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It is generally believed that the earliest progenitor of modern life was a set of chemical shapes or reactions that could reproduce itself. Although it is convenient to hypothesize that life began with a nucleic acid such as RNA, there is no a priori reason to believe that the earliest replicators were composed of compounds identical to those found in contemporary organisms. The difficulties inherent in forming nucleotide and oligonucleotide precursors under prebiotic conditions (Pace, 1991) have led many authors to propose that the earliest replicators may have been ‘unnatural’ nucleic acids that contained modified backbones or internucleotide linkages and replicated via complementary chemical shapes such as Watson-Crick base pairings (reviewed in Joyce, 1989; Ellington, 1993). However, while Watson-Crick style complementarity is clearly well-suited to copying chemical information, it is not necessary to envisage life. Wachtershauser (1988) has proposed a scenario in which the earliest replicators were chemical cycles, and Cairns-Smith (1982) has suggested that the first living systems were crystalline matrices, such as clays, that could grow and diversify. These non-Watson-Crick replicators were gradually replaced by nucleic acids in a ‘genetic takeover’ that led to modern life.