The once neat and tidy picture of how people first migrated to the new world has been thrown into turmoil by discoveries that push back the time of the first arrivals by thousands of years.

Discoveries of ancient skeletons, dwelling sites, language histories and genetic evidence all suggest that Asians migrated across the Bering Straits in successive waves over thousands of years and then fanned out into North and South America.Gone is the idea that Asians came across the Bering land bridge in a major migration 10,000 to 11,000 years ago and first settled at a site near Clovis, N.M.

"We know now there were at least three or four different waves of migration," Dennis Stanford of the Smithsonian Institution said Monday at a meeting of the American Association for the Advancement of Science. "But there really is not a consensus among the researchers about how and when it happened.

"The last two years have been an extremely exciting time of discovery in the study of the first Americans," he said.

A dwelling site in Monte Verde, in southern Chile, has been accepted by most scientists as the earliest confirmed place of sustained human habitation in the new world. It has been dated to 12,500 years ago. By some estimates, it would have taken more than 6,000 years for people to have journeyed by stages from the Bering Straits to Monte Verde.

That means the peopling of America started as early as 20,000 years ago, said Tom Dillehay, a University of Kentucky researcher who first excavated Monte Verde. He said there is some evidence that people may have lived in Chile as early as 33,000 years ago.

"I think there may have been a steady stream of people coming across from Asia from very early," Dillehay said.

He doubts that the migration was even stopped by the glacial maximum, a 9,000-year period during which the Alaska-Siberian corridor was clogged with mountains of ice starting about 22,000 years ago.

Ancient skeletons found in Nevada and Washington and in parts of South America suggest that the early Americans came from both Mongoloid and non-Mongolian Asian groups. Stanford said there are fundamental racial differences in the facial bones and in the teeth.

This supports the idea of suc-ces-sive waves of migration from Asia.

Johanna Nichols, a linguist at the University of California, Berkeley, said that a study of the original roots of the 140 languages spoken by Indians in North and South America show at least four waves of migration.

There is clear linguist evidence, she said, that people migrated from Asia 22,000 years ago, just before the age of glaciers.

After the ice retreated, Nichols said another group crossed over from Asia, while some of those who had first settled in South America migrated up to the central Plains of what is now the United States. She said the Sioux language, for instance, has its roots in the Indian speech of South America.

And finally, about 5,000 years ago, a wave of Asians moved into Alaska, Canada and Greenland, bringing with them the language now spoken by the Eskimo, she said.

Language changes at a specific rate, said Nichols, and for 140 distinct languages to evolve in the Americas could have taken as long as 40,000 years.

"I would be surprised if it was really that long ago, but that's what the linguist data suggest," she said.