A new analysis of DNA evidence offers a new twist on the popular "Out of Africa" theory of human origins: Modern humans interbred with other populations around the world for hundreds of thousands of years rather than replacing them.

Among the study's implications: the genes of people today carry vestiges of genes of Neanderthals and other extinct branches of the human family. According to the original Out of Africa theory, ancestors to today's human population migrated from Africa 100,000 years ago, and they replaced less modern populations in Europe and Asia.

The new study by population biologist Alan R. Templeton of Washington University in St. Louis suggests instead that there were at least two distinct migrations. The first wave occurred between 420,000 and 840,000 years ago, he said, and the second between 80,000 and 150,000 years ago.According to Templeton, the most recent migration, and perhaps both, were not "replacement events." Rather, he said DNA evidence shows evidence of interbreeding.

Templeton's study appears in this week's Nature journal. While the study offers no original data, it employs a unique statistical method that Templeton argues more reliably traces genetic trails over time. Templeton was lecturing in Israel and unavailable for comment. Other scientists said Templeton's conclusions could reconcile the Out of Africa theory with fossil evidence that suggests there was more than one migration wave. But they were less sure that Templeton's analytical model is entirely accurate.

In a separate review in Nature, Rebecca L. Cann, a molecular biologist at the University of Hawaii, suggested Templeton was "overambitious in the scale of his analysis" and perhaps too eager to contribute to the contentious evolutionary debate.

For his study, Templeton developed a computer program called GEODIS to analyze genetic material taken from thousands of present-day humans around the world.

Templeton's model analyzes DNA from 10 locations in each genetic sample. Previous statistical models that have targeted just one region of DNA _for example mitochondrial DNA that is passed from the mother to subsequent generations.

This narrow approach has resulted in incomplete results "with low statistical resolution," he argued.

In her rebuttal, Cann said the new model needs to be independently verified, and its conclusions compared to existing evidence on human origins from archaeology, linguistics and other scientific disciplines.

"Perhaps we will need a demonstration that GEODIS reveals the composite picture before we can settle on how to interpret the varied signals uncovered by Templeton's analysis on a global scale," she said.

Credit Source: Michael Newton

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