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No. 93: May-Jun 1994

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Molecular clock places humans in new world 22,000-29,000 bp

This is welcome news for anomalists who have been searching for a way to demolish the 12,000-BP (years Before Present) barrier erected across the Bering Land Bridge by the archeological establishment. But don't uncork the champagne yet, because molecular clocks are not like Big Ben.

Here's what has happened: A. Torroni and some colleagues at Emory University have analyzed the mitochondrial DNA (mtDNA) of members of seven linguistically related tribes in Central America called the "Chibcha speakers." Assuming that the homogeneous group separated from the other Amerind tribes 8,000-10,000 years ago, the Emory group found that their mtDNA had mutated at the rate of 2.3-2.9% per million years. (Note: this works out to 0.0022-0.0029% per thousand years -- a very small amount to measure accurately!) Next Torroni et al measured the mtDNA of 18 other tribes throughout the Americas and, using the mutation rate just mentioned, computed how long ago these peoples had diverged from a common ancestor. The result: 22,000-29,000 years ago. The Emory study was published in the February 1, 1994, issue of the Proceedings of the National Academy of Sciences.

All this is very well, but suppose that the tribes had split from that common ancestor before they even crossed the Bering land bridge into the New World, thereby starting the molecular clock too early? Or, perhaps Southeast Asians arriving by boat tossed sand into the gears of the vaunted molecular clocks?

So, be careful with this apparent anomaly. Molecular clocks are tricky.

(Holden, Constance; "Early American Gene Clock Gains Time," Science, 263: 753, 1994. Also: Anonymous; "DNA Dates for First Americans," Science News, 145:126, 1994.)

From Science Frontiers #93, MAY-JUN 1994. 1994-2000 William R. Corliss