In a discovery with profound implications for the study of early human history, scientists digging in the nation of Georgia have found 1.7-million-year-old fossil human skulls that show clear signs of African ancestry and so may represent the species that first migrated out of Africa.
The two relatively complete skulls, being described today in the journal Science, begin to put a face, in a sense, to the ancestors who responded to opportunity and necessity by leaving Africa and spreading out over much of the rest of the world.
Many paleoanthropologists hailed the discovery as a major advance in their field, and said the skulls were probably the most ancient undisputed human fossils outside Africa.
''It's the first good physical evidence we have of the identity of the first emigrants out of Africa,'' said Dr. Ian Tattersall, a paleontologist and evolutionary biologist at the American Museum of Natural History in New York City.
The international discovery team, led by Dr. Leo Gabunia of the Georgia National Academy of Sciences, concluded that the age and skeletal characteristics of the skulls linked them to the early human species Homo ergaster, who lived from 1.9 million to 1.4 million years ago and who some researchers think is the African version of Homo erectus.
The specimens were said to bear less resemblance to the typical Asian Homo erectus.
''We suggest that these hominids may represent the species that initially dispersed from Africa and from which the Asian branch of H. erectus was derived,'' the discoverers said in their report.
The findings contradicted the theory that human ancestors left Africa soon after they invented better stone axes and other tools of what archaeologists call the Acheulean culture.
The more than 1,000 stone tools found in the sediments with the two skulls were all pre-Acheulean, crudely chipped cobbles that had been made since human ancestors began knapping stone tools about 2.5 million years ago.
The earliest tools of the Acheulean style did not appear in Africa until 1.6 million years ago, and about 100,000 years later outside Africa, in Israel.
By contrast, the discovery team reported, the tools found with the Georgian skulls resembled tools found in the Olduvai Gorge of Tanzania and dated at about 1.8 million years.
So it was not technology, but biology or environment, that presumably set human ancestors off on their migrations, scientists now say.
Dr. Susan C. Anton, a paleoanthropologist at the University of Florida who was a member of the discovery team, said that by this time human ancestors had become more carnivorous and their diets pushed them to expand their home range to match the wider ranges of the animals they preyed on.
''With the appearance of Homo, we see bigger bodies that require more energy to run, and therefore need these higher quality sources of protein as fuel,'' Dr. Anton said of the adaptation to meat-rich diets.
As long as early human ancestors had smaller bodies and brains, Dr. Tattersall said, they lived mainly on plants and confined themselves to a limited range at the edge of forests, not too deep in or too exposed far out on the savanna.
Once they had stronger bodies and high-protein meat diets, they were able to spread out geographically and ecologically.
Dr. Alan Walker, a paleoanthropologist at Pennsylvania State University who specializes in searching for human fossils in Kenya, said he agreed with the interpretation of the Georgian skulls. ''The new fossils look exactly like early Homo skulls from Kenya,'' Dr. Walker said.
He said the implications of the new findings for human dispersal from Africa supported an idea he and his wife, Dr. Pat Shipman, an anthropologist, proposed in 1989.
They suggested that once the more apelike australopithecines evolved into the genus Homo and became carnivorous, they were forced to expand their home territory.
''Herbivores are restricted to where the plants are that they eat,'' Dr. Walker said. ''Carnivores are not so restricted. Meat is meat, and you often have to travel far to find it.''
The two skulls were uncovered last summer at Dmanisi, on a slope of the Caucasus Mountains 55 miles southwest of Tbilisi, the Georgian capital. At the same site in 1991, paleontologists found a jawbone of what was identified as a Homo erectus.
Finding the craniums -- one of a young adult male and the other of a female adolescent -- has seemed to quiet skeptics who had disputed the jawbone dating.
The discovery team included scientists from France, Germany and the United States, as well as Georgia.
Dr. Carl C. Swisher 3rd of the Berkeley Geochronology Center in California determined the approximate age of the skulls by geochemical and paleomagnetic analysis of the sediments and the presence of small rodent bones of a well-established age that were found with the human fossils.
Dr. Philip Rightmire, a specialist in Homo erectus at the State University of New York at Binghamton, said he was impressed by the new discovery's implications for ''a pretty quick, really wholesale dispersal of these people, along with other animals,'' from the time the new species emerged in East Africa.
There is fossil evidence that by the time the individuals the Dmanisi skulls belonged to were living in Georgia, others of their species had already traveled as far east as Java in southeast Asia.
Unlike some scientists, Dr. Rightmire classifies the African ergaster together with the Asian erectus in the same species. To him and many others, they are regional variants of the same species.
Other scientists, often referred to as ''splitters'' and including discoverers of the Georgian skulls, look at the same fossils and see distinct species in Africa and Asia, with the Dmanisi fossils much more closely related to its African roots.
In their report, the discoverers said, ''The Dmanisi site suggests a rapid dispersal from Africa into the Caucasus via the Levantine corridor, apparently followed by a much later colonization of adjacent European areas.''
It was somewhat surprising, the discoverers said, to find that the Dmanisi fossils bore so little similarity to later European lineages.
Being close to the boundary between Europe and Asia, Georgia might have been a crossroads of dispersal to the west in Europe as well as to southern and eastern Asia.
Scientists said the discovery left unchanged current interpretations of the origin of anatomically modern humans in Africa some 100,000 years ago.
Photo: The Georgian scientists David Lord Kipandize, left, Leo Gabunia, center, and Abesalom Vekua, at a table with two 1.7-million-year-old fossil skulls believed to be the most ancient human fossils found outside Africa. (Gouram Tsibakhashvili)(pg. A1) Map of Georgia shows location of Dmanisi: Two fossil human skulls were found near Dmanisi in Georgia. (pg. A22)