Sources of Organic Compounds Required for Primitive Life

Carbon compounds on the early Earth were not the simple mixture previously referred to as a “prebiotic soup.” Instead, there was a continuing input of organic material synthesized by geochemical and photochemical reactions in the volcanic crust and atmosphere; organic compounds were also being delivered during late accretion by the infall of interplanetary dust particles (IDPs), impacting comets, and asteroid-sized bodies. Compounds from both sources (terrestrial and not) then underwent chemical processing by volcanism, photochemistry, and mineral-driven oxidation–reduction reactions. Some of these processes were synthetic reactions that led to increasing complexity, but this was balanced by other processes such as hydrolysis and pyrolysis that degraded organic material into simpler compounds or tar-like polymers. Because the atmosphere contained no molecular oxygen, the organic compounds that formed were relatively stable as a dilute solution in the global ocean, but were also dissolved in freshwater hydrothermal pools in contact with mineral surfaces of volcanic land masses. In either case, a process was required by which the organic compounds could become sufficiently concentrated to undergo chemical reactions. Questions to be addressed: What are plausible sources of organic compounds? What is their composition and abundance? How would organic material be chemically processed on the early Earth? How can dilute organic solutes become sufficiently concentrated to undergo chemical reactions? Chapter 1 described how virtually all of the carbon now circulating in the biosphere as organic and inorganic compounds was delivered during accretion of planetesimals as the Earth formed, and it is reasonable to assume that Mars had a similar addition of carbon compounds and water after it cooled from primary accretion. On the Earth, organic substances delivered during primary accretion would have been destroyed by the heat of impacts and the moon-forming event, so the carbon compounds necessary for the origin of life were necessarily added after the Earth had cooled sufficiently for a global ocean to appear.