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A new study headed by geneticists from Harvard Medical school examined Neanderthals' DNA and how it affects modern human genetics. The researchers reported that the genes humans inherited from Neanderthals could be both adaptive and maladaptive.

"Now that we can estimate the probability that a particular genetic variant arose from Neanderthals, we can begin to understand how that inherited DNA affects us," said David Reich, professor of genetics at HMS and senior author of the paper. "We may also learn more about what Neanderthals themselves were like."

In this study, the research team examined genetic variants from multiple samples, which included 846 non-African people, 176 sub-Saharan African people and one 50,000-year-old Neanderthal. The researchers discovered that some parts of the modern non-African human genome had rich traces of Neanderthal DNA.

The researchers found a link between Neanderthal DNA present in human genomes and keratin production, which is tied to skin, hair and nail toughness. The team found more Neanderthal ancestry in genes that affect keratin filaments. The researchers also found that genetic variants tied to traits inherited from Neanderthals affect immune function today. They also contributed to certain behaviors, such as the ability to quit smoking.

"I expect that this study will result in a better and more systematic understanding of how Neanderthal ancestry affects variation in human traits today," said first author, Sriram Sankararaman.

Based from their analyses, the researchers also found that the areas with little evidence of Neanderthal ancestry were clustered together in two regions of the modern-day human's genome. These clusters were on the genes tied to male germline, which are the testes and the genes on the X chromosome. These two types of patterns have been tied to hybrid infertility.

"This suggests that when ancient humans met and mixed with Neanderthals, the two species were at the edge of biological incompatibility," said Reich, who is also a senior associate member of the Broad Institute and an investigator at the Howard Hughes Medical Institute. "It is fascinating that these types of problems could arise over that short a time scale."

The research was published in Nature.