Walking on two legs has long been a defining feature to differentiate modern humans, as well as extinct species on our lineage, from our closest living ape relatives: chimpanzees, gorillas and orangutans. This new research, based on analysis of fossil leg bones, provides evidence that a hominin species - believed to be either Paranthropus robustus or early Homo - regularly adopted highly flexed hip joints; a posture that in other non-human apes is associated with climbing trees.