Our tree-dwelling ancestors could also walk on two legs 3.6 mln yrs ago

Experiments by a team of anthropologists have shown that fossil footprints made 3.6 million years ago are the earliest direct evidence of early tree-dwelling hominids using the kind of efficient, upright posture and gait now seen in modern humans.

More than three million years ago, the ancestors of modern humans were still spending a considerable amount of their lives in trees, but something new was happening.

David Raichlen, an assistant professor in the University of Arizona School of Anthropology, and his colleagues at the University at Albany and City University of New York's Lehman College have developed new experimental evidence indicating that these early hominins were walking with a human-like striding gait as long as 3.6 million years ago.

A trackway of fossil footprints preserved in volcanic ash deposited 3.6 million years ago was uncovered in Laetoli, Tanzania, more than 30 years ago.

The most likely individuals to have produced these footprints, which show clear evidence of bipedalism, or walking on two legs, would have been members of the only bipedal species alive in the area at that time, Australopithecus afarensis.

A number of features in the hips, legs, and back of this group indicate that they would have walked on two legs while on the ground.

But the curved fingers and toes as well as an upward-oriented shoulder blade provide solid evidence that the species also would have spent significant time climbing in trees.

Since the Laetoli tracks were discovered, scientists have debated whether they indicate a modern human-like mode of striding bipedalism, or a less-efficient type of crouched bipedalism more characteristic of chimpanzees whose knees and hips are bent when walking on two legs.

To resolve this, Raichlen and his colleagues devised the first biomechanical experiment explicitly designed to address this question.

The team built a sand trackway in Raichlen's motion capture lab at the UA and filmed human subjects walking across the sand.

"Based on previous analyses of the skeletons of Australopithecus afarensis, we expected that the Laetoli footprints would resemble those of someone walking with a bent knee, bent hip gait typical of chimpanzees, and not the striding gait normally used by modern humans," Raichlen said.

"But to our surprise, the Laetoli footprints fall completely within the range of normal human footprints," he added.

"This more human-like form of walking is incredibly energetically efficient, suggesting that reduced energy costs were very important in the evolution of bipedalism prior to the origins of our own genus, Homo," Raichlen said.

"What is fascinating about this study is that it suggests that, at a time when our ancestors had an anatomy well-suited to spending a significant amount of time in the trees, they had already developed a highly efficient, modern human-like mode of bipedalism," said biological anthropologist Adam Gordon.