Fossils Reveal Clues on Human Ancestor
By JOHN NOBLE WILFORD

New York Times
September 20, 2007

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.

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See fossil location shown on map of area in southern Georgia near Armenia:
09/19/science/FOSSILmap.jpg