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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Earliest known life forms | 1/3 | https://en.wikipedia.org/wiki/Earliest_known_life_forms | reference | science, encyclopedia | 2026-05-05T07:15:42.867278+00:00 | kb-cron |
The earliest known life forms on Earth may be as old as 4.1 billion years (or Ga) according to biologically fractionated graphite inside a single zircon grain in the Jack Hills range of Australia. 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. The earliest direct known life on Earth are stromatolite fossils which have been found in 3.480-billion-year-old geyserite uncovered in the Dresser Formation of the Pilbara Craton of Western Australia. 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, and in 3.42 Ga hydrothermal vent precipitates from Barberton, South Africa. Much later in the geologic record, likely starting in 1.73 Ga, preserved molecular compounds of biologic origin are indicative of aerobic life. Therefore, the earliest time for the origin of life on Earth is at least 3.5 billion years ago and 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.
== Biospheres ==
Earth is the only place in the universe known to harbor life, where it exists in myriad environments. 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. 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, up to 41–77 km (25–48 mi) into the atmosphere, and includes soil, hydrothermal vents, and rock. Further, the biosphere has been found to extend at least 914.4 m (3,000 ft; 0.5682 mi) below the ice of Antarctica and includes the deepest parts of the ocean. In July 2020, marine biologists reported that aerobic microorganisms (mainly) in "quasi-suspended animation" were found in organically poor sediment 76.2 m (250 ft) below the seafloor in the South Pacific Gyre (SPG) ("the deadest spot in the ocean"). Microbes have been found in the Atacama Desert in Chile, one of the driest places on Earth. In February 2023, findings of a "dark microbiome" of microbial dark matter of unfamiliar microorganisms in the Atacama Desert were reported. Microbes have also been found in deep-sea hydrothermal vent environments which can reach temperatures over 400 °C. Microbial communities can also survive in cold permafrost conditions down to -25 °C. Under certain test conditions, life forms have been observed to survive in the vacuum of outer space. More recently, studies conducted on the International Space Station found that bacteria could survive in outer space.
== Geochemical evidence == The age of Earth is about 4.54 billion years; the earliest undisputed evidence of life on Earth dates from at least 3.5 billion years ago according to the stromatolite record. Some computer models suggest life began as early as 4.5 billion years ago. The oldest evidence of life is indirect in the form of isotopic fractionation processes. 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. 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.
The oldest disputed geochemical evidence of life is isotopically light graphite inside a single zircon grain from the Jack Hills in Western Australia. 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 and the Isua Supracrustal Belt (ISB) in Greenland. 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. 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. 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. However, the ISB's graphite is generally more accepted as biologic in origin after further spectral analysis. Metasedimentary rocks from the 3.5 Ga Dresser Formation, which experienced less metamorphism than the sequences in Greenland, contain better preserved geochemical evidence. Carbon isotopes as well as sulfur isotopes found in barite, which are fractionated by microbial metabolisms during sulfate reduction, are consistent with biological processes. However, the Dresser formation was deposited in an active volcanic and hydrothermal environment, and abiotic processes could still be responsible for these fractionations. 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 ===