In comparison with all the species that have ever existed, those alive today represent just a fraction. The vast majority of species in the history of the planet have become extinct. It's been estimated that a species lives between 1 and 5 million years, from its origin to its extinction. Nevertheless, the process of fossilization itself suggests that this estimate is likely to be far too optimistic, and that many species have much shorter lifespans, on the order of thousands of years. As a result, extinction occurs constantly in the grand sweep of life's history. A pattern known as background extinction is characterized by the extinction of one or a few species occurring only within a particular locale rather than globally.