Primitive Earth Environments: Organic Syntheses and the Origin and Early Evolution of Life

Abstract

The Precambrian is the longest and least understood period in the history of our planet. There is no direct evidence which bears on the primitive Earth environment, the synthesis of organic compounds and the origin of life on our planet, since the geological record only goes back to 3.8 Ga ago. To overcome this deficiency it is necessary to extrapolate backwards from 3.8 Ga ago to the time when the Earth was formed, ~4.6 Ga ago, and also make use of additional scientific approaches. These include models of the solar nebula and planet formation, comparative observations with other cosmic environments, and experimental simulation studies under plausible primitive Earth conditions, particularly in regard to the prebiological synthesis of organic compounds. A large number of interstellar organic molecules have been detected by radioastronomy in the interstellar medium, particularly in places where other solar systems may be in the process of formation. It is remarkable that ~10 of these molecules are known to be the precursors of the most important biochemical monomers. Furthermore, some of these monomers, e.g., amino acids, purines and pyri-midines, have also been detected in carbonaceous chondrites, which were presumably synthesized on the parent bodies of the meteorites or possibly in the solar nebula. Comparative studies on the terrestrial planets indicate that their primitive atmospheres resulted primarily from outgassing of CO 2 and other partially oxidized (CO, N 2 ) and reduced (H 2 , CH 4 , NH 3 , H 2 , S) volatiles. Furthermore, comets and meteorites are thought to have made a significant contribution to the reduced carbon precursors for the prebiological synthesis of biochemical compounds. A brief review of the experiments carried out in the laboratory under possible primitive Earth conditions shows that amino acids, purines, pyrimidines, monosaccharides, fatty acids and other compounds are readily obtained using different energy sources. However, the formation of oligopeptides, oligonucleotides and phospholipids requires more restrictive conditions. They have been synthesized by means of condensing agents, such as cyanamide, as well as by heating under relatively dry conditions of less than 100° C. Experiments have been carried out on the formation of liposomes from prebiotically synthesized phospholipids. Also, using 2-methylimidazolide derivatives of nucleotides, the template directed synthesis of polynucleotides has been demonstrated. In the latter experiments a high fidelity of complementary base pairing has been observed, which if further improved would approach that of enzyme catalyzed replication. However, it is not known how the prebiologically synthesized polymers became organized into a self-replicating system. At any rate the first living systems were probably very rudimentary heterotrophic micro-organisms which used organic compounds from the environment for both cell components and energy. They were probably followed by fermenters and eventually by autotrophic CO 2 reudcing micro-organisms similar to the methanogenic and photo-synthetic bacteria.