Creatures are becoming extinct at a frightening rate. So who cares?

Life began on earth around 3.8bn years ago. From the first, death and extinction were part of the story.

For more than 2bn years, the planet was occupied only by microbes - bacteria, fungi and yeasts - forming a thick slime in ponds, clinging on to ocean reefs, forming thick mats in rivers and eating minerals in rocks.

Around 600m years ago, complex life suddenly arrived, and so many new forms of multicellular creatures appeared - and disappeared - that evolutionary biologists called it the "Cambrian explosion". Mostly, they disappeared, randomly weeded out by evolutionary pressures.

Life went on, creatures came and went. Sometimes they disappeared en masse. There have been five major "extinction events" in the last 600m years, and a larger number of minor ones.

Every now and then, conditions would change, and species that couldn't shape up, shipped out. Each time, biodiversity would be dramatically reduced. Each time, life would bounce back over a 10m-year interval, with a new suite of creatures to thrive in new conditions.

Most of the creatures that have ever lived are now extinct, leaving only the imprint of their bones in sandy or muddy rocks as testament that they lived at all. Palaeontologists had seen the pattern so often they even arrived at a natural lifespan for a species: two million years, they said, maybe as many as 11m years. Life was an unfolding story, and extinctions were part of the background.

Until about 1980, that was the orthodoxy. Biologists calculated that there might be 2m species on the planet, and said that they had names and descriptions and specimens for more than 1.5m of them. They also said that thousands were threatened with extinction because of human activity.

Then one of them spread a collecting net under some tropical trees and sprayed toxic smoke aloft, and counted the things that fell into his collecting funnels.

The consequences were astonishing. So many strange little arthropods - little things with skeletons on the outside and jointy legs - fell into his hands that he had to think again. He proposed that, if the undreamed-of creatures of the canopy were any guide, there might be 10m species to be named. Or 30m.

At that point, experts began to realise the cost of razing the tropical forests, draining the mangrove swamps or polluting the coral reefs. If there were so many plants, mammals, birds, bees, beetles, reptiles, ants, spiders and fungi on the planet, then they must compete for tiny specialised niches. If a lot of forest was cleared, then thousands would go.

Campaigners, anxious to create a sense of public urgency about the dangers of extinction, began to make strident claims. Biologists started taking cool, measured looks. They found that the campaigners weren't being strident enough.

The great US biologist Edward O Wilson started as a doubter and changed his mind: he recently calculated that at least 27,000 creatures were disappearing from the tropical forests before the bulldozer and blowtorch every year.

Britain's chief scientific adviser, Sir Robert May, believes that creatures are now perishing at 1,000 times, or even 10,000 times, the "background rate" of extinction over the past 600m years.

Most biologists call what is now going on "the sixth great extinction". The tiger and the panda are only a tiny part of the story. Half of all the planet's occupants could perish in the next century or so.

The cruellest aspect is that all biologists now recognise that they will never know anything at all about most of the creatures that will slip into oblivion.