Tens of thousands of years ago, a Neanderthal woman met and procreated with a Denisovan man in what is now the Altai mountains.

A fragment of a bone of their offspring, a female and the first-known hybrid child of the distinctly different ancient humans, was recently found in a cave in Siberia.

Proof Of Neanderthal, Denisovan Interbreeding

Scientists have long suspected that groups of Neanderthals and Denisovans, the closest extinct relatives of modern humans, intermingled and had children together. Small amounts of both Neanderthal and Denisovan DNA has been found in humans today.

However, ancient human fossils do not come around often. According to CBC, there are only half a dozen Denisovan fossils and a few hundred Neanderthal fossils found so far.

That is why the authors of the study published this week in Nature call the fragment of the bone of a half-Denisovan, half-Neanderthal woman such a lucky find.

"We knew from previous studies that Neandertals and Denisovans must have occasionally had children together," said Viviane Slon, one of the three authors of the study. "But I never thought we would be so lucky as to find an actual offspring of the two groups."

The bone fragment was found in 2012 inside a Denisova Cave by Russian archaeologists. It was brought to Leipzig, Germany for genetic analysis and, based on its protein composition, was identified as a hominin bone.

Bence Viola of the University of Toronto added that the bone belonged to a 13-year-old woman who died around 90,000 years ago.

Fabrizio Mafessoni, who co-authored the study, further explained that the fragment recently found revealed more about the Neanderthals. He said that the mother side of the hybrid child was genetically closer to the Neanderthals from western Europe than the Neanderthals that lived within the Denisovan territory. This finding suggests that Neanderthals group migrated to and from western and eastern Eurasia.

The bone fossil also revealed that the Denisovan father had at least one Neanderthal ancestor before he, himself, intermingled with a Neanderthal female.

"So from this single genome, we are able to detect multiple instances of interactions between Neandertals and Denisovans," said Benjamin Vernot, the third co-author of the study.

Lost Relatives

However, despite the interbreeding, Neanderthals and Denisovans did not merge into a single group. For thousands of years, they remained distinct for various possible reasons.

Dr. Svante Paabo, a renowned geneticist, said it is likely that Neanderthals and Denisovans did not have a lot of opportunities to meet because their groups were sprawled in a vast landscape. He also thinks that it is possible that hybrid offsprings had reproductive disorders and had fewer children than those without the mixed DNA.

Dr. Paabo added that the study could reveal more information about what happened to Neanderthals and Denisovans. He mused that both groups disappeared when they mixed with the modern humans after they had migrated out of Africa.