History of life

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The history of life on Earth traces the processes by which living and extinct organisms evolved, from the earliest emergence of life to the present day. Earth formed about 4.5 billion years ago (abbreviated as Ga, for gigaannum) and evidence suggests that life emerged prior to 3.7 Ga.^[1][2][3] The similarities among all known present-day species indicate that they have diverged through the process of evolution from a common ancestor.^[4]

The earliest clear evidence of life comes from biogenic carbon signatures^[2][3] and stromatolite fossils^[5] discovered in 3.7 billion-year-old metasedimentary rocks from western Greenland. In 2015, possible "remains of biotic life" were found in 4.1 billion-year-old rocks in Western Australia.^[6][7] There is further evidence of possibly the oldest forms of life in the form of fossilized microorganisms in hydrothermal vent precipitates from the Nuvvuagittuq Belt, that may have lived as early as 4.28 billion years ago, not long after the oceans formed 4.4 billion years ago, and after the Earth formed 4.54 billion years ago.^[8][9] These earliest fossils, however, may have originated from non-biological processes.^{[1][10][7][11]}

Microbial mats of coexisting bacteria and archaea were the dominant form of life in the early Archean eon, and many of the major steps in early evolution are thought to have taken place in this environment.^[12] The evolution of photosynthesis by cyanobacteria, around 3.5 Ga, eventually led to a buildup of its waste product, oxygen, in the oceans. After free oxygen saturated all available reductant substances on the Earth's surface, it built up in the atmosphere, leading to the Great Oxygenation Event around 2.4 Ga.^[13] The earliest evidence of eukaryotes (complex cells with organelles) dates from 1.85 Ga,^[14][15] likely due to symbiogenesis between anaerobic archaea and aerobic proteobacteria in co-adaptation against the new oxidative stress. While eukaryotes may have been present earlier, their diversification accelerated when aerobic cellular respiration by the endosymbiont mitochondria provided a more abundant source of biological energy. Around 1.6 Ga, some eukaryotes gained the ability to photosynthesize via endosymbiosis with cyanobacteria, and gave rise to various algae that eventually overtook cyanobacteria as the dominant primary producers.

At around 1.7 Ga, multicellular organisms began to appear, with differentiated cells performing specialised functions.^[16] While early organisms reproduced asexually, the primary method of reproduction for the vast majority of macroscopic organisms, including almost all eukaryotes (which includes animals and plants), is sexual reproduction, the fusion of male and female reproductive cells (gametes) to create a zygote.^[17] The origin and evolution of sexual reproduction remain a puzzle for biologists, though it is thought to have evolved from a single-celled eukaryotic ancestor.^[18]

While microorganisms formed the earliest terrestrial ecosystems at least 2.7 Ga, the evolution of plants from freshwater green algae dates back to about 1 billion years ago.^[19][20] Microorganisms are thought to have paved the way for the inception of land plants in the Ordovician period. Land plants were so successful that they are thought to have contributed to the Late Devonian extinction event^[21] as early tree Archaeopteris drew down CO₂ levels, leading to global cooling and lowered sea levels, while their roots increased rock weathering and nutrient run-offs which may have triggered algal bloom anoxic events.

Bilateria, animals having a left and a right side that are mirror images of each other, appeared by 555 Ma (million years ago).^[22] Ediacara biota appeared during the Ediacaran period,^[23] while vertebrates, along with most other modern phyla originated about 525 Ma during the Cambrian explosion.^[24] During the Permian period, synapsids, including the ancestors of mammals, dominated the land.^[25]

The Permian–Triassic extinction event killed most complex species of its time, 252 Ma.^[26] During the recovery from this catastrophe, archosaurs became the most abundant land vertebrates;^[27] one archosaur group, the dinosaurs, dominated the Jurassic and Cretaceous periods.^[28] After the Cretaceous–Paleogene extinction event 66 Ma killed off the non-avian dinosaurs,^[29] mammals increased rapidly in size and diversity.^[30] Such mass extinctions may have accelerated evolution by providing opportunities for new groups of organisms to diversify.^[31]

Only a very small percentage of species have been identified: one estimate claims that Earth may have 1 trillion species, because "identifying every microbial species on Earth presents a huge challenge."^[32][33] Only 1.75–1.8 million species have been named^[34][35] and 1.8 million documented in a central database.^[36] The currently living species represent less than one percent of all species that have ever lived on Earth.^[37][38]