ORIGIN OF LIFE 127 Figure The molecular structures of RNA and two molecules that have been suggested as possible precursors of RNA PNA (peptide nucleic acid) uses a simpler backbone based on peptide bonds, and TNA replaces the five carbon sugar ribose with the four carbon sugar threose All three molecules can use the same set of four bases (attached at the positions marked B) and can base pair with RNA or DNA enclosed by a membrane Other models propose that networks of reactions that are the forerunners of modern metabolic processes preceded the development of genes All such models present a number of problems, not least in explaining how the transition to an RNA world could occur (Figure 4) Suffice it to say that no model of a pre-RNA world has yet achieved any degree of consensus, and the problem of how to build an RNA world out of the available prebiotic material remains unsolved Sources of Prebiotic Organic Molecules The Miller experiment, described above, provided the first demonstration of how organic molecules of biological relevance might be produced in the atmosphere of the primitive Earth Similar experiments have produced a wide range of organic molecules in this way However, to achieve a substantial yield, a strongly reducing atmosphere (i.e one containing gases such as hydrogen, methane, and ammonia) is Figure Outline of a possible series of steps in the develop ment of life from prebiotic molecules up to the modern DNA RNA protein basis for biological chemistry required Current ideas suggest that the atmosphere of the early Earth was instead composed mostly of carbon dioxide and nitrogen (see Atmosphere Evolu-