The account of the earliest beginnings of life offered in the preceding chapters is, perforce, hypothetical. Except for a few scraps of information obtained from extraterrestrial objects and from laboratory simulation experiments, all the steps in my reconstructed history are inferred by reasoning and educated guesswork from what is known of present-day life. With due regard to the weakness of such an approach, it may be profitable to look back over the account and try to discern certain key notions likely to be of more general significance and applicable to any scenario that others may propose.

Overview

Figure 12.1 shows a schematic overview of the early development of life as I have hypothesized it. The diagram depicts three main stages. First there is abiotic chemistry, a term used to designate the mechanisms that have generated the raw materials of life. These include, on one hand, the organic building blocks manufactured by cosmic chemistry (meant to include possible terrestrial processes of the kind studied by Miller and his followers) and, on the other, the energy-bearing compounds, pyrophosphates and thioesters, taken to be products of volcanic chemistry (with the help of cosmic chemistry for thioesters).

The second stage is protometabolism, which is the name given to the set of processes that have led from abiotic chemistry to the third stage, or metabolism, defined as the first set of reactions catalyzed by protein enzymes (and, perhaps, ribozymes), prefiguring present-day metabolism and, perhaps, already including certain central systems, such as the glycolytic chain and the Krebs cycle.