Clay hypothesis of the origin of life
A. Graham Cairns‑Smith formulated Clay hypothesis of the origin of life (Cairns-Smith 1982). He based this on the concept that the original structure that provided for transfer of information could have been a clay-type inorganic substance rather than an organic compound.
The microstructure of clay is formed by an irregular crystal, in which the individual series of silicate molecules lie above one another in regularly ordered layers. However, the overall structure of clay is in no way monotonous, as the layers copy the surface on which they lie and also contain a number of defects that are then copied in further layers of the molecule. The fact that the defects are thus copied ensures a certain mechanism of heredity. Clays containing various types of defects can be variously successful. Some grow and enclose further layers faster than other ones, some dry out faster and, after disintegration into smaller particles, can be readily dispersed by the wind and can thus “infect” other locations on which clays settle. A certain type of natural selection can thus occur between various types of clays.
Similar to nucleic acid in the genetic model of life, clays can also “learn” to cooperate with some other substances, for example with proteins, whose synthesis they can catalyze on their surface (Coyne 1985b; Ferris, Huang, & Hagan 1988; Ferris et al. 1996).
The Cairns-Smith hypothesis is certainly very interesting and inspiring; however, it is hard to imagine a way in which a transition could occur from a system of storing genetic information in a set of defects in pseudocrystalline clays to a system of storage of information in sequences of nucleotides in nucleic acids. The hypothesis also in no way resolves the problem of the evolution of the genetic code and proteosynthetic apparatus.