for National Geographic News
Computer analysis of a fossil thigh bone indicates that a chimp-size human-like creature walked on two legs as early as six million years ago. Walking on two legs, known as bipedalism, is considered by scientists to be a distinguishing characteristic in what sets humans apart from apes.
Until now, the most widely accepted date for the advent of bipedalism was about four million years ago. That's when the hominids known as Australopithecus anamensis, lived. Hominids include humans and extinct near humans.
"Dating the beginnings of bipedalism is very important in the human story because, for many experts, it would mark a clear divergence from the ancestral/ape pattern and show that the human lineage had really begun," said Chris Stringer, director of the Human Origins Program at the Natural History Museum in London.
Although this finding puts the development of bipedalism back by another two million years, it is not necessarily a surprise. The chimpanzee-human divergence has been estimated to have occurred between five and seven million years ago, based on genetic data.
"Now, for the first time, we have solid evidence dated to six million years ago of an intermediate creature between humans and the apes that demonstrated upright posture and bipedalism," said Robert Eckhardt, a developmental-genetics and evolutionary-morphology researcher at Pennsylvania State University. "And the dating of this fossil is unusually secure."
Morphology is a branch of biology that deals with the form and structure of animals and plants.
The fossil was found nearly four years ago in Kenya's Lukeino Formation. The thigh bone is about the same size as a chimpanzee's, but CT scans of the bone's interior show that its owner had adapted to walking on two legs.
"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," Eckhardt said.
The ratio in this fossil is one to three, suggesting a biomechanical transition to upright posture and bipedalism, the researchers say.
Eckhardt is one of a team of researchers reporting on the findings in the September 3 issue of the journal Science.
Walking on Two Legs
Scientists hoping to identify the last common ancestor shared by humans and apes think walking in an upright posture could be a key to connecting the dots of human evolution.
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