Neanderthals and humans interbred 47,000 years ago for nearly 7,000 years, research suggests

Neanderthal genes seen in modern humans may have entered our DNA through an interval of interbreeding starting about 47,000 years ago that lasted nearly 7,000 years, new research finds.

Neanderthals were among the closest extinct relatives of modern humans (Homo sapiens), with the ancestors of both lineages diverging about 500,000 years ago. More than a decade ago, scientists revealed that Neanderthals interbred with the ancestors of modern humans who migrated out of Africa. Today, the genomes of modern human populations outside Africa contain about 1% to 2% of Neanderthal DNA.

Researchers are still unsure about when and where Neanderthal DNA made its way into the modern human genome. For instance, did Neanderthals and modern humans intermingle at one specific place and time outside Africa, or did they interbreed at many places and times?

To solve this mystery, researchers analyzed more than 300 modern human genomes spanning the past 45,000 years. These included samples from 59 individuals who lived between 2,200 and 45,000 years ago and 275 diverse present-day modern humans. The scientists posted their findings on the BioRxiv preprint database. (As the study is currently under review for potential publication in a scientific journal, the study's authors declined to comment.)

The scientists focused on how much Neanderthal DNA they could see in these modern human samples. By comparing how the level of Neanderthal ancestry varied in modern human DNA across different locations and times, they could estimate when Neanderthals and modern humans interbred, and for how long.

Related: 'More Neanderthal than human': How your health may depend on DNA from our long-lost ancestors

The best explanation for most Neanderthal DNA seen in the modern human genome was a single major period of interbreeding about 47,000 years ago that lasted about 6,800 years, the researchers found.