Sunday, March 21, 2010

Early Human Tree-Dwellers Were Also Bipedal

Laetoli Footprints Preserve Earliest Direct Evidence of Human-like Bipedal Biomechanics, 2010. PLoS; will be published on Monday, March 22.
Fossil footprints made 3.6 million years ago are the earliest direct evidence of early hominids using the kind of efficient, upright posture and gait now seen in modern humans

Three dimensional scans of experimental footprints and a Laetoli footprint, A, modern human footprint walking with a normal, extended limb, B, modern human footprint walking with a bent-knee, bent-hip gait, C, Laetoli. footprint.
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 significance of those prints for human evolution has been debated ever since.

The only individuals that could have produced these footprints are Australopithecus afarensis, the only bipedal species alive in the area at that time. A. afarensis includes the famous fossil"Lucy," a cast of whose skeletal remains can be seen at the Cleveland Museum of Natural History.

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 Lucy and other members of her species also would have spent significant time climbing in trees.

To resolve how the fossil trackways were made, scientists this built a sand trackway in filmed human subjects walking across the sand. The subjects walked both with normal, erect human gaits and then with crouched, chimpanzee-like gaits. Three-dimensional models of the footprints were collected and examined. It was found that the relative depth of footprints at the heel and toe, and found that depths are about equal when made by a person walking with an erect gait. In contrast, the toe print is much deeper than the heel print when produced by a crouched gait, a product of the timing of weight transfer over the length of the foot.

"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." link