A million-year-old skull discovered in Ethiopia has added fuel to the scientific debate over whether the thick-browed, primitive human called Homo erectus was a single, widely scattered species or two separate ones in Africa and Asia. The scientists who found the fossil argue that it demonstrates a unified species, but others do not accept their view.

About two million years ago, the human family left its ancestral African home and spread throughout Eurasia. Until the late 1970s, scientists considered these migrants members of one species: Homo erectus, an upright creature which used distinctive stone tool technologies and was believed to be the immediate ancestor of modern humans.

But some anthropologists argue that Homo erectus features in specimens from China and Indonesian Java are different from those found in Africa and the Caucasus republic, Georgia. They believe the African and Georgian fossils are a separate species they call Homo ergaster, which they say evolved into modern humans while Asian Homo erectus died out.

Others disagree, like graduate student Henry Gilbert of the University of California at Berkley. He says the physical differences are really the result of Homo erectus evolution over time.

"What we had is a comparison between early African Homo erectus, or what is called Homo ergaster by some, and late Asian Homo erectus. People saw that there were numerous differences between these two forms, theorizing that there was some deep split and that the initial populations that migrated out of Africa were isolated from the rest of Homo erectus and formed a different species," he said.

Now, Mr. Gilbert and other U.S. and Ethiopian researchers challenge this notion with their discovery of a partially crushed skull top in a fossil-rich region 225 km. northeast of Addis Ababa. In a report in the journal Nature, they say its anatomy is like Asian Homo erectus, indicating a single species with a wide distribution.

"Now we're comparing stuff that's at the same time, late Asians and late Africans, said Mr. Gilbert. "What we see is they're pretty similar to the late Asians. All of a sudden, those deep Asian and African differences seem to go away."

Other anthropologists remain skeptical of this assertion.

"I find it interesting that they would like to take numerous different fossils that have been found over the million-and-a-half years that are very distinct morphology and lump them into one species," said Kenneth Mowbray at the American Museum of Natural History in New York. He points out that the physical similarities between the fossils from the two Homo species are insufficient to support a single species theory.

"The morphology is so different in the shape and size of the brow region and the eyes and the nasal structure and the shape and size of the skull, how they could be in the same species is just unfathomable because it's so diverse," Mr. Mowbray said.

The arguments reflect a debate raging since the middle 1970s, when new analytical tools helped scientists assess finer and finer features of primitive human remains. The trouble is, there are few fossil skulls to go on, as Henry Gilbert notes. "Fifty crania over a period of a couple million years are really not that big of a sample," Mr. Gilbert said.

This is a point with which Kenneth Mowbray can agree. "There is a lack of material from the fossil record from that million year [period] reaching over to that 500,000 year period," Mr. Mowbray says.

Yet from such meager resources, scientists are struggling to determine our lineage. Henry Gilbert says researchers would love to find remains of a large Homo erectus population from a single site to get a sense of the physical variation within the species.

"That's the big question: how much variation do you expect within a species versus how much variation differentiate species," he says.