We don't know. Since Linnaeus set up his classification system 240 years ago, taxonomists have described and named around 1.4 million kinds of animal. Over the past ten years, new species have been named at a rate of about 10,000 a year, and scientists tell us that there could be 30 million species waiting to be discovered. The reason for such a fantastic variety of animals is because, even within one species, no two animals are alike. In every species, there is a huge diversity of genetic material and it is the variation that allows evolution to take place and new species to appear.
Evolution occurs through a process of 'natural selection'.
As they spread through the whole population the species will change, eventually becoming distinct enough to be regarded as a new species, while its ancestor becomes extinct. Extinction Extinction is a natural process, and usually occurs slowly as new species evolve. But sometimes the loss of biodiversity is much more dramatic. Then, around 65 million years ago, earth saw the disappearance of the dinosaurs and the ammonites. The extinction of big mammals like mammoths and giant sloths, 15,000-35,000 years ago, coincided with the spread of human hunters and, sadly, this latest wave of extinction is gathering speed.