New clues to when visitors first came to America

February 01, 1994|By San Francisco Chronicle

Scientists seeking to learn when humans first began to populate the Americas have created an evolutionary "clock" that suggests the event must have occurred in the depths of an ice age nearly 30,000 years ago.

If their claim is correct, it will more than double the antiquity of the "first Americans." Most archaeologists have long contended that the ancestors of today's multitude of Indian tribes could not have arrived in the Western Hemisphere any earlier than 12,000 to 15,000 years ago.

In any event, the Indian tribes of the Americas, the true "Native Americans," do indeed go far back in time. Both anthropologists and archaeologists agree that they must have reached these lands during a long-lasting ice age when so much water was bound up in northern glaciers that the sea level of what is now the Bering Straight dropped to create a broad plain about 60 miles wide between Siberia and Alaska.

Across that plain, at least three migrating peoples must have moved in separate waves to spread their numbers south over the next millennia, the scientists contend. But when it all began has been the ongoing puzzle.

A research team headed by geneticists Antonio Torroni of Baylor University in Atlanta and James V. Neel of the University of Michigan has analyzed the genetic material of nearly 18 widely separated Indian groups in both North and Central America.

After focusing on seven modern tribes all speaking the same language, known as Chibcha in Costa Rica and Panama, the researchers came to several conclusions: The linguistic evidence and the Chibcha artifacts found in ancient settlements indicated that the Indians most probably had been part of a distinct genetic group until about 8,000 to 10,000 years ago, after which they began to diverge.

From two long-isolated tribes in the groups, the scientists obtained samples of a unique type of genetic material that all human beings carry. It is known as mitochondrial DNA, and it is carried within cells but outside the nucleus where chromosomes are the major carriers of heredity.

Mitochondrial DNA, however, is inherited from generation to generation only through the female line, and it was this fact that impelled the late Berkeley anthropologist Allan C. Wilson to create his famed "DNA clock" and argue that all modern human beings are descendants of a single early human female some 200,000 years ago -- humanity's single hypothetical "Eve."

Messrs. Torroni and Neel have now used Mr. Wilson's mitochondrial DNA concept to estimate the average rate at which mutations have occurred in the genes of the two isolated Indian tribes since their relatives dispersed, and the scientists then calculated the rate at which their "DNA clock" ticked -- mutation by mutation -- back to the time when the original groups began to disperse.

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