Extinction

Extinction (the complete disappearance of a species from Earth) is an important part of the evolution of life on Earth. The current diversity of species is a product of the processes of extinction and speciation throughout the previous 3.8 billion year history of life. Scientists assume between 5 and 50 billion species have lived at some time during the history of the Earth. Therefore, it is estimated that 99.9% of all the life that has existed on Earth is now extinct; a species is assumed to be extinct when there is no reasonable doubt that the last individual has died.



Sources
http://cnx.org/content/m12157/latest/ (CC BY)
http://www.teachersdomain.org/resource/tdc02.sci.life.eco.bioinvaders/ (fair use)