Mating with Neanderthals and another group of extinct hominids, Denisovans, strengthened the human immune system and left behind evidence in the DNA of people today, according to new research.
The findings add to the growing body of evidence that modern humans who left Africa around 65,000 years ago mated with Neanderthals and Denisovans -- two archaic species that lived in Europe and Asia.
The study, which appears in this week's Science, is among the first to show how the interbreeding shaped modern human genes and the attributes they pass to us.
Peter Parham, a professor of cell biology, microbiology and immunology at the Stanford University School of Medicine, and his team focused their analysis on "HLA" genes, which are fast-evolving vital components of the human immune system.
"The modern human populations who left Africa to colonize other continents were likely to have been small groups who started off with limited HLA diversity and suffered further reduction of HLA diversity due to disease," Parham told Discovery News. "Interbreeding with archaic humans introduced additional HLA variants into the modern human population that increased their genetic viability and capacity to resist infection."
He and his colleagues studied the genomes for Neanderthals and Denisovans, as well as the DNA of modern human populations. The organization Bone Marrow Donors Worldwide, as well as bone marrow registries from several countries, provided data on HLA genes.
The analysis shows that Neanderthal and Denisovan HLA genes now represent more than half of such immune system-related DNA in modern European and Asian populations. They also appear to have been later introduced into Africans.
The specific gene HLA-A, for example, is present in the Neanderthal and Denisovan genomes. It contributed this much to the following modern human populations: Up to 95.3 percent for Papua New Guineans, 80.7 percent for Japanese people, 72.2 percent for Chinese people, 51.7 percent for Europeans, and 6.7 percent for Africans.
Such percentages provide clues on how modern humans migrated and interbred. The scientists believe some modern humans migrated out of Africa 67,500 years ago. Interbreeding became evident 50,000 years ago.
"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," Parham said.
Some of the Europeans and Asians then went back to Africa around 10,000 years ago, bringing the newly acquired genes and their associated immunity boost with them.
Human history was "a lot more complex and interesting" than previously thought, Svante Paabo, director of the Department of Genetics at the Max Planck Institute for Evolutionary Anthropology, told Discovery News.
In separate research, Paabo and his team found that about 4 percent of the genomes of non-Africans are derived from Neanderthals and 4 to 6 percent of modern Melanesian genomes are derived from Denisovans.
This earlier research and the new study then suggest at least two possible scenarios: Either interbreeding was frequent and widespread, involving a lot of individuals, or the majority of native modern populations from certain regions are descended from individuals that did interbreed, even if such "seed" groups were relatively small. Parham suspects the latter is what happened.
While Europe and Asia might now be viewed as a hotbed of interbreeding, modern humans who stayed in Africa appear to have been active interbreeders as well. Neanderthals and Denisovans weren't present, but other archaic human groups likely were.
"Well established is that modern Africans have greater genetic diversity, overall, than the modern populations of other continents," Parham said. "This greater diversity is likely due to what was inherited from earlier forms of Homo, combined with interbreeding between different forms of Homo."
The early ancestors of all modern people, then, did not seem to shy away from breeding with different human species, actions that strengthened our immune systems and likely resulted in other benefits yet to be revealed.