From replicators to reproducers: the first major transitions leading to life.

A classification of replicators is proposed: life depends on replicators that can exist in an indefinitely large number of forms (unlimited heredity), and whose replication is modular rather than processive. The first template replicators would have increased at a rate less than exponential, because of self-inhibition arising from molecular complementarity. The result would be the survival of a varied population of replicators, rather than the victory of one type. This variability was important, because inaccurate copying meant that individual replicators were small (Eigen's paradox). The origin of cooperation between replicators, and the problem of molecular parasites, are discussed. Today, cooperation depends on cellular compartments, and on the linkage of genes on chromosomes, but we argue that at an earlier stage surface metabolism, in which replicators react only with neighbours, was important. The origin of translation and the genetic code is discussed. The essential step is the binding of amino acids to specific oligonucleotides. We suggest that this binding originated, not as a step in protein synthesis, but in the formation of coenzymes in a metabolically complex RNA world. Existing organisms are not replicators (that is, new individuals do not arise by copying), but reproducers that contain replicators. We outline Griesemer's concept of a reproducer, which brings out the essential role of development in evolution.