Neanderthals and anatomically modern humans co-existed and interbed with each other, possibly for thousands of years, and this genetic legacy can be observed today – around 2% of the DNA of non-African people in some parts of the world comes from Neanderthals.

In recent times, research has shown that some of those Neanderthal genes have contributed to traits in modern humans, such as our immunity to a certain diseases.

Now, a new study published in the American Journal of Human Genetics has found that genes from our evolutionary relative contribute to a much wider range of characteristics, including skin tone, hair colour, sleep patterns, mood and even our capacity to be a smoker or not.

Neanderthal genes are relatively rare, so the researchers examined a large dataset from the UK Biobank project, which archives genetic information from 112,000 people.

Previous studies have suggested that Neanderthal DNA may have had an influence on human genes involving skin and hair biology. However, it hasn't been clear what exactly was affected, according to Janet Kelso, an author of the study.

"We can now show that it is skin tone, and the ease with which one tans, as well as hair colour that are affected," said Kelso, a researcher from the Max Planck Institute for Evolutionary Anthropology.

The researchers found multiple different Neanderthal alleles – different versions of the same gene – that contributed to skin and hair tone in modern humans. But they also found that these alleles were associated with both lighter and darker skin tones, with the same being true for hair colour.

"These findings suggest that Neanderthals might have differed in their hair and skin tones, much as people now do," added Michael Dannemann, first author of the study.

According to Kelso, the traits influenced by Neanderthal DNA, including skin and hair pigmentation, mood, and sleeping patterns, are all linked to sunlight exposure. Neanderthals had been living in Eurasia for thousands of years before humans arrived around 100,000 years ago, and so were better-adapted to lower and more variable levels of ultraviolet radiation from the sun.

"Skin and hair colour, circadian rhythms and mood are all influenced by light exposure," the researchers said. "We speculate that their identification in our analysis suggests that sun exposure may have shaped Neandertal phenotypes and that gene flow into modern humans continues to contribute to variation in these traits today."