A newly discovered 1.8 million-year-old skull offers evidence that humanity's early ancestors emerged from Africa as a single adventurous species, not several species as believed, drastically simplifying human evolution, an internationalresearch team said Thursday.

The skull-the most complete of its kind ever discovered-is 'a really extraordinary find,' said paleoanthropologist Marcia Ponce de Leon at the University of Zurich's Anthropological Institute and Museum, who helped analyze it. 'It is in a perfectly preserved state.'

Unearthed at Dmanisi in Georgia-an ancient route in the Caucasus for the first human migrations out of Africa-the skull was found at a spot where partial fossils of four other similar individuals and a scattering of crude stone tools had been found several years ago. They all date from a time when the area was a humid forest where saber-tooth tigers and giant cheetahs prowled. Preserved in siltstone beneath the hilltop ruins of a medieval fortress, the remains are the earliest known human fossils outside Africa, experts said.

David Lordkipanidze, director of the Georgian National Museum, who led the team, reported the discovery in Science. The primitive skull was first uncovered on Aug. 5, 2005-his birthday. 'It was a very nice present,' he said.

Taken together, the finds at Dmanisi are especially important because experts in evolution could analyze the physical differences between individuals living in the same place at the same time almost 2 million years ago, when humankind first emerged from Africa to people the world, according to Yale University anthropologist Andrew Hill.

'It gives you a chance to look at variation for the first time,' said Dr. Hill, who was not involved in the discovery.

By comparing these five extinct creatures at Dmanisi to each other, and to other specimens from the same era in Africa, the researchers concluded that all of the primordial peoples of the Homo genus-the root-stock of the modern human family tree-likely belonged to just one species spreading out across the continents, not three or more as many experts have argued.

Their conclusion breaks with recent practice in the scholarly search for human origins. Typically, researchers have highlighted the differences between various human fossils-often assigning each new discovery to a separate species-and not grouping them by physical traits they had in common.