THE 9 billion humans inhabiting the earth today originated about 270,000 years ago from a population of a few thousand in one region of the world and then dispersed, says a recent study. This challenges the origin of species hypothesis which says that modern humans began evolving about a million years ago from a more primitive human ancestor, the Homo erectus, in different parts of the world simultaneously.

But these findings are consistent with the prevalent view among anthropologists about the origins of modern humans (Homo sapiens).

On the evolutionary scale, the 270,000-year date is considered close to the 200,000-year age claimedin 1987 for a common female ancestor, who was inevitably called "Eve". In both cases, the modern human DNA, which is considered to be a repository of evolution- ary information, was analysed.

The later study is based on the genetic structure of Y- chromosome taken from 38 men of different racial backgrounds from all over the world. It suggests that our common ancestors were probably an isolated tribe of a more primitive human species which did not mate with other groups. Because of this, their evolutionary changes made them different from their ancestors.

Says 'Walter Gilbert of Harvard University, "What's striking about our finding is that when we looked at all these guys, we didn't find any differences. If our species had evolved much earlier, you wouldn't find that. You'd expect differences."

The researchers did not examine the entire Y-chromosome of the subjects, but concentrated on a small segment containing 729 bases of genetic sequence. Called the "intron", this sequence lies within the gene but plays no known role in the gene's code. Robert L Dorit of the Yale University, said they chose this particular segment hoping to find some differences if stuciied in a wide variety of men.

They reasoned that these differences would help construct a family tree and provide an estimate of when the common ancestors lived. If the DNA segment in people differs in only one or a few bases, they are presumed more closely related than those people whose segments differ in more bases.

But, to their surprise, Dorit, Gilbert and Hiroshi Akashi of the University of Chicago, found no differences in the intron, even though they checked the DNA of men from every major racial and geographic region.