Reposted from:
http://www.nytimes.com/2007/09/20/science/20fossil.html?ref=science

The discovery of four fossil skeletons of early human ancestors in Georgia, the former Soviet republic, has given scientists a revealing glimpse of a species in transition, primitive in its skull and upper body but with more advanced spines and lower limbs for greater mobility.

The findings, being reported today in the journal Nature, are considered a significant step toward understanding who were some of the first ancestors to migrate out of Africa some 1.8 million years ago. They may also yield insights into the first members of the human genus, Homo.

Until now, scientists had found only the skulls of small-brain individuals at the Georgian site of Dmanisi. They said the new evidence apparently showed the anatomical capability of this extinct population for long-distance migrations.

"We still don't know exactly what we have got here," David O. Lordkipanidze, the excavation leader, said Monday in an interview on a visit to New York. "We're only beginning to describe the nature of the early Dmanisi population."

Other paleoanthropologists said the discovery could lead to breakthroughs in the critical evolutionary period in which some members of Australopithecus, the genus made famous by the Lucy skeleton, made the transition to Homo. The step may have been taken more than two million years ago.

"The Australopithecus-Homo transition has always been murky," said Daniel E. Lieberman, a paleoanthropologist at Harvard University. "The new discoveries further highlight the transitional and variable nature of early Homo."

The international team led by Dr. Lordkipanidze, director of the Georgian National Museum in Tbilisi, found several skulls and stone tools at Dmanisi in the 1990s. They were dated to 1.77 million years ago and resembled Homo erectus, the immediate predecessor of Homo sapiens. The fossils were tentatively assigned to the erectus species.

But erectus had been considered a species with more affinities to modern humans, with large bodies and long faces, smaller teeth and larger brains than predecessors. A young erectus man in Africa, dating to 1.5 million years ago, had a modern body and was almost six feet tall.

The Dmanisi specimens were quite different. Their skull sizes indicated that their brains were not much larger than the brain of a chimpanzee. Their brains were closer in size to those of Homo habilis, a poorly understood earlier ancestral species.

In the last few years, however, the researchers collected more extensive, well-preserved skeletal remains of an adolescent and three adults. Some of the fossils resembled those of later erectus specimens in Africa. The lower limbs and arched feet reflected traits "for improved terrestrial locomotor performance," the team reported.

Over all, the fossils were "a surprising mosaic" of primitive and evolved features. The small body and small craniums, the upper limbs, elbows and shoulders were more like the earliest habilis specimens.

"Thus, the earliest known hominids to have lived outside of Africa in the temperate zones of Eurasia did not yet display the full set" of evolved skeletal features, the scientists concluded.

In an accompanying article in Nature, Dr. Lieberman said the new findings, with other recent research on erectus and habilis fossils in Africa, showed that "early Homo was less modern and more variable than sometimes supposed."

A possible explanation, he said, was that the Dmanisi specimens "were simply smaller than their African relations." Or they may be a different species.

"My hunch," Dr. Lieberman wrote, "is that the Dmanisi and early African H. erectus fossils represent different populations of a single, highly variable species."

Ian Tattersall, a paleoanthropologist at the American Museum of Natural History, said that when the Dmanisi skulls came to light some scientists thought they represented a distinct species, which they called Homo georgicus. But others settled on an erectus designation.

"By tradition, erectus is the hominid in the middle, between earlier habilis and later Homo sapiens," Dr. Tattersall said. "This mind-set prevailed."

But more significant, he said, the Dmanisi skeletons may reveal how early human ancestors could move out of Africa. Once larger brains, better tools and evolved limb proportions were the probable explanations. Previous discoveries ruled out the first two, but provided no direct evidence for the third.

"It seems the limb proportions to traverse environments out of Africa were there at least 1.8 million years ago," he said.