The evidence, reported by a team that included U.S. university students in Friday's issue of the journal Science, comes from a rare anthropological discovery: their footprints.
Among those who helped uncover the tracks, preserved for the ages when a gently flowing river changed course and covered them with sand, were undergraduates from Rutgers University in New Jersey. Supervised by scientists, they used brushes to clear away sand bit by bit, retreating to their tents at night after days under the merciless Kenyan sun.
"You realize these are made by our human ancestors," marveled Andrew Du, now a Rutgers senior. "It's a pretty surreal experience."
The find has set off a small debate over whether the creatures who made these prints -- thought to be Homo erectus -- are truly the earliest "modern" walkers, as the authors propose. Other fossils show that by the time of these footprints, our ancestors probably had been spending most of the time on two feet for millions of years.
Yet there is no doubt that the new prints are a rare find, and that the creatures who made them were spending not most, but all of their time on two feet, said Nina Jablonski, head of the anthropology department at Penn State University.
Their long, efficient strides would have allowed them to stray from the wood's edge, crossing open spaces to find other sources of food and possibly to hunt, said Jablonski, who was not involved with the research.
This would, in turn, allow for the continued development of a larger brain. That process already was under way as early humans spent less time in trees, freeing their hands to accomplish more-complex tasks.
Harvard University anthropology professor Daniel Lieberman said the arched, springy feet that made these prints could even have enabled the creatures to run for long distances -- the first appearance, perhaps, of a trait that makes modern humans rare in the animal kingdom.
"Looking at these footprints, I get very excited," said Lieberman, who was not involved in the research.
The authors of the research first discovered what they thought might be footprints in 2005, while digging a trench to study the site's geology.
They noticed curious deformations in a layer about 8 feet below the surface and suspected they were seeing the cross section of footprints, said anthropologist David R. Braun, who is affiliated with Rutgers and the University of Cape Town, in South Africa. Three-dimensional images of the prints enabled detailed analysis of foot dimensions and the amount of pressure exerted by different parts of the feet.
Rutgers professor John W.K. Harris said several characteristics set these feet apart from those of an earlier human ancestor, the short-legged Australopithecus afarensis, whose 3.7 million-year-old prints were found in Tanzania in 1978.
First, the big toes in the new prints are pointed in a direction that is more nearly parallel to the axis of the foot, enabling a stride with a powerful push-off. Also, the arches in the new prints are more pronounced than those in the earlier prints, Harris said.
One of the excavators of those earlier prints, Tim White of the University of California, Berkeley, questioned whether the feet of the two human ancestors were all that different. He said the authors of the new paper are overstating the supposedly "primitive" characteristics of the earlier prints, making their new find seem more dramatic.
The new prints were dated in part by analyzing nearby bits of feldspar, a mineral formed by the cooling materials from a volcanic eruption.
0 comments:
Post a Comment