By 2018, eight years after the Denisovan genome was first published, the scientific community had developed a fairly clear (though still incomplete) picture of the two groups: the Neanderthals, well-known from fossil evidence but now also characterized at the genomic level; and the Denisovans, known almost entirely from DNA but understood as a separate and distinct lineage.

Both groups had existed for hundreds of thousands of years.

Both had expanded across vast territories.

And the genetic evidence, applied to both ancient individuals and modern human populations, had established that both groups had interbred with modern humans at various points during the period when modern humans were spreading out of Africa and across the world.

Modern humans of non-African ancestry carry, on average, between 1 and 3 percent Neanderthal DNA — the genetic legacy of encounters between anatomically modern humans and Neanderthals that occurred primarily in the Middle East and Europe somewhere between 40,000 and 60,000 years ago.

The Neanderthal DNA that modern humans carry includes variants associated with immune function, skin and hair characteristics, and susceptibility to certain diseases — evidence that the Neanderthal genetic contribution was not merely incidental but included variants that may have been useful to modern humans in the environments they encountered.