human ancestors," Dr Kristian J. Carlson, lead author from University of Southern California, said in a statement.

Although it was first found 27 years ago, Little Foot took a long time to extract from its unusually hard rock and proved difficult to date accurately. Estimates of its age have ranged from 2 to 4 million years – although a 2015 dating of 3.67 million, made using newer measures based on cosmic ray exposure, is now widely accepted. There is also still debate as to whether Little Foot was an Australopithecus africanus or a new species dubbed A. prometheus.

The small feet and legs that gave this fossil its name were previously assessed as suited to walking upright, suggesting pre-humans were well on the path to our modern lifestyle by this point.

Focusing on the upper limbs, however, Carlson found something different. The shoulder blades' large ridges, suited to having big muscles attached, suggest the arms were good for tree climbing and hanging from branches, but not for throwing things, he concludes in the Journal of Human Evolution. Other features, such as the apelike S-shaped collar bone, tell a similar story.

“Based on comparisons with living humans and apes, we propose that the shoulder morphology and function of Little Foot is a good model for that of the common ancestor of humans and chimpanzees 7 million to 8 million years ago." Carlson said. This means halfway from the time our ancestors split away from the great apes to eventually become us, our ancestors' arms and shoulders had barely changed.

Little Foot represents the oldest near-complete Australopithecus fossil. It's both older and more complete than the famous Lucy. Little Foot was found in the Sterkfontein caves, South Africa, whereas Lucy was from Ethiopia, enhancing the rivalry over which part of Africa represents humanity's cradle.

The fact a species still suited to living in trees died in a cave has been explained as her probably having fallen into a sinkhole and dying there in a pool of water.