The consensus among scientists has always been that lower levels of vitamin D at higher latitudes — where the sun is less intense — caused the lightening effect when modern humans, who began darker-skinned, first migrated north.

But other factors might be at work, a new study suggests. From the varying effects of frostbite to the sexual preferences of early men, a host of theories have been reviewed…

“One of the hypotheses is that men seem to prefer women with a light skin color, which can be regarded as a sign of youth and fertility,” Juzeniene told LiveScience. “Because light skin characterizes the early infant stage of primates, it may have become a visual cue that triggers appropriate adult behavior toward infants, i.e. decreased aggressiveness and increased desire to provide care and protection,” she said…

Frostbite was another causal effect investigated by the researchers.