New research on the limb bones of the famous Little Foot skeleton from the Wits Sterkfontein Caves suggests that a major change in how our ancestors moved through their environment took place between about three million and 1.8 million years ago.
The study, published in Science Advances, finds that Australopithecus, the group of hominins that includes Little Foot, combined regular tree-climbing with walking on two legs on the ground. However, by about 1.8 million years ago, early members of the genus Homo appear to have shifted much more strongly towards a life on the ground.
The findings are based on comparisons of the relative strength of arm and leg bones in fossil human ancestors, modern humans and living apes.
Professor Kristian Carlson of the University of Southern California, who is affiliated with the Evolutionary Studies Institute at Wits University, says the results point to a fundamental difference between Australopithecus and later Homo.
"Early human ancestors such as Australopithecus appear to have used trees as much as modern chimpanzees and gorillas do today, but when they were on the ground they were walking on two legs," Carlson says.
"That combination is important because there is no living species that provides a good behavioural comparison for them."
The researchers added new data from the approximately 3.67-million-year-old StW 573 skeleton, known as Little Foot, from the Wits Sterkfontein Caves in South Africa, and fossils from Dmanisi in Georgia dating to about 1.8 million years ago.
They compared the strength of bones in the arms and thighs, as well as strength patterns within the lower limbs.
Little Foot and other Australopithecus fossils show relatively strong arms compared with their thighs, a pattern closer to African apes than to modern humans. This suggests their arms were still regularly exposed to the loads associated with climbing and moving in trees.
But Little Foot's lower limb tells a different story. The relationship between the strength of the thigh and lower-leg bones is closer to the modern human pattern.
"This is one of the most surprising results," Carlson says. "The Australopithecus individual has a modern human-like signal within the lower limb, while the relationship between the arm and thigh is much more African ape-like."
He says the combination highlights how unusual Australopithecus locomotion was.
"These ancestors were not simply moving like chimpanzees, nor were they moving like modern humans. They seem to have combined substantial use of trees with regular bipedal walking on the ground."
In contrast, the Dmanisi fossils and other early Homo and Homo erectus individuals show limb-strength patterns much closer to those of modern humans. The researchers interpret this as evidence that, by about 1.8 million years ago, the human lineage had become far more committed to walking on two legs on the ground and was using trees much less.
The researchers propose that the difference may represent a "threshold" between the adaptive patterns of Australopithecus and Homo, similar to other major changes used to distinguish stages of human evolution.
What caused the shift remains uncertain. The authors suggest it could be linked to changes in ranging, food gathering and other pressures that favoured stronger lower limbs and greater dependence on life on the ground.
They also note that the change took place during a broad period in which brain size was increasing in the human lineage.
"We are not saying that one change caused the other," Carlson says. "But the timing is intriguing. We hope the possible relationship between changes in limb use, ranging behaviour and brain size will stimulate further discussion."
The study includes researchers from Wits University's Evolutionary Studies Institute and the Sterkfontein Caves research programme, together with international collaborators.
