It was big news in genetics when researchers discovered that some modern humans inherited DNA from other hominid species: Denisovans and Neanderthals. People of European and Asian ancestry now have between one and four percent of their DNA from these ancient hominids, which was first introduced 30,000 to 60,000 years ago when their ancient ancestors got down and dirty with these other species.
There’s been a lot of attention paid to the traits that this interbreeding introduced to modern humans, a process called adaptive introgression. While some of those traits may have had some positive benefits in the past, many are now considered negatives—allergies, addiction and depression. But a new study took another look at the Neanderthal genetic legacy and found there were some positive benefits as well, even if we don't understand all of them yet.
Sara Kaplan at The Washington Post reports that researchers Fernando Racimo, Davide Marnetto and Emilia Huerta-Sanchez wanted to take a closer look at adaptive introgression to confirm previous findings and to see if there were any positive traits that were overlooked. “Nobody has yet done a systematic survey of adaptive introgression around the world,” Racimo, a geneticist at the New York Genome Center and lead author of the study appearing in the journal Molecular Biology and Evolution tells Kaplan.
Using statistical tools, the team examined the genomes of 1,000 modern humans around the world from various regions and ethnicities, looking for sequences of DNA that seem to come from the Neanderthal or Denisovan. Through this study, they confirmed some previous findings. For instance, people in Tibet have a genetic variation, likely from Denisovans, that allows them to breathe more efficiently at high altitudes. But the gene is missing in the Han Chinese population, which lives nearby but at lower elevations.
They also found genes related to the production of fat tissue that come from Denisovans and are found in modern Native Americans and Eurasians reports Kaplan. That finding will be detailed in a future study. Though they did find many new snippets of DNA, but figuring out exactly why some sequences of Neanderthal DNA were once beneficial to Homo sapiens is not easy, especially since many traits come from a combination of genes.
In general, Racimo thinks that grabbing genes from archaic humans may have been a stepping stone to helping modern humans spread more quickly around the world. “Archaic humans expanded out of Africa before modern humans, so they had a lot more time to adapt to the particular conditions of Europe and Asia,” he tells Kaplan. “A shortcut to adapt to these conditions, instead of waiting for the mutations to occur, is to obtain the genetic material from these archaic human groups who were established for a long time.”
But along with genetic material, archaic humans probably gave us a something else: the sexually transmitted disease HPV16, a human papillomavirus that causes cervical and mouth cancer. Another study in the same issue of Molecular Biology and Evolution tracks the development of this disease and suggests it likely came from Neanderthals. According to a press release, when humans and Neanderthals split into two species, they each developed their own strains of HPV. But when humans entered Europe and Asia and began breeding with archaic humans, the Neanderthal and Denisovan versions of the viruses entered the human population and soon spread.
“The history of humans is also the history of the viruses we carry and we inherit,” lead author Ignacio Bravo of the French National Center for Scientific Research says in the press release. “Our work suggests that some aggressive oncogenic viruses were transmitted by sexual contact from archaic to modern humans.”
On the flip side, recent research also shows that Homo sapiens likely passed some nasty bugs to Neanderthals too, like stomach ulcers, tapeworms and tuberculosis. It’s thought those diseases may have weakened the Neanderthal population and in part lead to their extinction.
Bravo and his colleagues believe a higher percentage of Neanderthal and Denisovan DNA in a person’s genome my influence their chances of developing cancer from HPV, an idea that they hope to soon put to the test.