Life is defined as a chemical system capable of transferring its molecular information via self-reproduction and of evolving - probably originated from the reaction of reduced carbon-based organic matter on the primitive Earth. Primitive carbon was available as gaseous compounds, either oxidized (carbon dioxide, carbon monoxide) or reduced (methane). More complex organic molecules might have been formed by the action of UV light, shockwaves and electric discharges in a primitive atmosphere.
For example, Stanley Miller exposed a mixture of methane, ammonia, hydrogen and water to electric discharges to mimic the action of lightening on a primitive atmosphere. He obtained four of the twenty amino acids utilized in life today, via the intermediary formation of hydrogen cyanide and aldehydes. In the laboratory, hydrogen cyanide and formaldehyde have been shown to lead to many of the building blocks of the biopolymers, such as amino acids and nucleic acid bases.
However, the composition of the primitive Earth's atmosphere was
probably dominated by carbon dioxide and thus is only weakly reducing.
Very low levels of amino acids are formed from these gas mixtures.
Deep-sea hydrothermal systems as explored by French scientists at IFREMER
in Brest may also represent an environment for the synthesis of
prebiotic organic molecules. Reduced organic molecules have been
obtained in experiments carried out under conditions simulating the hydrothermal vents.
The collection and analysis of meteorites and micrometeorites, formed by the collisions of asteroids and delivered to the Earth, has allowed close examination of extraterrestrial organic material. Eight protein amino acids have been identified in the Murchison meteorite among more than 70 amino acids found therein. Some non-biological amino acids present contain 9% L-enantiomeric excesses (54.5% L), a finding which may provide a clue to the emergence of a homochiral (one-handed) life on Earth. Purines and pyrimidines, components of RNA, and amphiphilic molecules have also been detected. From recent collection and analysis of micrometerorites in Antarctica ice sheets, the amount of extraterrestrial carbon delivered to the Earth during the late bombardment phase has been estimated to 3x1019 g.
This material appears to have been one of the primary sources of reduced carbon compounds on the primitive Earth. This amount represents about 30 times the amount of carbon recycled on the surface of the Earth today. Organic molecules present in the protosolar molecular cloud not processed by the planetary accretion phases may have also been delivered to the primitive Earth via dust eroded from comets passing through the inner solar system.
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