and the organisms undergoing speciation and rapid evolution are found in small populations or geographically restricted habitats and therefore rarely being preserved as fossils.[234]

Extinction
Further information: Extinction


Tyrannosaurus rex. Non-avian dinosaurs died out in the Cretaceous–Paleogene extinction event at the end of the Cretaceous period.
Extinction is the disappearance of an entire species. Extinction is not an unusual event, as species regularly appear through speciation and disappear through extinction.[235] Nearly all animal and plant species that have lived on Earth are now extinct,[236] and extinction appears to be the ultimate fate of all species.[237] These extinctions have happened continuously throughout the history of life, although the rate of extinction spikes in occasional mass extinction events.[238] The Cretaceous–Paleogene extinction event, during which the non-avian dinosaurs went extinct, is the most well-known, but the earlier Permian–Triassic extinction event was even more severe, with approximately 96% of species driven to extinction.[238] The Holocene extinction event is an ongoing mass extinction associated with humanity's expansion across the globe over the past few thousand years. Present-day extinction rates are 100–1000 times greater than the background rate and up to 30% of current species may be extinct by the mid 21st century.[239] Human activities are now the primary cause of the ongoing extinction event;[240] global warming may further accelerate it in the future.[241]

The role of extinction in evolution is not very well understood and may depend on which type of extinction is considered.[238] The causes of the continuous "low-level" extinction events, which form the majority of extinctions, may be the result of competition between species for limited resources (competitive exclusion).[49] If one species can out-compete another, this could produce species selection, with the fitter species surviving and the other species being driven to extinction.[109] The intermittent mass extinctions are also important, but instead of acting as a selective force, they drastically reduce diversity in a nonspecific manner and promote bursts of rapid evolution and speciation in survivors.[242]

Evolutionary history of life
Main article: Evolutionary history of life
See also: Timeline of evolution and Timeline of human evolution
Origin of life
Further information: Abiogenesis and RNA world hypothesis
Highly energetic chemistry is thought to have produced a self-replicating molecule arou