Chimp-sized Hominid Walked Upright On Two Legs Six Million Years Ago

Finding: When did hominids begin to walk upright? Recent fossil evidence suggests that a chimp size hominid walked upright on two legs in Kenya's Tugen Hills, over 6 million years ago. That is about 3 million years earlier than "Lucy," the most famous early biped in our lineage.

How the finding occured:

A U.S. research team responsible for analysis of the CT scans of the internal structure of the fossil bone was responsible there is now solid evidence of the earliest upright posture and bipedalism securely dated to six million years.

The fossil the team studied is part of a left thighbone unearthed nearly four years ago by Senut and Pickford at their dig in the Kenyan Lukeino Formation. The fragment includes the intact head of the left thighbone -- the ball that is inserted into the hip socket joint -- plus the bony neck that connects the ball to the thighbone shaft as well as part of the thighbone shaft.
Measurements show that the fossil bone is about the same size as a chimpanzee's. However, CT scans of the interior of the bone reveal that the neck connecting the ball to the shaft is thinner on top than it is on the bottom, a sign the researchers say that the individual from which it came walked on two legs.

How does this compare to modern day apes and humans?

In present day chimps and gorillas, the thicknesses in the upper and lower parts of that bone are approximately equal. In modern humans, the bone on top is thinner than on the bottom by a ratio of one to four or more. The ratio in this fossil is one to three.

The key is the ratio

The ratio in the fossil is evidence for transition to an upright posture and habitual bipedal gait the researchers argue. In addition, because walking upright is the essential mark of a hominid, the ratio is functional evidence that the bones fossilized at Lukeino were from hominids.