Figure 8

Autocatalytic system of birds and nests. (a) Birds as simple catalysts of nests. No nest is needed to produce new birds. (b) The nesting bird phase can only continue if there is a nest present (possibly one that was built by the bird, as it is pictured here). Small circle indicates point where a nest is obligatory. (c) Formal version of (b), with A₁ as the bird, A₂ the nesting bird and B the nest. Note: Figure 7(b) is equivalent to (b) and (c) if E is an obligatory catalyst of RNA.