It just keeps on growing

Strands of Darwin's beard are a fitting display: our hair's story is entwined with that of evolution
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Charles Darwin
Photograph: Corbis

When Darwin died his family searched his desk for fallen strands of hair - sentimental mementos that have now attracted attention at an exhibition at the Natural History Museum.

Odd you might think, but hair has always been the purveyor of personality. For years I squirmed and screamed under the tyranny of my mother's mania about my hair - it had to be cleaned and brushed before I left the house - which I regarded as both unnecessary and an affront to my dignity. Until, at 13, I started to develop small wisps of hair on my upper lip and I began to share my mum's controlling obsession.

Hair is everywhere. There are virtually no organisms on the planet without hair; plants use it to collect water and to shade themselves from the sun; insects use it to navigate and sense their microscopic world; animals use it to keep warm and as a form of display. Hair, it seems, is an evolutionary inevitability. But it is only us that cuts it, plucks it, dyes it, laser-zaps it, plats it, spins it, weaves it, knits it, and wigs it. This is because every aspect of our hair is culturally loaded and is closely associated with different gender, societal, and religious groups. But the urge to control is different from being able to do so: to control hair is a sign of technological sophistication, a sign of power. There are back-to-basic urges in food production but not in hairdressing. No one wants to have their hair cut with a copper knife or a blunt, rusty pair of scissors. This is why it pleases us to have a razor associated with a scientific break-through, and why shampoo adverts look like they have been designed by Nasa.

Hair is a physical reminder of our animal nature, which we choose to tame and subdue through technology. But it is a never-ending battle; hair just keeps on growing, reminding us that our animal nature is constantly bubbling under the surface. It has been shown that the rate of beard growth is a strong indicator of a man's anticipation of sex. Thus, for a man to shave his face is to make a public expression not only that he has control over his sexual desires, but that he does so through technology. Underneath the clothes, the rules are completely different. Sensuality reigns and the animal spirit in men is admired; thus men don't shave their armpits or their genitals. Conversely, women are cast as pure, clean, virginal, and in control of their bodies (and reproduction): the overriding aesthetic of hairlessness signifies their technological sophistication.

Hair is constantly surprising in both its ability to cause disgust and delight. Finding hair in your soup is so revolting that whole sectors of the catering industry are set up to avoid that eventuality: your meal is prepared by someone wearing a hairnet. But there are times when finding a hair is wonderful, such as when you spot a hair of a loved one on your jacket while waiting for a plane at a far-away airport. The Victorians developed a whole genre of sentimental jewellery around encapsulating a lock of hair, and they were right about the role of hair in identity: like a fingerprint it is unique. These days it can provide a record of our blood chemistry for the past six months, and is used by archaeologists to probe the diet of our ancestors, by biochemists to drug-test the living, and by police to identify the guilty.

Thus to control hair is a symbolic act. Whether you are shaving your legs in the shower, or dyeing your grey hair, you are re-enacting the ritual of our ancestors to use technology to control cultural and sexual identity, and thus to evolve. It's utterly appropriate that Darwin's hair is on display: it embodies not just him but his life's work.

• Dr Mark Miodownik is director of the Materials Library in the Engineering Division of King's College London

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