3 Collective Autocatalysis

The basic notion of an ``autocatalyst'' is a molecular species which, in the presence of suitable reaction substrates, catalyses a reaction in which one or more molecules of that same catalyst molecular species are among the reaction products--over and above the original catalyst molecule itself. Note carefully the distinction here between the original catalyst molecule which--by definition--emerges unchanged from the catalysed reaction; and the one or more additional catalyst molecules which are among the reaction products.

If we assume that the uncatalysed reaction occurs at negligible rate then, in the absence of any catalyst molecules, the substrates will remain unreactive; but if the medium is seeded or innoculated with one or more catalyst molecules this would then trigger production of more catalyst so that the reaction would actually proceed at an accelerating rate until limited by the availability of the substrate. If the reaction takes place in a flow reactor, where substrate is continuously provided, and the reaction product removed, then the reaction could evidently sustain itself indefinitely.

Autocatalytic molecules may also be termed ``self-replicating'' in a reasonable sense. This has an obvious biological relevance in the exploitation of such molecules as genetic information carriers. This usage depends on the existence of a large set of distinct molecules (which can thus carry ``information'' via their distinctions) but which are all individually autocatalytic, thus allowing for the generation of informational ``copies'' for distribution to offspring.

The autocatalytic molecules which play this role, in modern organisms at least, are the nucleic acids. Note, however, that nucleic acids are themselves rather complex; and require complex substrates (nucleotides) and additional catalysts (replicases) in order to manifest their autocatalytic activity. So while autocatalytic molecules, per se, can be very useful and play a key role in modern organisms, it is doubtful whether such molecules could participate in the spontaneous emergence or origin of life.

In response to this, Stuart Kauffman has proposed that, while individually autocatalytic molecules might be rather unlikely to arise (or even sustain themselves) spontaneously, the same is not necessarily true for sets of molecular species which mutually catalyse each others production. The idea here is to consider sets of molecular species such that none of them need be individually autocatalytic (which may be an unfeasible requirement), but where production of every element of the set is effectively catalysed by at least one other member of the same set. In such a situation the whole set is said to be collectively autocatalytic.

In just the same way as sketched out for an individually autocatalytic molecular species, we can imagine seeding a flow reactor with a collectively autocatalytic set of species (at least one molecule from each member of the set). Again, provided we maintain a flow of some basic substrate molecules (the ``exogenous food set'' in Kauffman's terms) there should now be the possibility of an indefinitely sustained, self-reinforcing, reaction network, continuously producing more molecules of all the species in the set.

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Timestamp: 2003-03-28