Modern humans developed stronger immune systems after they interbred with Neanderthals.
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
Humans Vs. Neanderthals: How Did We Win?
Aug. 9, 2011 -- Up until about 30,000 years ago, humans shared the planet with Neanderthals, a relative so close to humans that our species interbred. In fact, some Neanderthal lives on in some of our DNA to this day.
But around then, Homo sapiens were already well into the process of displacing Neanderthals, an undertaking that had been some 20,000 to 40,000 years in the making.
How humans outpaced their relatives remains a mystery, but fossil evidence has left some clues about the scenarios that may have led to the downfall of Neanderthals.
No single smoking gun is likely responsible for the disappearance of Homo neanderthalensis. Here, we explore some of the factors that likely contributed to their decline.
In the end, Neanderthals may have been wiped out because they simply lost the numbers game.
As Homo sapiens moved from Africa into areas of southern Europe where Neanderthals had already been settled, the two species were placed in direct competition with one another.
Eventually outnumbered 10 to one, Neanderthals were pushed to less favorable areas where food and shelter were more difficult to find, according to a study published last month in the journal Science. Resource competition and interbreeding wiped out the Neanderthals in this scenario.
Forced into Cannibalism?
With Homo sapiens pushing Neanderthals to fringe settlements, it’s possible that resource competition between Neanderthal groups forced them to turn to cannibalism.
Fossil evidence suggests that may have been the case. Bones discovered in a cave in France show a group of Neanderthals defleshed the bones of others within their species for sustenance. They even ate humans.
As grisly as the practice was, cannibalism also took a hidden toll on those who hunted and consumed their own species: a fatal epidemic similar to mad cow disease that caused severe mental impairments and wiped out thousands. These series of events could have contributed to the disappearance of Homo neanderthalensis.
The Fitter Specimen
In a battle of the brawn, Neanderthals would surely come out ahead. But in a footrace over a long distance, humans had the advantage.
Humans were built for long-distance running, which allowed for hunting in hotter climates. Neanderthals, on the other hand, were strong and sturdy. They could run faster than humans, but only over a short sprint. As such, Homo neanderthalensis was better equipped for cooler climates.
Distance-running and endurance could have given prehistoric Homo sapiens an edge when they entered Neanderthal strongholds in Asia and Europe, and came into direct competition with their cousins.
The Big Bang Theory
Neanderthals may not have quietly faded away so much as they went out with a bang, according to a study published last September in Current Anthropology.
Around 40,000 years ago, a sequence of three major volcanic eruptions devastated Neanderthal homelands in Europe and Asia, speeding the demise of this species.
Homo sapiens, by contrast, lived on the fringes beyond the range of the volcanic ash clouds. In other words, simple geographic luck could have led early humans to overtake Neanderthals.
Neaderthals had brawn, but early humans had a leg up on brains.
Starting at birth, human and Neanderthal brains are similar. During the first year of life, however, the human brain begins more activity in neural circuitry.
Although this doesn't mean that Neanderthals weren't as intelligent as humans, the brains of Homo sapiens developed to support higher-order functions, such as creativity and communication.
Traces of Neanderthal creativity have been found, but no evidence has yet emerged to show they had a complex language of their own.
However, according to one study published in the journal Medical Hypotheses, this lack of cognitive complexity also may have meant that Neanderthals didn't suffer from the same mental disorders as humans.
This distinction, however, proved to be a net gain for humans and may have "helped early Homo sapiens survive in the process of natural selection," according to one report.
Humans Weren't to Blame
Neanderthals and humans were not in direct competition for too long, because Neanderthals disappeared earlier than once thought, according to one study published in May of this year in the Proceedings of the National Academy of Sciences,
In this scenario, Neanderthals disappeared around 39,700 years ago -- 10,000 years earlier than is commonly believed.
Since Homo sapiens arrived in the northern Caucasus region a few hundred years earlier, that didn't leave too much time for the two species to interact.
This theory discounts any human intervention in the decline of Neanderthal populations, but still leaves open the possibility of other extinction scenarios.