Over the last half billion years, life has become more diverse and complex. Although there seems to have been an increase in diversity through that time, the record is marked by several episodes of mass extinction. Five major events are recognized. The largest, at the end of the Permian, may have eliminated 70-90% of species. Each extinction provides opportunities for survivors. The biological modernization of our world comes after the extinction event at the Cretaceous-Tertiary boundary, about sixty five million years ago. Climate change appears to have been heavily involved in all of these events.