Cross-breeding by early humans with ancient populations such as Neanderthals was highly advantageous to the modern human immune system, according to a new study published in Science.
Common variants of the HLA (human leucoyte antigen) family, important for defending the body against pathogens such as viruses, appear to have been inherited from mating with our early relatives the Neanderthals and another ancient group, the Denisovans.
Professor Peter Parham, lead author of the study, said: 'Interbreeding with archaic humans introduced additional HLA variants into the modern human population that increased their genetic viability and capacity to resist infection'.
'Because archaic humans had lived in Asia and Europe for hundreds of thousands of years before the modern humans arrived, their HLA alleles almost certainly were adapted to the local infections and in this way further invigorated the immune systems of the recent modern migrants', he explained.
Scientists from Stanford University, USA compared DNA extracted from fossilised remains of Nethanderthals and Denisovans to information obtained from bone marrow donor registers. They were able to look at the diversity of the HLA genes in modern human populations and determine how much of it was due to cross-breeding.
The results show that early human HLA genes now represent more than half of the HLA variants in modern European and Asian populations.
Looking at one allele, HLA-A, the scientists estimated that Europeans owe roughly half their variants to interbreeding with Neanderthals and Denisovans; Asians 80 percent; people from Papua New Guinea up to 95 percent; and Africans just under seven percent.
However, John Hawks, an assistant professor of anthropology at the University of Wisconsin-Madison, according to the BBC commented: 'I'm cautious about the conclusions because the HLA system is so variable in living people'.
A previous study by the same group showed that modern humans share about four percent of their genetic code with Neanderthals.