Crossbreeding with Neanderthals and the Denisovans gave early man a much needed immune boost as he moved into new environments. It is unclear is the sex was consensual or violent.
Sexual encounters with archaic humans like the Neanderthals produced children who inherited key genes that have helped modern humans fight illness and disease, a study has found.
"The cross-breeding wasn't just a random event that happened, it gave something useful to the gene pool of the modern human," said Stanford University's Peter Parham, senior author of the study in the journal Science.
Equipped with knowledge of the genome of the Neanderthals and the Denisovans, of whom a tooth and a finger bone were discovered in a Russian cave last year, researchers scoured the data for hints of what genes crossed over.