Bipedalism takes a big step backward

Human ancestors began walking more like humans and less like apes long before modern humans began walking the Earth.

A new laser analysis involving footprints discovered in 1976 at Laetoli, Tanzania, suggest that 3.6 million years ago, hominids (proto-humans) were walking in a very humanlike way, extending their legs and using more-balanced foot mechanics than apes. Walking on two legs has been considered a key feature of human evolution since Charles Darwin, but scientists have not agreed on when humanlike walking began. A likely time frame seemed to be about 2.5 million years ago, since that is when the earliest members of our species emerged.

The new research places the start much earlier, when skeletal evidence suggests that human ancestors still spent significant amount of time in trees. The fossilized footprints from two and possibly three upright walkers in Laetoli were preserved in muddy volcanic ash about 3.6 million years ago and may have been made by Australopithecus afarensis, the hominid species of the famed Lucy skeleton. Past analysis of the prints have focused on stride length and compared the look of the footprints to that of human prints, without coming to a definite conclusion.

The researchers used laser scans to examine foot biomechanics of eight human volunteers walking normally and then with a crouched-ape stance. After creating three-dimensional models of those strides, they compared them to the Laetoli prints and found a clear similarity in the equal toe-heel balance of the human upright strides and the Laetoli prints. The crouched stance left much deeper toe imprints.

"Based on previous analyses of the skeleton 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," lead researcher David Raichlen, an anthropologist at the University of Arizona, said in a statement. "But to our surprise, the Laetoli footprints fall completely within the range of normal human footprints."

According to the study, which was published this month in the journal PLOS One, the "results provide the earliest unequivocal evidence of human-like bipedalism in the fossil record." The researchers, however, said there was no way to speculate on how early that bipedalism developed.
-- Margaret Shapiro

http://www.washingtonpost.com/wp-dyn/content/article/2010/03/29/AR2010032902894.html