6 Did the RNA World Exploit an Expanded Genetic Alphabet?

Abstract

SINGLE BIOPOLYMER LIFE FORMS BASED ON RNA
In terms of its macromolecular chemistry, life on Earth can be classified as a “two-biopolymer” system. Nucleic acid is the encoding biopolymer, storing information within an organism and passing it to its descendants. Nucleic acids also direct the biosynthesis of the second biopolymer, proteins. Proteins generate most of the selectable traits in contemporary organisms, from structure to motion to catalysis.

The two-biopolymer strategy evidently works rather well. It has lasted on Earth for several billion years, adapting in this time to a remarkable range of environments, surviving formidable geobiological (and perhaps cosmic) events that threatened its extinction, and generating intelligence capable of exploring beyond Earth.

The terrestrial version of two-biopolymer life contains a well recognized paradox, however, one relating to its origins. It is difficult enough to envision a nonbiological mechanism that would allow either proteins or nucleic acids to emerge spontaneously from nonliving precursors. But it seems astronomically improbable that both biopolymers arose simultaneously and spontaneously, and even more improbable (if that can be imagined) that both biopolymers so arose with an encoder-encoded relationship.

Accordingly, a variety of “single-biopolymer” models have been proposed as forms of life that antedated the two-biopolymer system. These (presumably) could have emerged more easily than a two-biopolymer system. Such models postulate that a single biopolymer can perform the catalytic and information repository roles and undergo the Darwinian evolution that defines life (Joyce 1994). For example, Rich (1962), Woese (1967), Orgel (1968), and Crick (1968) proposed that the...