imeline 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, the diversity of life is not a set of completely unique organisms, but organisms that share morphological similarities. Third, vestigial traits with no clear purpose resemble functional ancestral traits and finally, that organisms can be classified using these similarities into a hierarchy of nested groups – similar to a family tree.[249] However, modern research has suggested that, due to horizontal gene transfer, this "tree of life" may be more complicated than a simple branching tree since some genes have spread independently between distantly related species.[250][251]

Past species have also left records of their evolutionary history. Fossils, along with the comparative anatomy of present-day organisms, constitute the morphological, or anatomical, record.[252] By comparing the anatomies of both modern and extinct species, paleontologists can infer the lineages of those species. However, this approach is most successful for organisms that had hard body parts, such as shells, bones or teeth. Further, as prokaryotes such as bacteria and archaea share a limited set of common morphologies, their fossils do not provide information on their ancestry.

More recently, evidence for common descent has come from the study of biochemical similarities between organisms. For example, all living cells use the same basic set of nucleotides and amino acids.[253] The development of molecular genetics has revealed the record of evolution left in o

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