Sexual encounters with archaic humans like the Neanderthals produced children who inherited key genes that have helped modern humans fight illness and disease, said a published U.S. study.
"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 four percent of Neanderthal DNA and up to six percent of Denisovan DNA are present in some modern humans.
This 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.
Researchers traced the origin of one type, HLA-B*73, to the Denisovans, who likely mated with humans arriving in West Asia on their way out of Africa. The variant is rare in modern African populations but is common in people in west Asia.
"We think this had a lot to do with the pathogenic environment in different parts of the world," said Laurent Abi-Rached, a French researcher and lead author of the study.
"When modern humans came out of Africa, they were going into a new environment. This gave them an advantage. It was a rapid way of acquiring defense," he told AFP.
These ancient HLA genes have multiplied among modern populations and are seen in more than half of Eurasians today, said the study.
"If canoodling was the whole story, that's an awful lot of genes," said Milford Wolpoff, a paleoanthropologist at the University of Michigan who was not involved with the study but said he supported its findings.
"This is called multiregional evolution. We have been talking about this for 30 years," he told AFP.
"Many of the genes we find are doing something useful. The only answer for that is natural selection."
Neanderthals died off about 30,000 years ago. They and the Denisovans shared a common ancestor with modern humans about 400,000 years ago.
Modern humans eclipsed these ancient cousins when our contemporaries began expanding into Asia and Europe from Africa about 65,000 years ago.
Some mating must have occurred, given the evidence that lingers in our DNA, but even the latest findings have shed little light on the nature of those relationships -- whether violent or consensual, short or long-term.
"Even though there was probably interbreeding, that was not necessarily very frequent," said Abi-Rached.
"But it has played a major role in shaping modern human immunity."
Abi-Rached said he hopes further research will reveal more about the role that the immune genes may have played in protecting those who survived but also their role in auto-immune diseases that humans face today.
The work of studying the legacies left by ancient ancestors in our bodies could lead to new pathways for treatment of modern diseases, which has researchers excited about the potential of the emerging field.
"Most of the money in genetics is related to diseases," said Wolpoff.
"Paleoanthropology is just like politics. You follow the money."