The extinction of species has always been a natural part of evolution. The fossil record shows that since life originated about four billion years ago the vast majority of species that have existed are now extinct. Extinct species outnumber living ones by a factor of perhaps a thousand to one.
Scientists have identified five extinction events in Earth's history, with some so severe that more than 90 percent of all life forms were killed off. The last and most famous extinction was the Cretaceous-Tertiary event some 63 million years ago that killed off the dinosaurs and allowed the rise of mammals. It is thought to have been caused by an asteroid hitting Earth.
Today we are in the middle of a sixth extinction event that began about 50,000 years ago but is now accelerating in an alarming speed! This extinction event is not a natural event, but is caused directly or indirectly by humans.