Abiogenesis
Emergence of Earth

The age of the Earth is 4.54 Gya as found by radiometric dating of calcium-aluminium-rich inclusions in carbonaceous chrondrite meteorites, the oldest material in the Solar System. The Hadean Earth (from its formation until 4 Gya) was at first inhospitable to any living organisms. During its formation, the Earth lost a significant part of its initial mass, and consequentially lacked the gravity to hold molecular hydrogen and the bulk of the original inert gases. Soon after initial accretion of Earth at 4.48 Ga, its collision with Theia, a hypothesised impactor, is thought to have created the ejected debris that would eventually form the Moon. This impact would have removed the Earth's primary atmosphere, leaving behind clouds of viscous silicates and carbon dioxide. This unstable atmosphere was short-lived and condensed shortly after to form the bulk silicate Earth, leaving behind an atmosphere largely consisting of water vapor, nitrogen, and carbon dioxide, with smaller amounts of carbon monoxide, hydrogen, and sulfur compounds. The solution of carbon dioxide in water is thought to have made the seas slightly acidic, with a pH of about 5.5.

Condensation to form liquid oceans is theorised to have occurred as early as the Moon-forming impact. This scenario has found support from the dating of 4.404 Gya zircon crystals with high δ O values from metamorphosed quartzite of Mount Narryer in Western Australia. The Hadean atmosphere has been characterized as a "gigantic, productive outdoor chemical laboratory," similar to volcanic gases today which still support some abiotic chemistry. Despite the likely increased volcanism from early plate tectonics, the Earth may have been a predominantly water world between 4.4 and 4.3 Gya. It is debated whether or not crust was exposed above this ocean due to uncertainties of what early plate tectonics looked like. For early life to have developed, it is generally thought that a land setting is required, so this question is essential to determining when in Earth's history life evolved. The post-Moon-forming impact Earth likely existed with little if any continental crust, a turbulent atmosphere, and a hydrosphere subject to intense ultraviolet light from a T Tauri stage Sun, from cosmic radiation, and from continued asteroid and comet impacts. Despite all this, niche environments likely existed conducive to life on Earth in the Late-Hadean to Early-Archaean.

The Late Heavy Bombardment hypothesis posits that a period of intense impact occurred at ~3.9 Gya during the Hadean. A cataclysmic impact event would have had the potential to sterilise all life on Earth by volatilising liquid oceans and blocking the Sun needed for photosynthesising primary producers, pushing back the earliest possible emergence of life to after Late Heavy Bombardment. Recent research questions both the intensity of the Late Heavy Bombardment as well as its potential for sterilisation. Uncertainties as to whether Late Heavy Bombardment was one giant impact or a period of greater impact rates greatly changed the implication of its destructive power. The 3.9 Ga date arises from dating of Apollo mission sample returns collected mostly near the Imbrium Basin, biasing the age of recorded impacts. Impact modelling of the lunar surface reveals that rather than a cataclysmic event at 3.9 Ga, multiple small-scale, short-lived periods of bombardment likely occurred. Terrestrial data backs this idea by showing multiple periods of ejecta in the rock record both before and after the 3.9 Ga marker, suggesting that the early Earth was subject to continuous impacts that would not have had as great an impact on extinction as previously thought. If the Late Heavy Bombardment did not take place, this allows for the emergence of life to have taken place far before 3.9 Ga.

If life evolved in the ocean at depths of more than ten meters, it would have been shielded both from late impacts and the then high levels of ultraviolet radiation from the sun. Geothermically heated oceanic crust could have yielded far more organic compounds through deep hydrothermal vents than the Miller–Urey experiments indicated. The available energy is maximized at 100–150 °C, the temperatures at which hyperthermophilic bacteria and thermoacidophilic archaea live.
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