Studies have found that up to 2.1 percent of individuals who live outside of Africa have Neanderthal DNA indicating that Homo sapiens and the now extinct species of humans, the Neanderthals, have interbred.

Now, a 45,000 year old thigh bone found by an ivory carver in the banks of a river in Siberia in 2008 has offered experts with genetic evidence that hint on when the interbreeding between the two species happened. The bone belonged to an Ust'-Ishim, a member of an old population of modern humans whose genetic makeup was found to be more similar to those of present-day East Asians.

It was earlier estimated that the interbreeding occurred sometime between 37,000 and 86,000 years ago but the Ust'-Ishim man's thigh bone has helped an international genetics team narrow down the time frame from 50,000 to 60,000 years ago.

For the study published in the journal Nature on Oct. 22, Janet Kelso, a genetics expert from the Max Planck Institute for Evolutionary Anthropology in Germany, and colleagues collected DNA from the femur of the ancient man and conducted an analysis of his genetic map.

The genome sequence revealed that the bone had about 2.3 percent Neanderthal genes. In comparison, modern humans only have 2.1 percent or lower of these genes. It also showed that the Ust'-Ishim man was descended only 230 to 400 generation from the interbreeding of humans and Neanderthals 7,000 to 13,000 years before helping experts point out the date when modern humans mated with their closest relative between 50,000 and 60,000 thousand years ago.

"This individual derives from a population that lived before-or simultaneously with-the separation of the populations in western and eastern Eurasia and carries a similar amount of Neanderthal ancestry as present-day Eurasians," the researchers wrote. "However, genomic segments of Neanderthal ancestry are substantially longer than those observed in present-day individuals, indicating that Neanderthal gene flow into the ancestors of this individual occurred 7,000-13,000 years before he lived."

The DNA sequence revealed that the productive mating between the two human species occurred during the so-called Initial Upper Palaeolithic period, which is marked by the explosion of modern human culture with the emergence of musical instruments, artwork, jewelry and more advanced stone and bone tools across Eurasia.

"What we think may be the case is that the ancestors of the Ust'-Ishim man met and interbred with Neanderthals during the initial early admixture event that is shared by all non-Africans at between 50,000 and 60,000 years ago, and perhaps somewhere in the middle East," Kelso said.

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