Origin of life

The Earth is about 4.54 billion years old.^[258]^[259]^[260] The earliest undisputed evidence of life on Earth dates from at least 3.5 billion years ago,^[12]^[261] during the Eoarchean Era after a geological crust started to solidify following the earlier molten Hadean Eon. Microbial mat fossils have been found in 3.48 billion-year-old sandstone in Western Australia.^[14]^[15]^[16] Other early physical evidence of a biogenic substance is graphite in 3.7 billion-year-old metasedimentary rocks discovered in Western Greenland^[13] as well as "remains of biotic life" found in 4.1 billion-year-old rocks in Western Australia.^[262]^[263] Commenting on the Australian findings, Stephen Blair Hedges wrote: "If life arose relatively quickly on Earth, then it could be common in the universe."^[262]^[264] In July 2016, scientists reported identifying a set of 355 genes from the last universal common ancestor (LUCA) of all organisms living on Earth.^[265]

More than 99% of all species, amounting to over five billion species,^[266] that ever lived on Earth are estimated to be extinct.^[234]^[235] Estimates on the number of Earth's current species range from 10 million to 14 million,^[267]^[268] of which about 1.9 million are estimated to have been named^[269] and 1.6 million documented in a central database to date,^[270] leaving at least 80% not yet described.

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.^[10] The current scientific consensus is that the complex biochemistry that makes up life came from simpler chemical reactions.^[271]^[272] The beginning of life may have included self-replicating molecules such as RNA^[273] and the assembly of simple cells.^[274]

Common descent

All organisms on Earth are descended from a common ancestor or ancestral gene pool.^[168]^[275] Current species are a stage in the process of evolution, with their diversity the product of a long series of speciation and extinction events.^[276] 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. Fourth, organisms can be classified using these similarities into a hierarchy of nested groups, similar to a family tree.^[277]

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.^[278]^[279] To solve this problem and others, some authors prefer to use the "Coral of life" as a metaphor or a mathematical model to illustrate the evolution of life. This view dates back to an idea briefly mentioned by Darwin but later abandoned.^[280]

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.^[281] By comparing the anatomies of both modern and extinct species, palaeontologists 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.^[282] The development of molecular genetics has revealed the record of evolution left in organisms' genomes: dating when species diverged through the molecular clock produced by mutations.^[283] For example, these DNA sequence comparisons have revealed that humans and chimpanzees share 98% of their genomes and analysing the few areas where they differ helps shed light on when the common ancestor of these species existed.^[284]

Evolution of life

Prokaryotes inhabited the Earth from approximately 3–4 billion years ago.^[286]^[287] No obvious changes in morphology or cellular organisation occurred in these organisms over the next few billion years.^[288] The eukaryotic cells emerged between 1.6 and 2.7 billion years ago. The next major change in cell structure came when bacteria were engulfed by eukaryotic cells, in a cooperative association called endosymbiosis.^[289]^[290] The engulfed bacteria and the host cell then underwent coevolution, with the bacteria evolving into either mitochondria or hydrogenosomes.^[291] Another engulfment of cyanobacterial-like organisms led to the formation of chloroplasts in algae and plants.^[292]

The history of life was that of the unicellular eukaryotes, prokaryotes and archaea until about 610 million years ago when multicellular organisms began to appear in the oceans in the Ediacaran period.^[286]^[293] The evolution of multicellularity occurred in multiple independent events, in organisms as diverse as sponges, brown algae, cyanobacteria, slime moulds and myxobacteria.^[294] In January 2016, scientists reported that, about 800 million years ago, a minor genetic change in a single molecule called GK-PID may have allowed organisms to go from a single cell organism to one of many cells.^[295]

Soon after the emergence of these first multicellular organisms, a remarkable amount of biological diversity appeared over approximately 10 million years, in an event called the Cambrian explosion. Here, the majority of types of modern animals appeared in the fossil record, as well as unique lineages that subsequently became extinct.^[296] Various triggers for the Cambrian explosion have been proposed, including the accumulation of oxygen in the atmosphere from photosynthesis.^[297]

About 500 million years ago, plants and fungi colonised the land and were soon followed by arthropods and other animals.^[298] Insects were particularly successful and even today make up the majority of animal species.^[299] Amphibians first appeared around 364 million years ago, followed by early amniotes and birds around 155 million years ago (both from "reptile"-like lineages), mammals around 129 million years ago, Homininae around 10 million years ago and modern humans around 250,000 years ago.^[300]^[301]^[302] However, despite the evolution of these large animals, smaller organisms similar to the types that evolved early in this process continue to be highly successful and dominate the Earth, with the majority of both biomass and species being prokaryotes.^[146]