The Ancient Girl Whose DNA Revealed Her Parents Were Two Different Human Species

The classic example is the horse and the donkey, which can interbreed to produce mules, but mules are sterile — they cannot themselves reproduce.

This sterility is taken as evidence that horses and donkeys are distinct species with a reproductive barrier that natural selection has built between them.

By this definition — the biological species concept — Neanderthals and Denisovans should perhaps be considered the same species if they could produce fertile offspring together.

Denny herself tells us only that she was born and survived to a significant age; it does not tell us whether she herself was fertile or could reproduce.

But the broader genetic evidence — the presence of Neanderthal DNA in modern human genomes, the presence of Denisovan DNA in modern human genomes — implies that the children of such encounters were fertile and did reproduce, passing the genetic contributions of their ancient-human parents into the modern human gene pool.