For phoodoo. To discuss the Origin of Life.
Just to offer my own thoughts on the matter, as a red rag for phoodoo’s contempt, I think that all theories that require some kind of ‘takeover’ of one genetic system by another are dead in the water. That includes the Cairns-Smith ‘dust’ notion, but also ‘proteins-first’ theories.
The fundamental characteristic of life is replication. For evolution to occur, it further needs to have the capacity for exponential replication. The ‘Darwinian’ arena, which we should extend to include the zone of neutrality, involves competitive replication in a finite container. If the exponent is less than 1, replication will fizzle out. If greater than 1, it will fill the container, at which point the net exponent becomes, approximately, 1 – a steady state. Novel variants may arise with an exponent greater than 1, which means that, relatively speaking, the resident now has an exponent of less than 1. The new one will tend to supplant the old, until back at a net 1 again.
Now, there is a fundamental characteristic of the nucleic acids that renders them exponential replicators: complementarity. One double strand can generate 2 identical molecules. This is the basis of PCR, whereby a few rounds of replication can generate millions of copies of a single molecule. And of course, this gives the fundamental motor to organic increase, be it bacteria in a chemostat, the cells of a body, or clouds of midges.
Nucleic acids are dual-function molecules – not only does one part specify its entire complement, it also specifies, in exactly the same way, any fragmentary complement. A single base has optimal affinity for its complementary base, permitting template polymerisation, but a sequence of bases also has physical affinity for the complementary sequence. This includes binding RNAs that control gene expression, hairpin folds in a single strand, and also tRNA anticodons. The latter permits repeat specification. One can copy a strand faithfully, generating millions of copies, and from each of those copies produce the exact same protein. This is not available to ‘proteins-first’, for example. By lucky accident one may produce a useful peptide, but only one. This cannot be a replicator, and so cannot evolve. And even if it could, it does not seem plausible that a completely different kind of molecule could subsequently ‘take over’ the genetic function.
For such reasons, I tend towards ‘RNA-first’ theories of origins. There has been a ‘takeover’ of sorts, DNA for RNA, but DNA is merely a minor variant of RNA. Its lack of a 2′ oxygen, and its use of methylated uridine (ie, thymine) serve to stabilise the double strand, making it more useful as an information store and less useful as a catalyst. There has also been a ‘takeover’ of catalytic function (largely) by protein. But the genetic function, I think, traces right back to the earliest beginnings.
One is left with major problems, of course. Where did the nucleotide bases come from? How did they polymerise? How did complementary bases become stabilised out of a more complex mess? How was the vital coupling with cyclic energy generation mediated? How did replication commence?
The RNA World has many critics, and difficulties. Nonetheless, its central evolutionary logic renders it my favoured bet. Life is replication, at its very core, and nucleic acid displays a potential continuity in that regard that is strongly suggestive that its role is fundamental.