A new study, published in PNAS, has found evidence that human ancestors as recent as two million years ago may have regularly climbed trees.

Walking on two legs has long been a defining feature to differentiate hominins (close human relatives and ancestors) from our closest living ape relatives. This new research, based on analysis of fossil leg bones, provides evidence that a hominin species regularly adopted highly flexed hip joints; a posture that in other non-human apes is associated with climbing trees.

These findings came from analyzing and comparing the internal bone structures of two fossil leg bones from South Africa, discovered over 60 years ago and believed to have lived between 1 and 3 million years ago. The remains are likely to belong to either Paranthropus robustus or early Homo.

For both fossils, the external shape of the bones were very similar showing a more human-like than ape-like hip joint, suggesting they were both walking on two legs. The researchers examined the internal bone structure because it remodels during life based on how individuals use their limbs. Unexpectedly, when the team analysed the inside of the spherical head of the femur, it showed that they were loading their hip joints in different ways.

These results demonstrate that novel information about human evolution can be hidden within fossil bones that can alter our understanding of when, where and how we became the humans we are today.

Dr Matthew Skinner, based at the University of Kent’s School of Anthropology and Conservation, and an author on this article said: ‘It has been challenging to resolve debates regarding the degree to which climbing remained an important behavior in our past. Evidence has been sparse, controversial and not widely accepted, and as we have shown in this study the external shape of bones can be misleading. Further analysis of the internal structure of other bones of the skeleton may reveal exciting findings about the evolution of other key human behaviors such as stone tool making and tool use. Our research team is now expanding our work to look at hands, feet, knees, shoulders and the spine.’

Research article: Evidence for habitual climbing in a Pleistocene hominin in South Africa