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.
Most recent extinctions extinctions have been associated with European expansion in the 15th and 16th century. However, in some parts of the world some species are known to have become extinct before the arrival of the Europeans. For example, the Polynesians who colonised the Hawaiian Island in the 4th, 5th, and 6th centuries may have been responsible for the loss of around 50 of the 100 or so species of endemic land birds in the period between their arrival and that of the Europeans.
