Tuesday, November 21, 2006

Race, genes and skin color

Steve Jones:

The attempts in the 1970s to use our limited knowledge of human difference as an alibi for anti-racism struck me as quite wrong: certainly it seemed that most genetic changes across the world did not fit with traditional ethnic groups – but what, many biologists asked, if the story changed as science moved on? Would that give the green light to prejudice?

Well, science has moved on a lot since then, and DNA technology now shows a small but emphatic split between Africa and the rest of the world. It also reconstructs the escape from that land-mass, which happened surprisingly recently.

A consensus based on genes and fossils agrees that our ancestors stepped out of their native continent only 60,000 years ago (100,000 years after their origin). Africans are more diverse than others because of the bottlenecks the rest of us went through as we crossed the globe. Even so, overall differences are small and, like it or not, we all remain cousins, more or less, under the skin.

But what about the hue of that useful organ itself, with its social, political and legal implications? The story of skin colour has begun to emerge in black and white (or, as usual in biology, in several shades of grey) and it contains some real surprises.

The first key to colour came not from men but from fish. The zebrafish is a creature much used to study development. One mutation lacks the black melanin pigment that gives the animal its name. Its skin is almost transparent and allows the curious biologist to watch as the egg unfolds to give an adult. A quick search through human DNA for the same gene shows that we possess an almost precise match (so close, indeed, that the human version reinstates the fish's black stripes when injected into an embryo).

The gene plays a real part in our own pigment patterns, with a great shift in its structure across the globe. An amino acid called alanine is present at one site along its chain in 98 per cent of Africans, while in 99 per cent of Europeans that building block is replaced by another one known as threonine. That single change explains a considerable proportion of the difference in skin pigmentation between the two continents.

The golden fish does not tell the whole story. Two other genes in the melanin chain also have an entirely different identity in Africa and in Europe (although the segment of DNA which, when changed by mutation, causes red or blonde hair in Europeans does not play much part in the north-south patterns).

Something changed us on our journey from the sunshine of the tropics to the grey skies of the north, but quite what we are not certain. In Africa, skin damage caused by ultraviolet may have kept people black; but for the northern explorers vitamins were more important. Those with light skins were able to make vitamin D when exposed to sunshine and hence less liable to the soft-bone disease, rickets.

Simple enough; but there is a twist in the tale. Chinese and Japanese, who reached their part of the world somewhat later than did Europeans, also have light-coloured skins, but none of the three major European genes plays a part, for most East Asians have the African form of each of them.

The people of Asia, it seems, paled in quite a different fashion (one of which involves shifts in the structure of the gene which, when mutated, gives red-heads in Europe).

Gene implicated in human pigment variation

Researchers Identify Human Skin Color Gene

Zebrafish Researchers Hook Gene for Human Skin Color

Pigmentary Diversity: Identifying the genes causing human diversity

Fish help unlock mystery of our skin color


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