It turned out that this tiny tweak of just one letter in the genetic code didn't change the structure or function of the protein. Instead, it acted like a tiny thermostat, subtly ramping up or down the production of pigment in the hair follicle and nowhere else, Kingsley told Live Science.
The study convincingly ties the gene to hair color, Hopi Hoekstra, a geneticist at Harvard University who was not involved in the study, told Live Science in an email.
"The study is rigorous, elegant and airtight," she said.
From an evolutionary perspective, the range in human hair color is a puzzle, Hoekstra said.
Selection for different hair color could be a byproduct of other, more consequential genetic changes, she said. For instance, light-skin genes may have helped ancient humans survive in the low-light conditions of Northern Europe by enabling their skin to make more vitamin D, and light hair may have been an accidental consequence.
Sexual selection could also have allowed blond hair to spread.
"Lots of children have light hair — it's a color that's associated with youth," which may make blond hair more alluring, Kingsley said.
Or, given the relatively low prevalence of blond hair, the gene may have been subject to frequency dependent selection — meaning that golden tresses provided an edge in luring partners as long as they remained relatively uncommon.
"It may have been a celebrated trait because it was rare," Kingsley said.
Of course, blond hair could also have provided no evolutionary advantage, and simply persisted by random chance, he said.
The blond gene was detailed June 1 in the journal Nature Genetics.
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