Abiogenesis

• 1. The First Living Thing

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  NASA defines life with a single tight phrase: "a self-sustaining chemical system capable of Darwinian evolution." That definition sounds simple until you try to build one from rock and water. Abiogenesis, sometimes called biopoiesis, is the natural process by which life arises from non-living matter, such as simple organic compounds. The transition from non-life to life has never been observed in any experiment. Researchers can only propose stages for how it happened.

  Consider a problem that sits at the heart of the whole inquiry. A cell copies its DNA using the DNA polymerase enzyme. That enzyme is itself produced by translating the DNA polymerase gene. Neither the enzyme nor the DNA can be made without the other. So how does a system this tightly interlocked assemble itself one step at a time, when every part seems to need every other part to work at all?

  The last universal common ancestor of all modern life, known as LUCA, lived some 4 billion years ago. It was already a complex single-celled organism with hundreds of genes. It was not the first living thing. Between the true origin of life and LUCA lies an unknown span of gene transfers, extinctions, and adaptation. The chapters ahead trace how chemistry on a young planet might have crossed that gap, from the first stars that forged carbon to the warm pools and deep vents where life may have begun.

• 2. A Warm Little Pond

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  On the 1st of February 1871, Charles Darwin wrote to Joseph Hooker and let himself speculate. The original spark of life, he mused, may have formed in a "warm little pond, with all sorts of ammonia and phosphoric salts,-light, heat, electricity present, that a protein compound was chemically formed." Darwin added a sharp observation: such matter today would be "instantly devoured or absorbed," which would not have been the case before living creatures existed.

  Long before Darwin, the dominant view was spontaneous generation. From Aristotle until the 19th century, people believed lower animals like insects sprang from decaying organic matter. In 1665, Robert Hooke published the first drawings of a microorganism. In 1676, Antonie van Leeuwenhoek drew and described microorganisms, probably protozoa and bacteria, and by the 1680s convinced himself the old theory was wrong. In 1668, Francesco Redi showed that no maggots appeared in meat when flies were kept from laying eggs.

  Alexander Oparin in 1924 and J. B. S. Haldane in 1929 turned the pond into a theory. The earliest cells, they proposed, slowly self-organized from a primordial soup. Haldane pictured prebiotic oceans as a "hot dilute soup" in which organic compounds could form. In 1967, J. D. Bernal laid out three stages: the origin of biological monomers, the origin of biological polymers, and the evolution from molecules to cells.

  A rival idea, panspermia, dates back to Anaxagoras in the 5th century BC. It holds that life originated elsewhere in the universe and arrived here, perhaps carried by meteoroids, asteroids, or comets. There remains some interest in the possibility that life began on Mars and later transferred to Earth.

• 3. Cooking The Ingredients

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  In 1952, Stanley Miller and Harold Urey sealed a highly reducing mixture of gases, methane, ammonia, and hydrogen, together with water vapor, and ran it through a chemical experiment. Out came organic monomers, including amino acids, formed from inorganic precursors alone. It was a demonstration that the building blocks of life could appear spontaneously under conditions like those of early Earth.

  Bernal was not fully satisfied. He said it was not enough to explain how such molecules form. What was needed, he argued, was a physical-chemical account of "suitable sources and sinks for free energy." Later science complicated the picture further. The primitive atmosphere is now described as weakly reducing or neutral, which shrinks the variety of amino acids the original setup could yield. Adding iron and carbonate minerals, present in early oceans, restores a diverse array of amino acids.

  Alexander Butlerov showed in 1861 that the formose reaction creates sugars, including tetroses, pentoses, and hexoses, when formaldehyde is heated under basic conditions with divalent metal ions like calcium. R. Breslow proposed in 1959 that the reaction was autocatalytic. Nucleobases such as guanine and adenine can be built from simple sources like hydrogen cyanide and ammonia. Formamide, made from water and HCN, produces all four ribonucleotides when warmed with terrestrial minerals.

  Cold proves surprisingly productive. Freezing temperatures assist the synthesis of purines by concentrating precursors like HCN. In one striking result, seven amino acids and eleven types of nucleobases formed in ice when ammonia and cyanide were left in a freezer for 25 years. These low-temperature reactions run fast because eutectic freezing crowds impurities into microscopic pockets of liquid within the ice.

• 4. Seeds From Space

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  During the Late Heavy Bombardment, meteorites may have delivered up to five million tons of organic prebiotic material to Earth each year. Even now, 40,000 tons of cosmic dust fall to Earth annually. Space, it turns out, is a chemistry set of its own. Organic compounds are relatively common there, built in "factories of complex molecular synthesis" within molecular clouds and circumstellar envelopes.

  The amino acid glycine was found in material ejected from comet Wild 2, having earlier been detected in meteorites. Purine and pyrimidine nucleobases, including guanine, adenine, cytosine, uracil, and thymine, along with sugars, have turned up in meteorites. NASA studies suggest all four DNA nucleobases formed in outer space. Glycolaldehyde, a sugar molecule and RNA precursor, has been detected around protostars and on meteorites.

  Polycyclic aromatic hydrocarbons, abbreviated PAHs, are the most common and abundant polyatomic molecules in the observable universe and a major store of carbon. They seem to have formed shortly after the Big Bang and are associated with new stars and exoplanets. A star called HH 46-IR, resembling the early Sun, is wrapped in a disk containing cyanide compounds, hydrocarbons, and carbon monoxide.

  Carbon itself has a stellar pedigree. It formed mainly in white dwarf stars, which ejected carbon and oxygen across the universe as they died, allowing rocky planets to form. Soon after the Big Bang, roughly 14 Gya, only hydrogen, helium, and lithium existed. The first stars fused hydrogen and forged heavier elements up to Iron-56, with the heaviest born in supernovas. The cosmic dust permeating the universe still carries complex organics, amorphous solids with a mixed aromatic-aliphatic structure, that stars can create rapidly.

• 5. Building A Habitable World

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  The age of the Earth is 4.54 Gya, found by radiometric dating of calcium-aluminium-rich inclusions in carbonaceous chondrite meteorites, the oldest material in the Solar System. Soon after initial accretion at 4.48 Gya, a collision with a hypothesised impactor named Theia is thought to have thrown off the debris that became the Moon. That impact stripped away Earth's primary atmosphere.

  What remained was an atmosphere largely of water vapor, nitrogen, and carbon dioxide, with smaller amounts of carbon monoxide, hydrogen, and sulfur compounds. Carbon dioxide dissolving in water made the early seas slightly acidic, around a pH of 5.5. Liquid oceans may have condensed as early as the Moon-forming impact. That scenario draws support from 4.404 Gya zircon crystals with high oxygen-18 values, recovered from metamorphosed quartzite at Mount Narryer in Western Australia.

  The Hadean atmosphere has been called a "gigantic, productive outdoor chemical laboratory." Earth may have been a predominantly water world between 4.4 and 4.3 Gya, swept by intense ultraviolet light from a Sun in its T Tauri stage and battered by asteroid and comet impacts. Whether any crust rose above that ocean is still debated.

  The Late Heavy Bombardment hypothesis once imagined a single cataclysmic strike at 3.9 Gya that could have sterilized the planet by boiling its oceans. That 3.9 Gya date came from Apollo samples gathered mostly near the Imbrium Basin, which biased the record. Impact modelling now suggests many small, short-lived bombardment periods instead, with terrestrial ejecta layers appearing both before and after the 3.9 Gya marker. A young Earth under continuous lighter impacts had far less power to wipe the slate clean.

• 6. The Earliest Traces

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  In 2017, the earliest physical evidence of life was reported as microbialites in the Nuvvuagittuq Greenstone Belt of Northern Quebec, set in banded iron formation rocks at least 3.77 and possibly as old as 4.32 Gya. The micro-organisms could have lived within hydrothermal vent precipitates, soon after oceans formed around 4.4 Gya. They resemble modern hydrothermal vent bacteria. Later research disputed the reading, arguing abiotic processes in silica-rich waters or volcanic ejecta might explain the same observations.

  Biogenic graphite appears in 3.7 Gya metasedimentary rocks from southwestern Greenland. Near the Isua supracrustal belt, rocks from Akilia Island dating to 3.7 Gya show biogenic carbon isotopes. Within the Isua belt, graphite inclusions trapped in garnet crystals connect to oxygen, nitrogen, and possibly phosphorus. Carbon isotope ratios on graphite inclusions from the Jack Hills zircons suggest life could have existed from 4.1 Gya.

  The Pilbara region of Western Australia holds the Dresser Formation, with rocks 3.48 Gya containing layered structures called stromatolites. Their modern counterparts are made by photosynthetic micro-organisms, including cyanobacteria. Parts of the formation preserve hot springs on land, while other regions appear to have been shallow seas. In the same region, pyrite-bearing sandstone from a fossilized beach held rounded tubular cells that oxidized sulfur by photosynthesis without oxygen.

  A 2024 study inferred LUCA's age as around 4.2 Gya, within a range of 4.09 to 4.33 Gya, by analysing pre-LUCA gene duplicates. That places LUCA much sooner after the origin of life than previously thought. A molecular clock analysis suggests LUCA emerged prior to 3.9 Gya, hinting that life took hold quickly in geological terms.

• 7. Membranes And Money

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  The largest unanswered question in evolution is how simple protocells first arose and then differed in their contribution to the next generation, setting evolution in motion. The lipid world theory proposes that the first self-replicating object was lipid-like. Phospholipids form lipid bilayers in water under agitation, but those molecules were not present on early Earth. Other membrane-forming amphiphilic long-chain molecules were, and such bodies can expand by inserting lipids and split into two offspring of similar size.

  Irene Chen and Jack W. Szostak suggest that elementary protocells can give rise to primitive differential reproduction, competition, and energy storage. Competition for membrane molecules would favor stabilized membranes, hinting at a selective advantage for cross-linked fatty acids and even modern phospholipids. A membrane is exactly what a cell needs to create its own electrochemical gradient. Fatty acid vesicles in alkaline vent conditions can be stabilized by isoprenoids made through the formose reaction.

  In 1961, Peter Mitchell proposed chemiosmosis as a cell's primary system of energy conversion. The mechanism now powers energy conversion in micro-organisms and in the mitochondria of eukaryotes, which produce adenosine triphosphate, or ATP, the energy currency of the cell. ATP synthesis runs on protons moving across a closed membrane where the ATP synthase enzyme sits. In the first organisms, that proton gradient could have come from the difference between flow out of a hydrothermal vent and the surrounding seawater.

  Life requires a loss of entropy as molecules organize into living matter. That does not break the second law of thermodynamics. A living organism creates order in its body at the expense of more entropy elsewhere, in heat and waste. A boundary is therefore needed to separate ordered life from chaotic non-living matter, and a functional protocell capable of Darwinian evolution has not yet been built in any laboratory.

• 8. Vents, Springs And Ice

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  William Martin and Michael Russell proposed that life began in metal-sulphide-walled compartments at submarine hydrothermal vents, structures that could act as precursors to cell walls. These vents form where hydrogen-rich fluids rise from below the sea floor through serpentinization of olivine, meeting carbon-dioxide-rich ocean water at a pH interface. The redox reactions there, with molecular hydrogen meeting carbon dioxide, supply a sustained chemical energy source. Russell showed that alkaline vents create an abiogenic proton motive force, a chemiosmotic gradient he called ideal for abiogenesis.

  Not everyone is convinced the vents are the cradle. Vents do not concentrate prebiotic materials, since strong dilution by seawater leaves little residence time to accumulate compounds. All modern cells rely on phosphates for the nucleotide backbone and potassium for protein formation, yet both were scarce in Archaean oceans. No studies have experimentally demonstrated the synthesis of sugars, amino acids, nucleotides, or membrane-forming fatty acids under plausible vent conditions.

  Surface bodies of water offer something the open ocean cannot: wet-dry cycles that concentrate prebiotic compounds and enable condensation reactions to build macromolecules. Lakes and ponds also receive phosphate from weathering of continental apatite-containing rocks. Mulkidjanian and co-authors argue that only hot springs, like those at Kamchatka, provide the ionic balance cells universally need, especially a high potassium to sodium ratio. A phylogenomic analysis found that LUCA's intracellular fluid matches the ionic composition of hot springs.

  Cold-start theories take the opposite tack. Stellar models predict the early Sun was about 25% weaker than today, which would favor an icy planet, though strong evidence points to liquid water alongside icy poles. Water under ice would be shielded from ultraviolet light while staying connected through cracks. Experiments simulating Europa-like conditions near 20 degrees Celsius have synthesised amino acids and adenine.

  There is also a case for going underground. The continental crust, threaded with tectonic fault zones, could offer a stable, well-protected setting for long-term prebiotic evolution. At depths around 1000 meters, carbon dioxide sits near the phase transition between supercritical and gaseous states, letting lipophilic molecules accumulate and precipitate. One quirk of life narrows the search further. Living organisms are homochiral, with amino acids almost always left-handed and sugars right-handed, and some amino acids delivered by meteorites already show a chiral asymmetry of the same sign as life on Earth.