Fig. 3

A putative RNA world. The three domains of modern life, eubacteria, archaebacteria, and eukaryotes, clearly have a last common ancestor that was already metabolically complex. The chemical nature of the “metabolic fossils” conserved between these domains suggests that this antecedent arose from a RNA world in which ribozymes were the principle catalysts. The likely extinction of many early molecular lineages obscures any attempt to identify one or more origins of life