Several hypotheses argue that RNA was the only genetically encoded component of biological catalysis during an earlier episode of life on Earth. Some others view that statement as true for the very first form of life on Earth (the RNA-first hypothesis). Others have argued that the first form of life on Earth was supported by genetic molecules that had structures quite different from the structure of DNA or RNA.
Some have even argued that the original genetic material was mineral, not organic.1,2 They suggest that a truly primitive replicator might have been a layered inorganic mineral, crystallizing from solution and in the process amplifying some particular permutation of stacking: either identical layers stacked on top of each other in different orientations or stacks of two or more chemically different layers. The “information” would be the particular stacking sequence of a crystal displayed like a bar code on its edges and maintained and extended through crystal growth with ions, or small molecular units, adding only to the edges. The stacking sequences would also specify particular phenotypic properties that would allow Darwinian competition.
As is evident in Section 5.7, a case can be made that the earliest forms of life on Earth contained no macromolecules at all and that heredity was carried by monomers3—still another route for future exploration.
It has been more than 50 years since Stanley Miller first explored electrically induced chemical reactions that might convert simple gases into small organic molecules.4 The production of amino acids was especially easily demonstrated. More recently, the highly reducing atmosphere used by Miller has fallen out of favor as representative of the likely atmosphere on early Earth (although Kasting has shown that the impact of a large asteroid with iron causes a transient reducing atmosphere5). Even with more contemporary models of early planetary atmospheres, however, electrical discharge, ultraviolet radiation, and other sources of energy are suitable for creating organic species. For example, Box 5.1 lists compounds, called “tholins,” produced from relatively oxidizing environments under these conditions.
Organic Compounds Identified in Tholin Mixtures
SOURCE: Derived from Sagan, C., Khare, N.B., Bandurski, L.E., and Batholomew, N., 1978, Ultraviolet-photoproduced organic solids synthesized under simulated Jovian conditions: Molecular analysis, Science 199:1199-1201; Sagan, C., and Khare, N.B., 1979, Tholins: Organic chemistry of interstellar grains and gas, Nature 277:102-107; and Pietrogrande, M.C., Coll, P., Sternberg, R., Szopa, C., Navarro-Gonzalez, R., Vidal-Madjar, C., and Dondi, F., 2001, Analysis of complex mixtures recovered from space missions: Statistical approach to the study of Titan atmosphere analogues tholins, J. Chromatogr. A. 939:69-77.