WASHINGTON (AP) - Armed only with primitive stone tools, the first human migrants left Africa around 1.7 million years ago, boldly starting a dispersal that eventually would people the world. An international team of scientists uncovered the fossils of two partial human-like skulls from the site of an ancient river bluff in the Republic of Georgia. The specimens, identified as from a pre-human species known as early Homo erectus or Homo ergaster, have been age-dated at 1.7 million years, making them the oldest human ancestral fossils ever found outside of Africa, the cradle continent for all of humanity. ``This is the first time that we have a firm date about the very early immigrants from Africa,'' said Reid Ferring, a scientist from the University of North Texas in Denton and co-author of the study. ``These are the earliest ones we know about.''
Ferring said the fossils are clearly related to the early Homo erectus or Homo ergaster specimens found in Africa and dated at about 1.8 million years. Earlier studies had suggested that it was the late Homo erectus human species that first left Africa about one million years ago, but the new discovery advances that migration to a significantly earlier date. ``This shows that humans were far more adaptable and mobile and aggressive in their exploration at a much earlier stage than we thought,'' he said. A report on the study appears Friday in the journal Science. Other experts said the discovery, found under the ruins of a medieval castle in Dmanisi, Georgia, provides stunning new understanding of when humans left their ancestral continent, Africa, and began exploring and settling beyond. ``This may be the major human evolutionary discovery in the last 10 years, if not in the last 20 years,'' said Ofer Bar-Yosef, a noted Harvard University expert on early humans.
``This is the earliest real human fossil found in Eurasia,'' said Bar-Yosef. ``These are real African skulls and there is no disagreement in the community.'' Co-author Susan Anton, a University of Florida at Gainesville expert on early humans, said the fossils were discovered with primitive stone tools - little more than rock flakes - that were first developed in Africa about 2.4 million years ago.
Earlier studies had suggested that human migration from Africa came after the development, about 1.6 million years ago, of the more sophisticated stone implements called Acheulean tools. But Anton said the new finding shows it was ``biology, not new technology, that prompted the dispersal.
``These are hominids with bigger brains and bodies than the ones who came before,'' said Anton. ``The ability to disperse may have something to do with this biological change.'' She said the early Homo erectus also ate more meat than earlier human forms. The new diet may have helped prompt a search for better hunting grounds, said Anton. Dating of the fossils is key to the conclusions, and co-author Carl C. Swisher III of the Berkeley Geochronology Center in California said the age of 1.7 million years is nailed down in three different ways.
A lava layer found just below the fossils was chemically dated at 1.8 million years. Found with the human remains were fossils of a rodent that is known to have lived only between 2 and 1.6 million years ago. And, finally, sediment found with the fossils bear the signature of change in the Earth's magnetic polarity that is known from other studies to have occurred about 1.78 million years ago. ``Everything fits on the age,'' said Swisher. ``There is no discrepancy.'' © Associated Press, credit Ron Schaffner, Creature Chronicles.