Most atmospheric models generally predict a warm early Earth with high levels of CO2 or other greenhouse gases. In the absence of greenhouse warming, however, the Earth's oceans would have been frozen because of a 30% less luminous sun (62). Our kinetic data on the stability of the nucleobases indicate that a cold or frozen early Earth would be more favorable for the accumulation of the nucleobases and therefore for the origin of life. An early frozen Earth may have been melted numerous times as a result of a large meteor or comet impacts (63). However, very large impactors could boil the Earth's oceans. The rates of hydrolysis at 100°C, for all of the nucleobases measured, suggest that an ocean-boiling impact event would completely decompose the nucleobases in addition to a number of other biologically important compounds. This would require the whole prebiotic process to begin again. Ocean-boiling impacts therefore are more damaging to prebiotic chemistry than to an early biosphere (64-66), where the survival of a single organism (e.g., in a crustal environment) would be sufficient to reestablish the entire ecosystem. Other stability problems also point to a low-temperature origin of life and early evolution in the pre-RNA and RNA world. These include the stability of ribose (67), the decomposition of nucleosides (28, 68), and the hydrolysis of the phosphodiester bonds of RNA (23). Similar stability considerations would apply to any alternative pre-RNA backbone, e.g., peptide nucleic acids. All of these factors point to a low-temperature accumulation of organic compounds on the primitive Earth and a low-temperature origin of life. Therefore, atmospheric models suggesting a cool early Earth (
0°C) rather than a warm one (12, 13) need to be considered.
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