When Homo sapiens crossed paths with their Neanderthal cousins tens of thousands of years ago in Europe, they also encountered dangerous new pathogens—and, though interbreeding, the genes to fight those infections, a new study suggests. As the researchers report today (October 4) in Cell, genes for virus-recognizing proteins are relatively common among the tiny percentage of modern humans’ DNA that originated in Neanderthals.
The paper’s authors, Dmitri Petrov of Stanford University and his former postdoc, David Enard of the University of Arizona, note that the H. sapiens who left Africa for Europe tens of thousands of years ago would likely have encountered pathogens that Neanderthals had long been exposed to. They reasoned that descendants of H. sapien–Neanderthal interbreeding events who carried Neanderthal genes for pathogen-fighting proteins would have been more likely to survive and pass the genes along.
“It made much more sense for modern humans to just borrow the already adapted genetic defenses from Neanderthals rather than waiting for their own adaptive mutations to develop, which would have taken much more time,” says Enard in a statement released by Stanford.
To see whether this was the case, Enard and Petrov compiled a list of 4,534 genes in modern European and East Asian human genomes that code for proteins that interact with viruses, and compared it to Neanderthal DNA. They came up with 152 sequences for virus-interacting proteins that had come from Neanderthals. Both the number of these virus-fighting DNA sequences that have persisted, relative to other types of genes, and their relatively long length suggest that they’ve conferred an evolutionary advantage, the authors write.
“Pathogens have been a big driver in human adaptations,” Emilia Huerta-Sanchez, a population geneticist at Brown University who was not involved in the study, tells The Atlantic.
The study also found that proteins coded for by the Neanderthal genes interact with HIV, Influenza A, and Hepatitis C, all viruses that keep their genetic material as RNA rather than DNA. Enard notes in the statement that the RNA in the ancient viruses that drove the selection for the Neanderthal genes was far too fragile to survive to the present day, but “because we know which genes interact with which viruses, we can infer the types of viruses responsible for ancient disease outbreaks.”