Summing up: The Seven Clues
Cairns-Smith (A.G.)
Source: Cairns-Smith - Seven Clues to the Origin of Life - A Scientific Detective Story
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  1. First clue: from biology
    • Genetic information is the only thing that can evolve through natural selection because it is the only thing that passes between generations over the long term. Although held in a genetic material, genetic information is not itself substance. It is form. But it is a sort of form that can outlast substance, because it is replicable. Evolution can only begin once there is this kind of form — when the conditions exist for the replication of genetic information.
    • This first clue was by far the most important. It directed our attention to the real issue: it set the question. And it was to suggest an answer, if only in general terms, as to what the very first organisms must have been like. They must have been ‘naked genes', or something close to that. This clue first appeared early on — in chapter 2.
  2. Second clue: from biochemistry
    • DNA is a suburban molecule far from the centre of the present biochemical pathways. The same can be said of RNA. Biochemically as well as chemically these are evidently difficult molecules to make: it takes many steps to manufacture even just their nucleotide units from the simpler central molecules of biochemistry. All this suggests a comparatively late arrival for these now undisputed rulers.
    • The second clue seemed to be in conflict with the first, which had clearly implied a working genetic material from the start. (The resolution of this conflict was to be the way forward.) The second clue was a long time in coming; there had been some detailed facts to be presented, and two red herrings to be removed first. Foreshadowed in chapter 6, this clue finally appeared towards the end of chapter 7.
  3. Third clue: from the building trade
    • To make an arch of stones needs scaffolding of some sort; something to support the stones before they are all in place and can support each other. It is often the case that a construction procedure includes things that are absent in the final outcome. Similarly in evolution, things can be subtracted. This can lead to the kind of mutual dependence of components that is such a striking feature of the central biochemical control machinery.
    • This third clue alerted us to the likelihood of a missing agent, an earlier 'scaffolding' — an earlier design of organism at the start of evolution. And it seemed very possible that these first organisms would have been based on a genetic material no longer present at all in our biochemistry. The clues were coming thick and fast now: this one early in chapter 8.
  4. Fourth clue: from the nature of ropes
    • None of the fibres in a rope has to stretch from one end to the other, so long as they are sufficiently intertwined to hold together sideways. The long lines of succession that alone connect us to distant ancestors are like multi-fibred ropes in that what are passed on between generations are collections of genes ('intertwined' because they correspond to viable organisms and it is thus in their mutual interest to stay together). But new 'gene fibres' may be added and others subtracted without breaking the overall continuity.
    • This fourth clue was about ways and means. It suggested to us how organisms based on one genetic material could gradually evolve into organisms based on an entirely different genetic material. This was the central clue, in more ways than one. It appeared in the middle of chapter 8.
  5. Fifth clue: from the history of technology
    • Primitive machinery is usually different in its design approach (and hence in materials of construction) from later advanced counterparts. The primitive machine has to be easy to make from immediately available materials; and it must work, more or less, with minimum fuss. The advanced machine simply has to work well, but it does not have to be particularly easily assembled: it can be made from diverse specialist components working in collaboration — and usually is.
    • This fifth clue led us to suspect that the first, unevolved (necessarily 'low-tech') organisms would have been very different from the (manifestly 'high-tech') organisms of today. Most probably their materials of construction would have been very different too. This clue appeared late in chapter 8.
  6. Sixth clue: from chemistry
    • Crystals put themselves together, and in a way that might be suitable for 'low-tech' genetic materials. Even the most primitive kind of gene-printing process would have to be fairly precise and involve the coming together of a fair number of atoms. Big organic molecules show little sign of having the appropriate self-control. On the other hand there are several cases where the replication of complex information can be imagined as taking place through crystal growth processes.
    • The sixth clue gave a sense of direction to our search for primitive biochemical materials. The significance of this clue emerged gradually through chapter 9 — and it took much of the rest of the book to develop. It first came into sharp focus at the start of chapter 10.
  7. Seventh clue: from geology
    • The Earth makes clay all the time, as you can see from the huge amounts of it that are carried in rivers. The minerals of clay are tiny crystals that grow from water solutions derived from the weathering of hard rocks. Not only for primitive genes, but also for other primitive control structures such as 'low-tech' catalysts and membranes, these kinds of inorganic crystals seem to be much more appropriate than big organic molecules.
    • The seventh clue depends for its significance on all the others. It is certainly no new idea that this most earthly of materials, clay, should have been the stuff of first life — it is in the Bible. What is new is our understanding of just how interesting, varied and complicated this sort of stuff is when looked at under a super-powered magnifying glass. The seventh clue appeared in chapter 11.
  8. These, then, are my seven best clues to the origin of life. Only the first could be said to represent an important insight in itself (and it is by no means new); and only the second is at all technical. The others are commonplace. But then as Holmes said:
    … 'The interplay of ideas and the oblique uses of knowledge are often of extraordinary interest.’

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