Sexual encounters with archaic humans like the Neanderthals produced children who inherited key genes that have helped modern humans fight illness and disease, a US study has said.
“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 per cent of Neanderthal DNA and up to six per cent 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-B73, 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 defence,” he remarked.
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 palaeo-anthropologist 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 said.
“Many of the genes we find are doing something useful. The only answer for that is natural selection.”