If you¿ve been following the debate over modern human origins lately, you might feel as though you're watching an academic boxing match. One week the Out of Africa model¿the leading view, which proposes a recent African origin for our species¿delivers a punch; the next week the Multiregionalism theory¿which holds that humans arose in Africa around two million years ago, spread throughout the Old World and evolved as a single species¿returns the blow. Earlier this year, Multiregionalism gained some ground, but a study published today in the journal Science seems to offer strong new support to the Out of Africa scenario.
When it comes to genetic evidence in particular, Out of Africa has claimed the lion's share of support. But many of the studies have focused on so-called mitochondrial DNA (mtDNA), prompting critics to counter that the results reflect the history of the genes contained in that small piece of DNA, not the history of the population from which the DNA was taken. The new findings, however, come from an analysis of characteristic DNA sequences, known as markers, on the Y chromosome.
Geneticist Li Jin of the University of Texas and his colleagues examined Y chromosomal DNA from more than 12,000 males belonging to 163 populations in Asia and Oceania. Multiregionalism posits that archaic humans such as Neandertals contributed to the modern human gene pool. Thus modern Southeast Asian populations, for example, should retain some ancient lineages inherited from the archaic populations that once inhabited that region¿an argument that the multiregionalists have bolstered with fossil evidence. But Jin and his colleagues only found markers associated with a gene mutation known to have arisen in an African ancestor sometime between 35,000 and 89,000 years ago. This, the authors write, "indicates that modern humans of African origin completely replaced earlier populations in East Asia."
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With support coming from both mitochondrial and Y-chromosomal DNA, the Out of Africa case might seem a lot stronger. But other genes may one day tell different tales. "You can nail it down from the perspective of the Y and mtDNA," University of Arizona geneticist Michael Hammer told Science, "but that still leaves us at the doorstop of the nuclear genome."
