Ancient Human Ancestor Lived in Trees and Walked on Two Feet

Our human ancestors evolved their bipedalism over time, transitioning from a lifestyle of dwelling in trees to one on the ground. Now, though, researchers have uncovered a bit more about how one of our oldest human ancestors moved. It turns out that the six-million-year-old "Millenium Man" was bipedal but lived in the trees. The findings may provide addition insight to the origins of human bipedalism.

Millenium Man is a fossil from East Africa. It's considered to be one of the best candidate species for what might be called the earliest hominins. Yet its status as a hominin has long been questioned by some scientists. Technically it's a Miocene ape, which is a fossil relative of the ape-human lineage. Most of these Miocene apes have body shapes somewhere between living monkeys and apes. Most walked on their fours in the trees instead of suspending themselves below branches.

The new discovery comes after the examination of the femur bone of this specimen, known as Orrorin tugenensis. The scientists completed 3D geometric morphometric analyses on the shape and characteristics of the femur. This revealed that its morphology was "intermediate" between fossil apes and later human ancestors.

"We discovered that Orrorin's femur is surprisingly 'intermediate' in both age and anatomy between quadrupedal Miocene apes and bipedal early human ancestors," said Sergio Almecija, one of the researchers, in a news release. "Our paper provides quantitative results on the Orrorin femur as a unique mosaic and stresses the need to incorporate fossil apes into future analyses and discussions dealing with the evolution of human bipedalism, an investigation that should stop considering chimpanzees as default living 'starting point' models."

In fact, it's likely that the species lived in trees but could also walk on two legs. According to the analyses, the Orrorin femur is most similar overall to Miocene ape Proconsul nyanzae, but also closely linked to Astralopithecus afarensis, better known as "Lucy."

"Living apes have long and independent evolutionary histories of their own, and their modern anatomies should not be assumed to represent the ancestral condition for our human lineage," said William Jungers, one of the researchers, in a news release. "But we need a better understanding of the paleobiology of Miocene apes in order to properly inform us as to how and when walking on two legs became part of our heritage."

The findings are published in the journal Nature Communications.