Earliest known life forms

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Evidence of possibly the oldest forms of life on Earth has been found in hydrothermal vent precipitates. Champagne vent white smokers.jpg
Evidence of possibly the oldest forms of life on Earth has been found in hydrothermal vent precipitates.

The earliest known life forms on Earth may be as old as 4.1 billion years old (or Ga) according to biologically fractionated graphite inside a single zircon grain in the Jack Hills range of Australia. [2] The earliest evidence of life found in a stratigraphic unit, not just a single mineral grain, is the 3.7 Ga metasedimentary rocks containing graphite from the Isua Supracrustal Belt in Greenland. [3] The earliest direct known life on land may be stromatolites which have been found in 3.480-billion-year-old geyserite uncovered in the Dresser Formation of the Pilbara Craton of Western Australia. [4] Various microfossils of microorganisms have been found in 3.4 Ga rocks, including 3.465-billion-year-old Apex chert rocks from the same Australian craton region, [5] and in 3.42 Ga hydrothermal vent precipitates from Barberton, South Africa. [1] Much later in the geologic record, likely starting in 1.73 Ga, preserved molecular compounds of biologic origin are indicative of aerobic life. [6] Therefore, the earliest time for the origin of life on Earth is at least 3.5 billion years ago, possibly as early as 4.1 billion years ago — not long after the oceans formed 4.5 billion years ago and after the formation of the Earth 4.54 billion years ago. [7]

Contents

Biospheres

Earth is the only place in the universe known to harbor life where it exists in multiple environments. [8] [9] The origin of life on Earth was at least 3.5 billion years ago, possibly as early as 3.8-4.1 billion years ago. [2] [3] [4] Since its emergence, life has persisted in several geological environments. The Earth's biosphere extends down to at least 10 km (6.2 mi) below the seafloor, [10] [11] up to 41–77 km (25–48 mi) [12] [13] into the atmosphere, [14] [15] [16] and includes soil, hydrothermal vents, and rock. [17] [18] Further, the biosphere has been found to extend at least 914.4 m (3,000 ft; 0.5682 mi) below the ice of Antarctica [19] [20] and includes the deepest parts of the ocean. [21] [22] [23] [24] In July 2020, marine biologists reported that aerobic microorganisms (mainly) in "quasi-suspended animation" were found in organically-poor sediments 76.2 m (250 ft) below the seafloor in the South Pacific Gyre (SPG) ("the deadest spot in the ocean"). [25] Microbes have been found in the Atacama Desert in Chile, one of the driest places on Earth, [26] and in deep-sea hydrothermal vent environments which can reach temperatures over 400°C. [27] Microbial communities can also survive in cold permafrost conditions down to -25°C. [28] Under certain test conditions, life forms have been observed to survive in the vacuum of outer space. [29] [30] More recently, studies conducted on the International Space Station found that bacteria could survive in outer space. [31] In February 2023, findings of a "dark microbiome" of unfamiliar microorganisms in the Atacama Desert in Chile, a Mars-like region of planet Earth, were reported. [32]

Geochemical evidence

The age of Earth is about 4.54 billion years; [33] [34] [35] the earliest undisputed evidence of life on Earth dates from at least 3.5 billion years ago according to the stromatolite record. [36] Some computer models suggest life began as early as 4.5 billion years ago. [37] [38] The oldest evidence of life is indirect in the form of isotopic fractionation. Microorganisms will preferentially use the lighter isotope of an atom to build biomass, as it takes less energy to break the bonds for metabolic processes. [39] Biologic material will often have a composition that is enriched in lighter isotopes compared to the surrounding rock it's found in. Carbon isotopes, expressed scientifically in parts per thousand difference from a standard as δ13C, are frequently used to detect carbon fixation by organisms and assess if purported early life evidence has biological origins. Typically, life will preferentially metabolize the isotopically light 12C isotope instead of the heavier 13C isotope. Biologic material can record this fractionation of carbon.

Zircons in metaconglomerates from the Jack Hills in Australia show carbon isotopic evidence for early life. Quartz-pebble metaconglomerate (Jack Hills Quartzite, Archean, 2.65 to 3.05 Ga; Jack Hills, Western Australia) 1 (26668804034).jpg
Zircons in metaconglomerates from the Jack Hills in Australia show carbon isotopic evidence for early life.

The oldest disputed geochemical evidence of life is isotopically light graphite inside a single zircon grain from the Jack Hills in Western Australia. [40] [41] The graphite showed a δ13C signature consistent with biogenic carbon on Earth. Other early evidence of life is found in rocks both from the Akilia Sequence [42] and the Isua Supracrustal Belt (ISB) in Greenland. [43] [44] These 3.7 Ga metasedimentary rocks also contain graphite or graphite inclusions with carbon isotope signatures that suggest biological fractionation.

The primary issue with isotopic evidence of life is that abiotic processes can fractionate isotopes and produce similar signatures to biotic processes. [45] Reassessment of the Akilia graphite show that metamorphism, Fischer-Tropsch mechanisms in hydrothermal environments, and volcanic processes may be responsible for enrichment lighter carbon isotopes. [46] [47] [48] The ISB rocks that contain the graphite may have experienced a change in composition from hot fluids, i.e. metasomatism, thus the graphite may have been formed by abiotic chemical reactions. [45] However, the ISB's graphite is generally more accepted as biologic in origin after further spectral analysis. [43] [44]

Metasedimentary rocks from the 3.5 Ga Dresser Formation, which experienced less metamorphism than the sequences in Greenland, contain better preserved geochemical evidence. [49] Carbon isotopes as well as sulfur isotopes found in barite, which are fractionated by microbial metabolisms during sulfate reduction, [50] are consistent with biological processes. [51] [52] However, the Dresser formation was deposited in an active volcanic and hydrothermal environment, [49] and abiotic processes could still be responsible for these fractionations. [53] Many of these findings are supplemented by direct evidence, typically by the presence of microfossils, however.

Fossil evidence

Fossils are direct evidence of life. In the search for the earliest life, fossils are often supplemented by geochemical evidence. The fossil record does not extend as far back as the geochemical record due to metamorphic processes that erase fossils from geologic units.

Stromatolites

Stromatolites are laminated sedimentary structures created by photosynthetic organisms as they establish a microbial mat on a sediment surface. An important distinction for biogenicity is their convex-up structures and wavy laminations, which are typical of microbial communities who build preferentially toward the sun. [54] A disputed report of stromatolites is from the 3.7 Ga Isua metasediments that show convex-up, conical, and domical morphologies. [55] [56] [57] Further mineralogical analysis disagrees with the initial findings of internal convex-up laminae, a critical criteria for stromatolite identification, suggesting that the structures may be deformation features (i.e. boudins) caused by extensional tectonics in the Isua Supracrustal Belt. [58] [59]

Stromatolite fossil showing convex-up structures. Stromatolite - National Museum of Nature and Science, Tokyo - DSC07686.JPG
Stromatolite fossil showing convex-up structures.

The earliest direct evidence of life are stromatolites found in 3.48 billion-year-old chert in the Dresser formation of the Pilbara Craton in Western Australia. [4] Several features in these fossils are difficult to explain with abiotic processes, for example, the thickening of laminae over flexure crests that is expected from more sunlight. [60] Sulfur isotopes from barite veins in the stromatolites also favor a biologic origin. [61] However, while most scientists accept their biogenicity, abiotic explanations for these fossils cannot be fully discarded due to their hydrothermal depositional environment and debated geochemical evidence. [62]

Most archean stromatolites older than 3.0 Ga are found in Australia or South Africa. Stratiform stromatolites from the Pilbara Craton have been identified in the 3.47 Ga Mount Ada Basalt. [63] Barberton, South Africa hosts stratiform stromatolites in the 3.46 Hooggenoeg, 3.42 Kromberg and 3.33 Ga Mendon Formations of the Onverwacht Group. [64] [65] The 3.43 Ga Strelley Pool Formation in Western Australia hosts stromatolites that demonstrate vertical and horizontal changes that may demonstrate microbial communities responding to transient environmental conditions. [66] Thus, it is likely anoxygenic or oxygenic photosynthesis has been occurring since at least 3.43 Ga Strelley Pool Formation. [67]

Microfossils

Claims of the earliest life using fossilized microorganisms (microfossils) are from hydrothermal vent precipitates from an ancient sea-bed in the Nuvvuagittuq Belt of Quebec, Canada. These may be as old as 4.28 billion years, which would make it the oldest evidence of life on Earth, suggesting "an almost instantaneous emergence of life" after ocean formation 4.41 billion years ago. [68] [69] These findings may be better explained by abiotic processes: for example, silica-rich waters, [70] "chemical gardens," [71] circulating hydrothermal fluids, [72] and volcanic ejecta [73] can produce morphologies similar to those presented in Nuvvuagittuq.

Archaea (prokaryotic microbes) were first found in extreme environments, such as hydrothermal vents. Halobacteria.jpg
Archaea (prokaryotic microbes) were first found in extreme environments, such as hydrothermal vents.

The 3.48 Ga Dresser formation hosts microfossils of prokaryotic filaments in silica veins, the earliest fossil evidence of life on Earth, [74] but their origins may be volcanic. [75] 3.465-billion-year-old Australian Apex chert rocks may once have contained microorganisms, [76] [77] although the validity of these findings has been contested. [78] [79] "Putative filamentous microfossils," possibly of methanogens and/or methanotrophs that lived about 3.42-billion-year-old in "a paleo-subseafloor hydrothermal vein system of the Barberton greenstone belt, have been identified in South Africa." [1] A diverse set of microfossil morphologies have been found in the 3.43 Ga Strelley Pool Formation including spheroid, lenticular, and film-like microstructures. [80] Their biogenicity are strengthened by their observed chemical preservation. [81] The early lithification of these structures allowed important chemical tracers, such as the carbon-to-nitrogen ratio, to be retained at levels higher than is typical in older, metamorphosed rock units.

Molecular Biomarkers

Biomarkers are compounds of biologic origin found in the geologic record that can be linked to past life. [82] Although they aren't preserved until the late Archean, they are important indicators of early photosynthetic life. Lipids are particularly useful biomarkers because they can survive for long periods of geologic time and reconstruct past environments. [83]

Lipids are commonly used in geologic studies to find evidence of oxygenic photosynthesis. Common lipids lmaps.png
Lipids are commonly used in geologic studies to find evidence of oxygenic photosynthesis.

Fossilized lipids were reported from 2.7 Ga laminated shales from the Pilbara Craton [84] and the 2.67 Ga Kaapvaal Craton in South Africa. [85] However, the age of these biomarkers and whether their deposition was synchronous with their host rocks were debated, [86] and further work showed that the lipids were contaminants. [87] The oldest "clearly indigenous" [88] biomarkers are from the 1.64 Ga Barney Creek Formation in the McArthur Basin in Northern Australia, [89] [90] but hydrocarbons from the 1.73 Ga Wollogorang Formation in the same basin have also been detected. [88]

Other indigenous biomarkers can be dated to the Mesoproterozoic era (1.6-1.0 Ga). The 1.4 Ga Hongshuizhuang Formation in the North China Craton contains hydrocarbons in shales that were likely sourced from prokaryotes. [91] Biomarkers were found in siltstones from the 1.38 Ga Roper Group of the McArthur Basin. [92] Hydrocarbons possibly derived from bacteria and algae were reported in 1.37 Ga Xiamaling Formation of the NCC. [93] The 1.1 Ga Atar/El Mreïti Group in the Taoudeni Basin, Mauritania show indigenous biomarkers in black shales. [94]

Genomic evidence

By comparing the genomes of modern organisms (in the domains Bacteria and Archaea), it is evident that there was a last universal common ancestor (LUCA). LUCA is not thought to be the first life on Earth, but rather the only type of organism of its time to still have living descendants. In 2016, M. C. Weiss and colleagues proposed a minimal set of genes that each occurred in at least two groups of Bacteria and two groups of Archaea. They argued that such a distribution of genes would be unlikely to arise by horizontal gene transfer, and so any such genes must have derived from the LUCA. [95] A molecular clock model suggests that the LUCA may have lived 4.477—4.519 billion years ago, within the Hadean eon. [37] [38]

RNA replicators

Model Hadean-like geothermal microenvironments were demonstrated to have the potential to support the synthesis and replication of RNA and thus possibly the evolution of primitive life. [96] Porous rock systems, comprising heated air-water interfaces, were shown to facilitate ribozyme catalyzed RNA replication of sense and antisense strands and then subsequent strand-dissociation [96] . This enabled combined synthesis, release and folding of active ribozymes [96] .

Further work on early life

Extraterrestrial origin for early life?

The theory of panspermia speculates that life on Earth may have come from biological matter carried by space dust or meteorites. Porous chondriteIDP.jpg
The theory of panspermia speculates that life on Earth may have come from biological matter carried by space dust or meteorites.

While current geochemical evidence dates the origin of life to possibly as early as 4.1 Ga, and fossil evidence shows life at 3.5 Ga, some researchers speculate that life may have started nearly 4.5 billion years ago. [37] [38] According to biologist Stephen Blair Hedges, "If life arose relatively quickly on Earth ... then it could be common in the universe." [99] [100] [101] The possibility that terrestrial life forms may have been seeded from outer space has been considered. [102] [103] In January 2018, a study found that 4.5 billion-year-old meteorites found on Earth contained liquid water along with prebiotic complex organic substances that may be ingredients for life. [104]

Early life on land

As for life on land, in 2019 scientists reported the discovery of a fossilized fungus, named Ourasphaira giraldae , in the Canadian Arctic, that may have grown on land a billion years ago, well before plants are thought to have been living on land. [105] [106] [107] The earliest life on land may have been bacteria 3.22 billion years ago. [108] Evidence of microbial life on land may have been found in 3.48 billion-year-old geyserite in the Pilbara Craton of Western Australia. [109] [110]

Earliest known life forms

See also

Notes

    Related Research Articles

    <span class="mw-page-title-main">Proterozoic</span> Geologic eon, 2500–539 million years ago

    The Proterozoic is the third of the four geologic eons of Earth's history, spanning the time interval from 2500 to 538.8 Mya, the longest eon of the Earth's geologic time scale. It is preceded by the Archean and followed by the Phanerozoic, and is the most recent part of the Precambrian "supereon".

    <span class="mw-page-title-main">Archean</span> Geologic eon, 4031–2500 million years ago

    The Archean Eon, in older sources sometimes called the Archaeozoic, is the second of the four geologic eons of Earth's history, preceded by the Hadean Eon and followed by the Proterozoic. The Archean represents the time period from 4,031 to 2,500 Ma. The Late Heavy Bombardment is hypothesized to overlap with the beginning of the Archean. The Huronian glaciation occurred at the end of the eon.

    <span class="mw-page-title-main">Stromatolite</span> Layered sedimentary structure formed by the growth of bacteria or algae

    Stromatolites or stromatoliths are layered sedimentary formations (microbialite) that are created mainly by photosynthetic microorganisms such as cyanobacteria, sulfate-reducing bacteria, and Pseudomonadota. These microorganisms produce adhesive compounds that cement sand and other rocky materials to form mineral "microbial mats". In turn, these mats build up layer by layer, growing gradually over time. This process generates the characteristic lamination of stromatolites, a feature that is hard to interpret, in terms of its temporal and environmental significance. Different styles of stromatolite lamination have been described, which can be studied through microscopic and mathematical methods. A stromatolite may grow to a meter or more. Fossilized stromatolites provide important records of some of the most ancient life. As of the Holocene, living forms are rare.

    <span class="mw-page-title-main">Geomicrobiology</span> Intersection of microbiology and geology

    Geomicrobiology is the scientific field at the intersection of geology and microbiology and is a major subfield of geobiology. It concerns the role of microbes on geological and geochemical processes and effects of minerals and metals to microbial growth, activity and survival. Such interactions occur in the geosphere, the atmosphere and the hydrosphere. Geomicrobiology studies microorganisms that are driving the Earth's biogeochemical cycles, mediating mineral precipitation and dissolution, and sorbing and concentrating metals. The applications include for example bioremediation, mining, climate change mitigation and public drinking water supplies.

    <span class="mw-page-title-main">Paleoarchean</span> Second era of the Archean Eon

    The Paleoarchean, also spelled Palaeoarchaean, is a geologic era within the Archean Eon. The name derives from Greek "Palaios" ancient. It spans the period of time 3,600 to 3,200 million years ago. The era is defined chronometrically and is not referenced to a specific level of a rock section on Earth. The earliest confirmed evidence of life comes from this era, and Vaalbara, one of Earth's earliest supercontinents, may have formed during this era.

    <span class="mw-page-title-main">Geobiology</span> Study of interactions between Earth and the biosphere

    Geobiology is a field of scientific research that explores the interactions between the physical Earth and the biosphere. It is a relatively young field, and its borders are fluid. There is considerable overlap with the fields of ecology, evolutionary biology, microbiology, paleontology, and particularly soil science and biogeochemistry. Geobiology applies the principles and methods of biology, geology, and soil science to the study of the ancient history of the co-evolution of life and Earth as well as the role of life in the modern world. Geobiologic studies tend to be focused on microorganisms, and on the role that life plays in altering the chemical and physical environment of the pedosphere, which exists at the intersection of the lithosphere, atmosphere, hydrosphere and/or cryosphere. It differs from biogeochemistry in that the focus is on processes and organisms over space and time rather than on global chemical cycles.

    <span class="mw-page-title-main">Sulfur cycle</span> Biogeochemical cycle of sulfur

    The important sulfur cycle is a biogeochemical cycle in which the sulfur moves between rocks, waterways and living systems. It is important in geology as it affects many minerals and in life because sulfur is an essential element (CHNOPS), being a constituent of many proteins and cofactors, and sulfur compounds can be used as oxidants or reductants in microbial respiration. The global sulfur cycle involves the transformations of sulfur species through different oxidation states, which play an important role in both geological and biological processes. Steps of the sulfur cycle are:

    <span class="mw-page-title-main">Gunflint chert</span> Geologic formation in Minnesota and Ontario

    The Gunflint chert is a sequence of banded iron formation rocks that are exposed in the Gunflint Range of northern Minnesota and northwestern Ontario along the north shore of Lake Superior. The Gunflint Chert is of paleontological significance, as it contains evidence of microbial life from the Paleoproterozoic. The Gunflint Chert is composed of biogenic stromatolites. At the time of its discovery in the 1950s, it was the earliest form of life discovered and described in scientific literature, as well as the earliest evidence for photosynthesis. The black layers in the sequence contain microfossils that are 1.9 to 2.3 billion years in age. Stromatolite colonies of cyanobacteria that have converted to jasper are found in Ontario. The banded ironstone formation consists of alternating strata of iron oxide-rich layers interbedded with silica-rich zones. The iron oxides are typically hematite or magnetite with ilmenite, while the silicates are predominantly cryptocrystalline quartz as chert or jasper, along with some minor silicate minerals.

    <span class="mw-page-title-main">Great Oxidation Event</span> Paleoproterozoic surge in atmospheric oxygen

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    <span class="mw-page-title-main">Oldest dated rocks</span> Includes rocks over 4 billion years old from the Hadean Eon

    The oldest dated rocks formed on Earth, as an aggregate of minerals that have not been subsequently broken down by erosion or melted, are more than 4 billion years old, formed during the Hadean Eon of Earth's geological history, and mark the start of the Archean Eon, which is defined to start with the formation of the oldest in-tact rocks on Earth. Meteorites that were formed in other planetary systems can pre-date Earth. Particles from the Murchison meteorite were dated in January 2020 to be 7 billion years old.

    <span class="mw-page-title-main">Isua Greenstone Belt</span> Archean greenstone belt in southwestern Greenland

    The Isua Greenstone Belt is an Archean greenstone belt in southwestern Greenland, aged between 3.7 and 3.8 billion years. The belt contains variably metamorphosed mafic volcanic and sedimentary rocks, and is the largest exposure of Eoarchaean supracrustal rocks on Earth. Due to its age and low metamorphic grade relative to many Eoarchaean rocks, the Isua Greenstone Belt has become a focus for investigations on the emergence of life and the style of tectonics that operated on the early Earth.

    Early Earth is loosely defined as encompassing Earth in its first one billion years, or gigayear (Ga, 109 y), from its initial formation in the young Solar System at about 4.55 Ga to some time in the Archean eon in approximately 3.5 Ga. On the geologic time scale, this comprises all of the Hadean eon, starting with the formation of the Earth about 4.6 billion years ago, and the Eoarchean, starting 4 billion years ago, and part of the Paleoarchean era, starting 3.6 billion years ago, of the Archean eon.

    <span class="mw-page-title-main">Pilbara Craton</span> Old and stable part of the continental lithosphere located in Pilbara, Western Australia

    The Pilbara Craton is an old and stable part of the continental lithosphere located in the Pilbara region of Western Australia.

    The history of life on Earth traces the processes by which living and fossil organisms evolved, from the earliest emergence of life to present day. Earth formed about 4.5 billion years ago and evidence suggests that life emerged prior to 3.7 Ga. Although there is some evidence of life as early as 4.1 to 4.28 Ga, it remains controversial due to the possible non-biological formation of the purported fossils.

    <span class="mw-page-title-main">Microbial mat</span> Multi-layered sheet of microorganisms

    A microbial mat is a multi-layered sheet of microorganisms, mainly bacteria and archaea, or bacteria alone. Microbial mats grow at interfaces between different types of material, mostly on submerged or moist surfaces, but a few survive in deserts. A few are found as endosymbionts of animals.

    <span class="mw-page-title-main">Warrawoona Group</span>

    The Warrawoona Group is a geological unit in Western Australia containing putative fossils of cyanobacteria cells. Dated 3.465 Ga, these microstructures, found in Archean chert, are considered to be the oldest known geological record of life on Earth.

    The Boring Billion, otherwise known as the Mid Proterozoic and Earth's Middle Ages, is the time period between 1.8 and 0.8 billion years ago (Ga) spanning the middle Proterozoic eon, characterized by more or less tectonic stability, climatic stasis, and slow biological evolution. It is bordered by two different oxygenation and glacial events, but the Boring Billion itself had very low oxygen levels and no evidence of glaciation.

    <span class="mw-page-title-main">Archean life in the Barberton Greenstone Belt</span> Some of the most widely accepted fossil evidence for Archean life

    The Barberton Greenstone Belt of eastern South Africa contains some of the most widely accepted fossil evidence for Archean life. These cell-sized prokaryote fossils are seen in the Barberton fossil record in rocks as old as 3.5 billion years. The Barberton Greenstone Belt is an excellent place to study the Archean Earth due to exposed sedimentary and metasedimentary rocks.

    Stanley Awramik is an American biogeologist and paleontologist. He is best known for his work related to the Precambrian. In 2013, he was inducted as a fellow of the Geological Society of America.

    Tanja Bosak is a Croatian-American experimental geobiologist who is currently an associate professor in the Earth, Atmosphere, and Planetary Science department at the Massachusetts Institute of Technology. Her awards include the Subaru Outstanding Woman in Science Award from the Geological Society of America (2007), the James B. Macelwane Medal from the American Geophysical Union (2011), and was elected an AGU fellow (2011). Bosak is recognized for her work understanding stromatolite genesis, in addition to her work in broader geobiology and geochemistry.

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