pansion 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 around 4 billion years ago, and half a billion years later the last common ancestor of all life existed.[243] The current scientific consensus is that the complex biochemistry that makes up life came from simpler chemical reactions.[244] The beginning of life may have included self-replicating molecules such as RNA[245] and the assembly of simple cells.[246]

Common descent
Further information: Common descent and Evidence of common descent


The hominoids are descendants of a common ancestor.
All organisms on Earth are descended from a common ancestor or ancestral gene pool.[177][247] Current species are a stage in the process of evolution, with their diversity the product of a long series of speciation and extinction events.[248] The common descent of organisms was first deduced from four simple facts about organisms: First, they have geographic distributions that cannot be explained by local adaptation. Second,