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## The Problem of Species Labels
The scientific literature and popular accounts of Denisova 11 have used various framings—hybrid, crossbreed, daughter of two species—and each of these framings carries interpretive weight that deserves attention.
The word “species” is doing conceptual work that the biological reality doesn’t always support. The classical definition of a species, the Biological Species Concept developed by Ernst Mayr, defines a species as a group of organisms that can interbreed and produce fertile offspring, and that are reproductively isolated from other such groups. By this definition, Neanderthals and Denisovans were the same species: they did interbreed, they did produce offspring, and at least some of those offspring were apparently fertile—because Denisovan genomes carry Neanderthal ancestry traceable to encounters several thousand years in the past, which would require those early hybrids to have reproduced and passed on their mixed ancestry.
The same argument applies to modern humans and Neanderthals: the 1 to 2 percent of Neanderthal DNA found in non-African modern humans is there because the children of Neanderthal-modern human encounters survived, reproduced, and contributed their genomes to the populations from which all non-African people eventually descended.
In practice, paleoanthropologists use species labels as a shorthand for recognizable genetic and morphological clusters rather than as strict claims about reproductive isolation. “Neanderthal” and “Denisovan” designate populations that were genuinely distinct in their genomes and, to whatever extent we can determine, in their physical forms—populations that diverged hundreds of thousands of years ago and spent most of that time apart from each other. The fact that they occasionally interbred when they encountered each other does not erase the distinctiveness of their long separation.