Coincidentally — or not — the genetic evidence suggests that modern humans interbred with Neanderthals during this challenging time for the latter 50,000–60,000 years ago. The fact that their mating resulted in children is itself a critical piece of evidence about Neanderthal identity.
Around 150 years ago, Neanderthals and Homo sapiens were designated as two distinct species. This was largely based on differences in skeletal anatomy and material cultures.
"However," Pääbo said, "genetics has now allowed us to show that Neanderthals and Denisovans interbred with early modern humans — and with each other — and had at least some fertile offspring. Therefore, by some definitions, modern humans, Neanderthals, and Denisovans are not distinct species."
He continued, "What is clear is that modern humans, Neanderthals, and Denisovans are three genetically distinct groups of humans."
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One of the most surprising findings from the new study is that the genome for one of the two early-modern humans that was analyzed "had large amounts of Neanderthal ancestry, suggesting quite recent Neanderthal admixture happening in Europe," Kelso said. "But we do not detect any indication of a recent gene flow from modern humans in the five late Neanderthals analyzed in this study."
It is possible that Neanderthal DNA affected anatomically modern humans more than the other way around. Future research could help to solve that puzzle.
Hajdinjak and her team plan to continue to identify, screen, and sequence the genomes of archaic humans, with substantial effort going into obtaining DNA from older specimens across wide geographic areas. Additionally, they hope to identify more Denisovans outside of Denisova Cave, the place where remains of these early humans were discovered in the Altai mountains of Siberia, Russia.
"Later mixture between different groups in different parts of the world certainly contributes to the genetic diversity of modern humans today,” Hajdinjak said, “and future studies may be able to give us a clearer picture of how early population mixtures contributed to the evolution of modern humans."