About 8,000 years ago, a wave of change began to flow west across Europe, largely following the Mediterranean coast. Archaeological sites from the time tell of how early Europeans stopped living as hunter-gatherers and started farming, herding animals and settling in villages. That much is certain. But just how this transition took place has remained a hotly debated question. Did farmers from the Middle East move into the region, or did the locals learn for themselves? And if Neolithic farmers did move west from the Fertile Crescent, did they replace the Paleolithic locals? According to a new study published in last week's issue of Science,the Paleolithic locals held their ground.

An international team of researchers, led by Ornella Semino of the University of Pavia and Giuseppe Passarino of Stanford University, tackled the question by studying the Y chromosome, transmitted only from father to son, in 1,007 European men from different geographic areas. On the basis of 22 different variations, or genetic markers, on the Y chromosome, they reconstructed a family tree and found that more than 95 percent of the men belonged to only 10 groups, or haplotypes. Two haplotypes come from a marker called M173, which probably arrived in Europe from Asia about 30,000 years ago and might be linked to the Aurignac culture. Two other haplotypes rely on marker M170, which probably came from the Middle East about 20,000 to 25,000 years ago and coincides with the Gravettian culture. In all, the descendents of these Paleolithic markers accounted for 80 percent of the Y-chromosome variants in modern European men.

As for signs of Neolithic farmers migrating from the Middle East in more recent times, the analysis found that only 20 percent of the current male population bore such markers. The results answer some questions but raise others because they correspond only in part with earlier studies of mitochondrial DNA in European women (mitochondria are inherited through the female line only). A striking difference, for example, is that a female haplogroup that probably arrived from the Middle East 45,000 years ago does not have an equivalent in men. And the female Neolithic haplotypes are not concentrated along the Mediterranean coastline, as are the mens. The researchers suggest that maybe the women traveled along different routes.