FIGURE 1.

A scheme describing the events that happen successively in the modeled system (A–C) and that might happen thereafter (D,E), which as a whole could be read as a proposed process for the origin of the RNA world. In each subgraph, new events are drawn in black, while old events are in gray. (A) Formation of nucleotides (L-shaped) from raw materials (dots) without ribozyme catalysis. (B) Nonenzymatic template-directed synthesis of RNA. The template molecules are synthesized by random ligations. (C) The emergence of a nucleotide synthetase ribozyme. The synthesized nucleotides around the ribozyme may be priorly used by itself in its replication (arrow). (D) The emergence of an amphiphilic molecule synthetase ribozyme, which promotes the formation of the protocell's membrane (arrow), thus limiting the diffusion of the synthesized nucleotides. The synthesized nucleotides may also be used by the amphiphilic molecule synthetase ribozyme (arrow). (E) The emergence of a template-directed RNA synthetase ribozyme, which speeds the replication of the RNA species in the protocell. The three ribozymes cooperate to facilitate the replication (reproduction) of the whole protocell.