A US study says sexual encounters with archaic humans like the Neanderthals produced children who inherited key genes that have helped modern humans fight illness and disease.
"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.
Scientists already knew that about 4 per cent of Neanderthal DNA and up to 6 per cent of Denisovan DNA are present in some modern humans.
The study took a close look at a group called HLA class I genes, which help the immune system adapt to fight off new pathogens that could cause various infections, viruses and diseases.
Now I know why I am still around.