Skin colour is determined by genes, not race
For centuries, skin colour has held powerful social meaning — a defining characteristic of race. “If you ask somebody, ‘ What are the main differences between races?’, they’re going to say skin colour,” said Sarah A Tishkoff, a geneticist at the University of Pennsylvania.
On Thursday, she and her colleagues showed this to be a profound error. In ‘Science’, the researchers published the first large-scale study of the genetics of skin colour in Africans. The researchers pinpointed eight genetic variants in four narrow regions of the human genome that strongly influence pigmentation — some making skin darker, and others making it lighter.
These genes are shared across the globe, it turns out; one of them, for example, lightens skin in both Europeans and hunter-gatherers in Botswana. The gene variants were present in humanity’s distant ancestors, even before our species evolved in Africa 300,000 years ago. The widespread distribution of these genes and their persistence over millenniums show that the old colour lines are essentially meaningless, the scientists said. The research “dispels a biological concept of race”, Tishkoff said.
Humans develop colour much as other mammals do. Special cells in the skin contain pouches, called melanosomes, packed with pigment molecules. The more pigment, the darker the skin.
Skin colour also varies with the kind of pigments: Melanosomes may contain mixtures of a brown-black called eumelanin and a yellow-red called pheomelanin.
To find the genes that help produce pigments, scientists began by studying people of European ancestry and found that mutations to a gene called SLC24A5 caused cells to make less pigment, leading to paler skin. “We knew quite a lot about why people have pale skin if they had European an- cestry,” said Nicholas G. Crawford, a co-author of the study. “But there was very little known about why people have dark skin.” Since the early 2000s, Tishkoff has studied genes in Africa, discovering variants important to everything from resistance to malaria to height. African populations vary tremendously in skin colour, and she reasoned powerful genetic variants must be responsible.
Studying 1,570 people in Ethiopia, Tanzania and Botswana, Tishkoff and her colleagues discovered a set of genetic variants that account for 29% of the variation in skin colour. (The remaining variation seems tied to genes yet to be discovered.)
The eight gene variants discovered in Africans turned out to be present in many populations outside the continent. By comparing the DNA of these people, the team was able to estimate how long ago the genes appeared.They turned out to be immensely old. A variant for light skin — found in both Europeans and the San hunter-gatherers of Botswana — arose roughly 900,000 years ago, for example.
Even before there were Homo sapiens, then, our forebears had a mix of genes for light and dark skin. Some populations may have been dark-skinned and others light-skinned; or maybe they were all the same colour.
In all, the new study provides “a deeper appreciation of the genetic palette that has been mixed and matched through evolution”, said Nina Jablonski, an expert on skin colour at Pennsylvania State University.