DNA dates for first Americans.

A new analysis of mitochondrial DNA indicates that the first settlers of the Americas crossed a land bridge from Asia at least 22,000 to 29,000 years ago.

Antonio Torroni, a geneticist at Emory University in Atlanta, and his colleagues identified the presence or absence of 14 mitochondrial DNA sequences among members of seven Central American Indian tribes that speak the same language. They next calculated the rate at which these random genetic variations occurred, based on estimates that the tribes diverged from a single group between 8,000 and 10,000 years ago. Torroni's group then applied this rate to mitochondrial DNA samples from 18 Indian tribes spread throughout the Americas.

The researchers, whose study appears in the Feb. 1 PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES, classify these and other tribes as "Amerinds," descendants of the New World's initial occupants (SN: 6/9/90, p.360).

Still, opinions differ on the rate at which mitochondrial genes change. Another mitochondrial DNA study has dated the first Americans to 15,000 years ago at most.

Many archaeologists support the latter estimate and note that potential American settlers did not occupy Siberia until 35,000 years ago. But confirmation of a 500,000-year-old date for a human site under investigation in Siberia may offer crucial support to the Atlanta scientists (SN: 2/5/94, p.84).