when did the first bacteria appear on earth
Bacteria The ancestors of modern bacteria were unicellular microorganisms that were the first forms of life to appear on Earth, about 4 billion years ago. For about 3 billion years, most organisms were microscopic, and bacteria and archaea were the dominant forms of life.[20][21] Although bacterial fossils exist, such as stromatolites, their lack of distinctive morphology prevents them from being used to examine the history of bacterial evolution, or to date the time of origin of a particular bacterial species. However, gene sequences can be used to reconstruct the bacterial phylogeny, and these studies indicate that bacteria diverged first from the archaeal/eukaryotic lineage.[22] The most recent common ancestor of bacteria and archaea was probably a hyperthermophile that lived about 2.5 billionâ€“3.2 billion years ago.[23][24]
Abiogenesis Nonetheless, Earth remains the only place in the Universe known to harbour life,[29][30] and fossil evidence from the Earth informs most studies of abiogenesis. More than 99% of all species of life forms, amounting to over five billion species,[31] that ever lived on Earth are estimated to be extinct.[32][33] The age of the Earth is about 4.54 billion years;[34][35][36] the earliest undisputed evidence of life on Earth dates from at least 3.5 billion years ago,[37][38][39] and possibly as early as the Eoarchean Era (between 3.6 and 4.0 billion years ago), after geological crust started to solidify following the molten Hadean Eon. In May 2017, evidence of the earliest known life on land may have been found in 3.48-billion-year-old geyserite and other related mineral deposits (often found around hot springs and geysers) uncovered in the Pilbara Craton of Western Australia.[40][41][42][43] However, there have been a number of discoveries that suggested the earliest appearance of life on Earth was even earlier. Currently, microfossils within hydrothermal vent precipitates dated from 3.77 to 4.28 billion years old found in Quebec, Canada may be the oldest record of life on Earth, suggesting life started soon after ocean formation 4.4 billion years ago.[1][2][44][45][46] According to biologist Stephen Blair Hedges, "If life arose relatively quickly on Earth â€¦ then it could be common in the universe."[47][48]
Life Since its primordial beginnings, life on Earth has changed its environment on a geologic time scale. To survive in most ecosystems, life must often adapt to a wide range of conditions. Some microorganisms, called extremophiles, thrive in physically or geochemically extreme environments that are detrimental to most other life on Earth. Aristotle was the first person to classify organisms. Later, Carl Linnaeus introduced his system of binomial nomenclature for the classification of species. Eventually new groups and categories of life were discovered, such as cells and microorganisms, forcing dramatic revisions of the structure of relationships between living organisms. The cell is considered the structural and functional unit of life.[8] There are two kinds of cells, prokaryotic and eukaryotic, both of which consist of cytoplasm enclosed within a membrane and contain many biomolecules such as proteins and nucleic acids. Cells reproduce through a process of cell division, in which the parent cell divides into two or more daughter cells.
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