This puts Neanderthals, Denisovans, and modern humans in an unusual biological category: distinct enough to be classified as different species by traditional morphological and phylogenetic criteria, but apparently capable of interbreeding and producing fertile offspring.

The biological species concept, which was developed primarily from observations of contemporary organisms, may not apply cleanly to the kinds of gradual population divergence that characterize ancient human evolutionary history.

The second implication concerns the nature of the ancient human world.

Before the ancient DNA revolution of the past two decades, paleoanthropologists thought of ancient human species as relatively discrete, geographically bounded groups: Neanderthals in Europe and western Asia, modern humans spreading from Africa, Homo erectus in eastern Asia.

The interactions between these groups were imagined as limited — a few encounters at the margins of their ranges, perhaps, with the archaic forms eventually outcompeted or replaced by modern humans.