Human geneticists have joined carbon daters, anthropologists,archeologists and geologists to help untie one of the knottiestmysteries of human evolution: How long ago did Homo sapienscross the Siberia-Alaska land bridge to begin populating North,Central and South America?
Scientists from the various disciplines, who have explored thisques-tion for most of the 20th century, cluster in two conflictingcamps and glare at each other over a 17,000-year gap. The"late-entry" believers claim that the Amerindian precursorsarrived in the New World around 13,000 years ago. Upholdersof the "early-entry" faith date their advent at circa 30,000years ago.
Results of the latest foray by molecular geneticists into thisfray ap-pear in the current issue of Proceedings of the NationalAcademy of Science (PNAS), published Tuesday. Titled"Mitochondrial DNA 'clock' for the Amerinds and itsimplications for timing their entry into North America," thereport tentatively times that entry at 22,414 to 29,545 years,give or take several millennia.
Medical geneticist James Neel of the University of MichiganMedical School collected most of the blood samples fromCentral American Indian tribes. The paper's first author,molecular geneticist Antonio Torroni of Emery University inAtlanta, performed the mitochondrial DNA analyses.
Neel told BioWorld that "in the U.S., there's a substantial groupof an-thropologists who believe that you cannot date thepresence of man in the New World by carbon-14 dating ofcampsites or tools until about 15,000 years ago." He recalls thediscovery in 1926 of the famous Folsom arrow-head embeddedbetween the ribs of an extinct bison in the northeast corner ofNew Mexico. "That was pretty good archeological evidence,"Neel ob-served, " that Indians were there when that bison died13,000 years ago. Dating is not so strong for earlier sites."
But he adds: "Another school says, 'Wait a minute! There aresome sites in South America where the dates look more like30,000 years ago.' "
Now geology enters the picture. During the intermittent iceages, glaciers locked up so much sea water that SIberia andAlaska PP today sepa-rated by the Bering Strait's 53 miles ofocean PP were joined by a dry-land bridge hundreds of mileswide. It lasted from 14,000 to 31,000 or 32,000 years ago, Neelsaid, time enough to accommodate both the early- and late-entry schools of thought.
As for the paper's genetically derived dates, he concluded, "asfate would have it, they ended up more or less in between. Thestatistical error is so large (though indeterminate) that we can'trule either out. But we're closer to the early than the latearrival."
Neel and his colleagues tested 18 Amerind tribes, the Chibchaby name, who are thought to have descended from the "firstwave" of SIberian ice-age immigrants. The Chibcha represent aformer federation of early native American tribes that heldsway in Central America and Colombia, roughly midwaybetween the Aztecs and Mayas to the north and the Incas tothe south.
To measure the rate at which their DNA mutated over thecenturies PP the "minute-hand" on the geneticists' human-evolution clock PP the re-searchers probed the Indians'mitochondrial DNA. Mitochondria, the or-ganelles in every cellthat generate its energy supply, have their own DNA, RNA andribosomes, very much like the free-living bacteria from whichthey are supposed to have descended.
Mitochondrial DNA (mtDNA), lacking a DNA repair mechanism,mutate much more rapidly than genomic nucleic acid, so theyserve as the clock's main spring.
The evolutionary geneticists estimate that mtDNA has anucleotide substitution rate in Amerinds of 0.022 to 0.029percent per 10,000 years.
To detect allelic variants in blood taken over eight years ofstudy from 325 individuals who speak the Chibchan language,the paper's authors probed their mtDNA for restrictionfragment length polymorphisms (RFLPs) by 14 sequence-cleaving enzymes. These endonucleases permitted screening forvariation of 15 to 20 percent of the mtDNA sequence perindividual PP about 2900 nucleotides.
When amplified by PCR and electrophoresed, they revealedthat the mtDNA of modern Amerinds cluster in four groups ofrelated genetic types, called haplotypes A, B, C and D. Thesereflect the irreducible patterns into which their evolving genevariants tend to fall. These haplotype groups must havedescended from the gene patterns prevailing at the time theirmost remote ancestors crossed the land bridge to becomefounders of the Amerind populations in the Americas. Three ofthe four haplogroups are also present in the veins andmitochondria of modern Siberian tribes.
The geneticists then subjected this mutational data to a versionof statistical analysis, cladistics, which calibrated divergence ofthe ever-changing mitochondrial sequences in these tribesthrough time and space. Their data gave arrival times for eachof the four haplotypes, based on the estimate of percentvariations per 10,000 years. When averaged, they yielded thebottom-line of 22,414 to 29,545 years in the PNAS paper.
A more basic bottom line ends a commentary in PNAS whichreviewed the tortured state of dating the ice-age Alaskaninflux. "These findings," wrote anthropologist/geneticistKenneth Weiss of Penn State, "show the potential of geneticmethods to address prehistorical questions. Better ge-netic datawill improve our resolving power, but genetics alone will notsuffice... (It) will require the combined application of all theanthropological sciences."
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.