122 



If we take it as a fact that such clay-surface charge patterns are 

 replicable, and that changes in them can induce stable changes in 

 their progeny, it is hard to deny the self-replicating quality. Notice 

 one very substantial difference from the biopolymer model: replica- 

 tion in dimension is different. The clay particles replicate two- 

 dimensional patterns (the laboratory examples show that some 

 10 6 ionic sites are replicated) while the nucleic acids instead replicate 

 a linear sequence that is one dimensional. 



Naturally there is a very long path from such curious "living" 

 mineral particles to the cells we ascribe to or even find in the fossil 

 stromatolites. The gap can be closed only by hard work in the labo- 

 ratory and by new ideas. But it is interesting to put forward an opti- 

 mistic, if vague, scenario about how the scheme might have gone 

 forward in the gap between 4.5 and 3.5 or 3.8 b.y. ago. Of course, 

 this is meant only as a pedagogical example. 



One such scenario would go as follows. Once the early Earth 

 had well-differentiated into core, mantle, and crust, the atmosphere 

 would be mainly carbon dioxide, nitrogen, and water, with some 

 minor constituents like hydrogen sulfide. This atmosphere would not 

 give rise to a soup of monomers, even locally. The interaction 

 between atmosphere, sea water, and the silicious, iron-rich crust of 

 the Earth, would lead rather to copious formation of clays. The iron- 

 rich clays replicated during many cycles of inundation and dryness, 

 mutated, and began to fix carbon dioxide photochemically, using 

 solar UV and ferrous ion. This could lead to sugars, to the citric-acid 

 cycle, and even to fatty acids. In a later stage, the fixation of molecu- 

 lar nitrogen occurred as well, and the surface formation of amino 

 acids and nucleotides became possible. The evolving clays began to 

 polymerize these surface monomers. In this system, the nucleic 

 acids became coupled to the polypeptides through a genetic code. 

 From this complex surface-borne system, a newly self-enclosed sys- 

 tem based on nucleic acids and proteins began a new and indepen- 

 dent evolution of its own, free of solid substrate — a protocell with 

 its membrane consisting of some lipid-rich layers. 



It is evident that in such ideas we have the beginnings of a rich 

 and promising experimental campaign which is complementary to 

 the search for coupling between the simpler biopolymers held in 

 solution. 



