They're on your skin, in your gut and under your nails. The vegetables in your garden cohabit peacefully with them. They dwell in the intestines of cows, exchanging the ability to digest cellulose for sanctuary from the cruel outdoors. In the air, on the land, even in the harshest, most extreme parts of the world where angels fear to tread, they make up more than two-thirds of the world's biomass - and yet people are generally ignorant of their existence or, at best, have thoroughly warped notions of the role they play in the big drama of life.

We are, of course, talking about micro-organisms.