Verified by 
Neanderthals and early humans interbred, which is why people of non-African descent harbor about 2 percent Neanderthal DNA. Although a large proportion of this Neanderthal DNA has deleterious or nonadaptive effects, some of these traits contribute to modern human adaptation to new environments, according to the results of a study published in the American Journal of Human Genetics. These Neanderthal-derived traits reflect skin tone, hair color, mood, and more.
In the study, Michael Dannemann and Janet Kelso mined baseline phenotypes for 112,000 individuals from the UK Biobank. The Biobank stores genetic data along with data on physical appearance, diet, sun exposure, behavior, and disease.
With respect to skin and hair colors, the researchers discovered that Neanderthal alleles related to both dark and light tones present in modern-day humans. The authors indicated that the ease with which one tans may be related to Neanderthal alleles.
“We found that skin and hair traits are over-represented among the most significant associations with archaic alleles,” wrote the authors. “However, when we compared the contribution of alleles of Neanderthal origin with the contributions of alleles of modern human origin, we found that both archaic and non-archaic variants contribute equally to skin and hair phenotypes, consistent with a neutral contribution from Neanderthals and with the idea that Neanderthals themselves were likely to be variable with respect to these traits. In fact, for most associations, Neanderthal variants do not seem to contribute more than non-archaic variants.”
In other words, although Neanderthal alleles contribute heavily to skin and hair tones, modern humans likely contributed equally, In fact, only 4 identified traits—all behavioral—were more heavily derived from Neanderthals. These traits include chronotype, loneliness/isolation, frequency of unenthusiasm or disinterest in the last 2 weeks, and smoking status. Of note, chronotype is a behavioral characteristic related to circadian rhythms.
What do all these Neanderthal genetic contributions have in common? According to the authors, the association may have to do with sunlight exposure. Neanderthals inhabited Eurasia for more than 200,000 years and were better adapted to lower UVB levels and variation in sunlight exposure than were modern humans who migrated from Africa around 100,000 years ago. Skin and hair colors, circadian rhythms, and mood are all affected by sunlight exposure, per the authors.
Ultimately, sun exposure could have played a role in Neanderthal phenotypes, with these genes flowing into the modern genome, thus yielding the respective variation in traits we see today.
