Up to 150,000 sprout from each human head. Every strand grows at roughly one centimetre a month, making the gross product per head about 10 miles of hair each year.

After four years or so a strand reaches the end of its life and is shed: each human loses around 50 hairs a day. Uncut, it can reach huge lengths. The world record is held by a Hindu holy man in the late 1940s whose tresses were estimated at seven to eight metres (23ft to 26ft).

It can be cut, bobbed, braided, frizzed, combed out or gathered up. But it can only be artificially extended. Each strand is highly structured from the molecular to the cellular level, says Frédéric Leroy of L'Oréal Research, the sponsor of Hair, a new exhibition at the Natural History Museum in London.

"It is this structure which gives hair its exceptional, even unique properties," he said. "It's very easy to bend it, but very difficult to extend it."

Hair is strong. A single strand could hold 100g (3oz) in weight: the combined hair of a whole head could support 12 tonnes, or the weight of two elephants. Weight for weight, it is not as strong as steel: more like aluminium, or reinforced glass fibres or Kevlar, which is used to make bulletproof vests.

"So nature made these kinds of composite materials long before men could do it," said Dr Leroy.

Hair follicles contain specialised cells called melanocytes which make melanin, the sun-protecting pigment that keeps hair lustrous and coloured.

But these tend to decrease with time, according to Bruno Bernard, the head of hair biology research at Clichy in Paris, which explains the inexorable emergence of silver threads.

The melanin is still there, but not in enough quantities to keep hair from at least seeming grey. And of course there is a widely distributed phenomenon of male pattern baldness which begins with a thinning of the temples and ends with an eventual unsolicited tonsure.

Hair is also home to wildlife: notoriously head lice spread rapidly among schoolchildren. A female will live for a month laying eight eggs a day, according to Chris Lyal, an entomologist at the museum. "It means a fast build-up of lice are quite active, shifting from head to head quite readily. It also means they evolve quite fast, so they can develop resistance to insecticide, for example."

Hair in Asia is more likely to grow thick and long. Hair in Africa is likely to be fine and crinkly, entangling to provide insulation against the fierce sun.

But humans are - compared with their primate relatives - apparently largely hairless. In fact human body hair is so fine that compared with a chimpanzee or gorilla humans appear hairless.

Hair on the head could have survived as protection for a biped that now only walked upright, according to Christophe Soligo, an anthropologist at the museum. It could also serve as a sexual signal. "If my hair is shiny and healthy, I am all shiny and healthy and therefore our kids are going to be shiny and healthy. There are links between reproductive success and attractiveness."

· Hair is at the Natural History Museum, London SW7 from May 29 to September 26