IN recent years, millions of people have been astonished, even thrilled, to learn from popular genetic tests that their DNA is laced with Neanderthal genes.

Those genes were first discovered in 2010 in a study of Neanderthal fossils. From DNA recovered from the bones, researchers deduced that modern human interbred with Neanderthals some 60,000 years, ago after leaving Africa.

As a result, the genes of non-Africans today are 1 percent to 2 percent Neanderthal. People of African ancestry, it was thought, have little to no Neanderthal DNA.

Using a new method to analyze DNA, however, a team of scientists has found evidence that significantly reshapes that narrative.

Their study, publish recently, in the journal Cell, concludes that a wave of modern humans left Africa far earlier than had been known : some 200,000 years ago.

These people interbred with Neanderthals, the new study suggests. As a result, Neanderthals were already carrying genes from modern humans when the next big migration from Africa occurred, about 140,000 years later.

The scientists also found evidence that people living somewhere in western Eurasia moved back to Africa and interbred with people whose ancestors had never left.

The new study suggests that all Africans have a substantially greater amount of Neanderthal DNA than earlier estimates.
''The legacy of gene flow with Neanderthals likely exists in all modern humans, highlighting our shared history,'' the author concluded.

''Overall, I find this a fantastic study,'' said Omer Gokcumen, a geneticist at the University of Buffalo, who was not involved in the research. The research offers a view of human history ''almost as a spider web of interactions, rather than a tree with distinct branches.''

But while evidence has been building that modern humans left Africa in waves and that those migrations began much earlier that once thought, some scientists disputed that evidence that people of Africa descent may be carrying Neanderthal genes.

David Reich, a geneticist at Harvard Medical School, praised much of the study but said he had doubts how extensive the flow of DNA back to Africa could have been.

''It looks like this is a really weak signal,'' he said of the data.

The honor and serving of the latest global operational research on Genes, DNA and past interactions,of humankind continues. The World Students Society thanks author Carl Zimmer.


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