The transition from abiotic to biotic chemistry: When and where?

The origin of life on Earth was marked by the transition from purely chemical reactions to autonomous self-replicating molecules capable of evolving by natural selection into ones of increasing efficiency and complexity. Two views on how this happened are presently popular (1): A) organic compounds in the primordial oceans, derived from "home grown" synthetic reactions and the infall of organic rich materials from space, underwent polymerization which produced increasingly complex molecules, some of which by chance where capable to catalyzing their own self-replication; and B) a primitive type of "metabolic life" characterized by a series of self-sustaining chemical reactions based on organic compounds made directly from simple constituents arose in the vicinity of mineral-rich hydrothermal systems. In the first scenario, organic compounds would need to be concentrated in order to polymerize. This could be accomplished by absorption onto mineral surfaces followed by polymerization, a process that has been demonstrated in the laboratory. Since absorption onto minerals involves the formation of weak non-covalent bonds, it would be most efficient at cool temperatures. Concentration could also be accomplished by evaporation of shallow water deposits, such as tidal lagoons, and by eutectic freezing of seawater, which could have taken place if the early Earth was extensively ice covered. Low temperatures are also favorable for the survival of organic compounds and thus the "primordial soup" origin of life scenario would most likely have taken place if the early Earth was chilly rather than boiling hot. Because of the reduced luminosity of the Sun, the early Earth may have been totally ice covered during its early history and it was under these conditions the first self-replicating molecular entities originated from the prebiotic mix of organic compounds. The second scenario could have conceivably taken place in any type of environment as long as the reactant/product molecules survived long enough to be part of the reaction chain although most researchers who have advanced this scenario favor hydrothermal temperatures. Of the various reactions that have so far been proposed and investigated none have been demonstrated to be autocatalytic. In addition, the reactions are probably not unique to hydrothermal temperatures and would also occur at lower temperatures albeit at slower rates. Based on the estimated Arrhenius activation energies for the synthesis/decomposition reactions of the reactant/product molecules it is likely that they would have been more favorable at lower temperatures. This stability argument is especially important as the autocatalytic reactions advanced to the point of synthesizing informational molecules such as nucleic acids which have very short life times at elevated temperatures. Thus even "metabolic life" as it evolved into biochemistry as we know it would likely only have been feasible if the early Earth was cool. If the transition from abiotic chemistry to biochemistry on the early Earth indeed required cool temperatures, the transition could have occurred during cold, quiescent periods between large bolide impacts. The first life that arose, regardless of the process, may not have survived subsequent bolide impacts, however. Life may have originated several times before surface conditions became tranquil enough for periods sufficiently long to permit the survival and evolution of the first living entities into the first cellular organisms found in the fossil record 3.5 billion years ago. 1. C. Wills and J. L. Bada, 2000. "The Spark of Life: Darwin and the Primeval Soup" (Perseus Publishing, Cambridge MA) 291 pp.