THERE'S NO SUCH THING AS SPECIES:

Multiple groups of humans once co-existed with Homo sapiens, including Neanderthals and the mysterious Denisovans. And we did more than simply live alongside them -- traces in our DNA reveal that our ancestors also interbred with other human species.

Now, for the first time, researchers have found direct evidence of this interbreeding in the form of a 13-year-old girl from Russia's Altai Mountains. Her mother was a Neanderthal and her father was a Denisovan, making her a first-generation hybrid of human species.

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