AS LONG AS there has been life on earth, there has been extinction. In fact, natural selection predicts that extinction is the ultimate fate of every species that has ever existed on this planet. Although today as many as 40 to 50 million species exist in the world, between 5 and 50 billion species have existed during the course of life on earth. This means that the millions of species alive today represent less than 1 percent of the total number of all species. The average life of a plant or animal species is "only" 4 million years.

Nature and extinction

Earth history is conventionally described by a geologic time scale divided into periods of millions of years. Modern scientists believe major mass extinctions occurred at the end of the Ordovician, Devonian, Permian, Triassic, and Cretaceous periods.

The most well known extinction occurred at the end...