Inorganic protolife

Which came first, the living state or the molecules (proteins, nucleic acids and membrane components) on which it depends? Nearly everyone would say that the molecules came first and the living state somehow arose from them. But alternative is logically possible. The first living things might have been made of entirely different materials, which were subsequently replaced by nucleic acids and proteins.

Cairns-Smith drew attention more than 30 years ago to the "replicating" properties of certain kinds of clay. Kaolinite, for example, forms large flattish crystals that stack like playing cards. Defects formed in a kaolinite crystal when one atom is replaced by another can be replicated: a new crystal formed adjacent to it repeats the defect, maintaining the same overall shape. Thus, kaolinite behaves superficially like DNA. It replicates itself, so long as it is supplied with the right ingredients; and it passes on its acquired defects to subsequent "generations". However, although kaolinite's crystal structure is quite complicated by mineral standards, it is incomparably simpler than DNA and therefore far more likely to form by ordinary physical and chemical processes. It was probably common on the prebiotic Earth.

Cairns-Smith suggests that life began as a replicating clay mineral system. Many clay minerals bind organic molecules such as nucleic acid bases and amino acids, which profoundly alter their properties. Montmorillonite, for example, becomes soft and pliant in the presence of some organic compounds but hard and brittle in the presence of others. At the same time, the clays catalyse reactions among the bound organic molecules. Under some conditions, they might catalyse polymerisation. Over millions of years, suggests Cairns-Smith, organic components attached to the replicating mineral system became steadily more complex and played an increasingly important part in the replication process, until they replaced the original clay mineral altogether. He calls this hypothesis "genetic takeover".

The idea is attractive for a number of reasons. It deals directly with the most profound question of all: how did undirected physico-chemical processes generate the inevitably high complexity and "semantic content" of the first living organism? It places the origin of life in a solid rather than a liquid environment, which is consistent with the production of order - and cells are certainly ordered. The minerals required were probably abundant on the primitive Earth. The hypothesis is chemically plausible. Some commentators have observed that Martian dust clouds are rich in Montmorillonite. Most of all, the idea is attractive because it encourages us to separate the question of life's origins from the chemistry of life as we know it today. We are not obliged to accept the intuitive "molecules first, cells after" approach.

However, nothing in Cairns-Smith's work suggests that his clay systems could have come close to a living state as we characterised it in chapter 10. Could a structure of kaolinite or Montmorillonite (with or without organic additives) have had reciprocally dependent internal structure, metabolism, and internal transport processes - an internal state - however rudimentary? Could it have responded in organised ways to external stimuli or exhibited anything analogous to control of gene expression? Even if the answers are "No" (which they probably are), the Cairns-Smith model might still help to explain how replicating nucleic acids came into being; but unless the answers are "yes", we cannot regard the proposed clay mineral structures as "living".

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