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Where do we come from?: a humbling look at the biology of life's origin - includes related articles
Skeptical Inquirer, Sept-Oct, 1999 by Massimo Pigliucci
The general path leading to the origination of life seems to have been something like this:
1. Primordial soup (simple organic compounds formed from atmospheric gases with the aid of various sources of energy)
2. Nucleo-proteins (similar to modern tRNAs)
3. Hypercycles (primitive and inefficient pathways, emergent properties)
4. Cellular hypercycles (more complex cycles, eventually enclosed in a primitive cell made of lipids)
5. Progenote (first self-replicating, metabolizing cell, possibly made of RNA and proteins, with DNA entering the picture later on)
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How plausible is all this? It certainly is conceivable from the standpoint of modern biology. The problem is that each step is difficult to describe in detail from a theoretical standpoint, and so far (with the exception of the formation of organic molecules in the soup) has proven remarkably elusive from an empirical perspective. It looks like we have several clues, but the overall puzzle is proving to be one of the most difficult for scientific analysis to solve. The chief reason for such difficulty could be - as I mentioned earlier - that all we have is one example to go by. Or it may simply be that the events in question are so far remote in time that there is very little we can be certain about, making any attempt at empirical investigation hopelessly vague. Consider that the fossil record shows completely formed, "modern-looking" bacterial cells a few hundred million years after the formation of Earth - about 3.5 billion years ago. This tells us that whatever happened before that happened quickly, but there is no record of it. Finally, it could very well be that we are missing something fundamental here. It may be that the study of the origin of life has not yet seen its Einstein or Darwin, and that things are going to change just around the corner, or never.
From Dust to Dust...
A contemporary discussion of the question of the origin of life cannot be complete without the inclusion of A.G. Cairns-Smith's theory of clay crystals. I hope this will not be the case for much longer (except as a footnote of historical value). Don't get me wrong, I am familiar with Cairns-Smith's research and writing, and I find it excellent. But everybody can make a mistake, and I think the clay theory clearly falls within the cracks of Cairns-Smith's career, as ingenious and superficially enticing as it may be.
Briefly, the idea is that life didn't originate with either nucleic acids or proteins. The original replicators and catalyzing agents were actually crystals found everywhere in the clay that lay around the primitive Earth. There are four cardinal points of Cairn-Smiths hypothesis. First, crystals are structurally much simpler than any biologically relevant organic molecule. Second, crystals grow and reproduce (i.e., they can break because of mechanical forces, and each resulting part continues to "grow"). Third, crystals carry information and this information can be modified. A crystal is a highly regular structure, which tends to propagate itself (therefore, it carries information). Furthermore, the crystal can incorporate impurities while it's growing. These impurities alter the crystal's structure and can be "inherited" when the original piece breaks (hence, the information can be modified). Fourth, crystals have some minimum capacity of catalyzing (i.e., accelerating) chemical reactions.
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