Africa's east coast netted ancient humans.

Modern humans lived on the African coast of the Red Sea about 125,000 years ago, pushing back the date for the earliest seaside settlement by at least 10,000 years, a new study finds. The discovery raises the possibility that ancient humans left Africa by following the Red Sea coast into southern Asia as well as by trekking up the eastern Mediterranean coast into central Asia.

The excavation, directed by geologist Robert C. Walter of Mexico's Centro de Investigacion Cientifica de Educacion Superior in Ensenada, has unearthed stone tools and shellfish remains in an exposed reef that straddles Eritrea's Red Sea shore. To assign a date to the artifacts, the scientists measured radioactive isotopes in fossil coral surrounding them.

Until now, the oldest coastal occupations by modern Homo sapiens--located at two South African cave sites--dated to between 115,000 and 100,000 years ago. Those discoveries include bone fragments from early H sapiens, whereas the Eritrean site has so far yielded no human skeletal remains.

Still, fossil finds elsewhere suggest that modern H. sapiens lived in eastern Africa by about 130,000 years ago, Walter says.

Global warming around 150,000 years ago may have dried out inland water sources and sent humans scurrying to the coast, where they learned to make a living from the sea, the researchers propose in the May 4 NATURE.

"The eventual dispersal of humans out of Africa was due to increased human competition for marine resources, possibly during hyper-arid conditions," they conclude. Moreover, the Eritrean finds support the theory that modern humans originated in Africa shortly after 200,000 years ago and then spread elsewhere, replacing Neandertals in the process, Walter's team asserts.

The scientists excavated a reef situated near the Eritrean village of Abdur. Ancient earth movements had pushed the reef above the water line, making it a convenient perch for residents harvesting shellfish.

Abundant stone tools, such as sharpened flakes, appeared in a section of the reef. These artifacts lay among shells of oysters, clams, and crustaceans that had been broken open. Excavation at the edge of the reef also uncovered shellfish leftovers, as well as bones of land animals such as elephants, rhinoceroses, and hippopotamuses.

Such finds indicate that when early H. sapiens left Africa, they hugged the coast into southern Asia, remarks anthropologist Christopher B. Stringer of the Natural History Museum in London in the same issue of NATURE. Stringer advocates a single evolutionary source in Africa for modern humans after 200,000 years ago.

Erik Trinkaus, an anthropologist at Washington University in St. Louis, calls the new find "interesting, but irrelevant to modern-human origins." There is evidence that Neandertals also gathered shellfish, so H. sapiens didn't achieve a unique milestone at Abdur, he argues. Trinkaus maintains that Neandertals and modern humans interbred to some extent (SN: 5/8/99, p. 295).