Skin: A Natural History

  • Nina G. Jablonski
University of California Press: 2006. 290 pp. $24.95, £15.95 0520242815 | ISBN: 0-520-24281-5

Biology is a historical science. Ask a 'why?' question about biology, as Nina Jablonski keeps doing in her book Skin, and you invite an evolutionary answer. She also tells us everything we might want to know about skin; perhaps more than some people want to know. She then goes on to take informed guesses as to why skin is the way it is and, by implication, why it is not like something else. Skin's appearance, its form and function, questions of how and why it works, and sometimes doesn't, have been thrashed out over a billion or so years at — to borrow her words — the “negotiating table of evolution”.

For Alexander Pope, the “proper study of mankind” may have been “man”, but Jablonski, as befits a modern biologist, thinks otherwise. Understanding starts, and possibly finishes, with comparisons, between humans and our biological relatives and neighbours, both near and not-so-near. We may share virtually all our genes with chimpanzees, but those we don't share are responsible for a lot of differences, reproductive, linguistic and cognitive. Skin genes, for example, which are responsible among other things for colour, body hair and the number of sweat glands, may well explain why chimps are still confined to African jungles, whereas we, their closest relatives, have already been to the Moon.

The skin's characteristics have been thrashed out at “the negotiating table of evolution”.

Skin is not just about biology, but also the way we live. Our skin is the visible, immediate personal territory where biology most obviously gives way to culture. Jablonski quotes Franz Kafka, who had the right idea, viewing the skin as “not only a garment but also a straitjacket and fate”. People go to a lot of trouble and expense to alter their appearance and change their fate. From war-paint and cosmetics to tanning, bleaching, tattooing, ritual scarring, botox, body piercing and 'nipping and tucking', there is someone making money out of it. And it does not necessarily have to stop just because someone is dead, as some enterprising Ancient Egyptian undertaker realized.

Some forms of personal make-over and disguise teach a salutory lesson: that culture comes at a biological price, paid from the genetic legacy bequeathed you by evolution. You are, let's say for the sake of argument, a fair-skinned northern European. But it has become the thing to show off a nice tan (Jablonski fingers fashionista Coco Chanel as the perpetrator of this particular vanity), and that means lying about without clothes in hot sun in latitudes rather nearer the Equator. The trouble is, the reason you are fair is a good historical one, indeed a matter of life and death for your ancestors in the Europe of 50,000 or so years ago. And that fact has implications for modern day Sun-worshippers, some of whom discover that mortality still starts with the skin.

At the core of Jablonski's theme is the skin's ability to multi-task: it protects, controls temperature, senses the world around you, and shows people how you really feel, as opposed to what you choose to tell them. But skin is also a chemical factory, fuelled in part by solar radiation. It manufactures vitamin D, without which you can neither extract calcium from your diet nor incorporate it in your bones — posing something of a challenge to survival. Here's an evolutionary conundrum. Ultraviolet light, which damages DNA directly and also destroys the folic acid essential for its synthesis, is, ironically, the energy source needed to make vitamin D. In equatorial Africa, our ancestral home, evolution engineered a nice compromise that allowed humans to leave the sheltering forest canopy and begin global colonization. Melanins that absorb ultraviolet afforded enough protection for the DNA, but left scope for the necessary production of vitamin D. When our dark-skinned ancestors started to migrate to the rest of the world, they first colonized regions that also had the strong sunlight to which their skin was already well adapted.

But dark skin was not adapted to the lower intensity of sunlight in northern Europe. It was simply over-protected, leading to problems producing vitamin D. Jablonski argues that Europe could only have been colonized in the wake of a genetic mutation that altered both the nature and the distribution of melanins in skin, producing fair skin with a tendency for freckles. The European climate selected for a gene that might well have been lethal back in equatorial Africa. Evolutionary negotiation achieved a new compromise.

These historical events have reverberated down the years, from biological prehistory into human documented history. On the one hand there have been rocketing frequencies of skin cancers in fair-skinned people exposed to too much sun; on the other, rickets became a problem in both the white-skinned populations of sun-deprived, smoke-polluted industrial Britain, and the later, darker-skinned immigrants to a postindustrial but still relatively unsunny northern Europe. These issues hint at the medical truth that any deep understanding of our ideas of 'wellness' and 'illness' is only likely to come from the central concepts of evolutionary theory: reproductive fitness and adaptation. It is amazing that medicine does not make much more use of evolutionary ideas. It is surely a sea change that is long overdue.