Philadelphia -- The long and bitter controversy over just when and how humans first entered the New World may never be settled, but new evidence suggests that the date was many thousands of years earlier than scientists had suspected.

Most anthropologists agree that the earliest Americans migrated across an ancient land bridge now covered by the Bering Sea. The date of that first migration had long been thought to have occurred about 11,500 years ago -- based on fluted stone spear points found at a site near Clovis, N.M. So ``Clovis Man'' had been the standard model of the earliest humans in the Americas.

Yesterday, however, a group of anthropologists and linguists pushed that date far, far back. Some researchers now say the first migration happened 30,000 years ago, others say it was 40,000 ago, and one -- in what amounts to pure speculation -- places the date at the very dawn of modern humanity: perhaps 250,000 years ago.

Much of this new inquiry into humankind's origins in the New World is based on the discovery 20 years ago of the oldest known human settlement in the Americas. The site at Monte Verde, Chile, about 500 miles south of Santiago, is at least 12,500 years old. The age of the site had long been challenged, but it was finally confirmed by radiocarbon techniques last year.

The scientists exploring human antiquity discussed their theories yesterday at the annual meeting of the American Association for the Advancement of Science, which is celebrating the 150th anniversary of its founding.

Johanna Nichols, a University of California at Berkeley linguist who studies the links between ancient and modern languages, believes that there were at least four separate waves of migration from Asia to America in the ancient past. By tracing the rate at which languages spread and change as primitive peoples slowly migrated for thousands of miles, Nichols estimates that the people who crossed the Bering land bridge must have begun the first migration southward well before 22,000 years ago.

At that time much of North America was covered by a thick layer of ice, a glacial period that lasted until at least 14,000 years ago. Nichols believes that those icy conditions must have impeded settlement southward from the Bering crossing. Nevertheless, he said, ``a large and diverse number of languages were already spoken in the American tropics and subtropics,'' which suggests that the migrations from Siberia must have begun well before the ice age made travel impossible.

Thus, Nichols calculates, the very first people in the New World could have arrived as far back as 40,000 to 50,000 years ago and migrated down to the southernmost reaches of South America. Thousands of years later their descendants may have moved back northward into New Mexico and Nevada, where the discovery of their artifacts first gave rise to the theory that Clovis Man was the first migrant from Siberia.

Tom D. Dillehay, the University of Kentucky anthropologist who first validated the antiquity of the Monte Verde site, said yesterday that he is puzzled by new excavations beneath the site that have revealed evidence of human bones and tools that appear to be 30,000 years old or more. Another site in Peru, dated at about 11,600 years old, has also unearthed complex stone tools that appear to be at least 30,000 years old, Dillehay said. Neither of the two new finds have yet been explained, he said.

At Emory University in Atlanta, Theodore Schurr and his colleagues have analyzed a specialized type of DNA carried only in the cells of women and transmitted across succeeding generations. Schurr and Emory geneticist Douglas C. Wallace have analyzed this mitochondrial DNA from Native American women in the American Southwest and also from aboriginal Indian groups from west of the Bering Sea. The scientists concluded that the genetic material represents four distinct population lineages that entered the New World between 20,000 and 40,000 years ago.

The oldest suggestion for movement from Siberia to North America, however, came from Robson Bonnichsen, director of the Center for the Study of the First Americans at Oregon State University. Three years ago Bonnichsen sponsored a scientific visit to Berkeley by a Siberian anthropologist who had discovered stone tools along a river near Irkutsk that he estimated were 1.8 million years old -- a spectacular find that would have challenged long-held evidence that the human race itself had first evolved from earlier apelike animals in Africa and only in Africa.

American experts using the newest radioisotope dating techniques quickly challenged the antiquity of the tools, however, and established that they were about 250,000 years old. But some of those tools did appear to show the same tool-making techniques suggested by similar tools found in North America, Bonnichsen said. That evidence, he said, ``at least raises the possibility'' that mankind's earliest entry into the Americas happened 200,000 years earlier than anyone had dared to suggest.