The Science of Discworld IV Judgement Da - By Terry Pratchett, Ian Stewart Page 0,60
the details in many sources, but for present purposes we just need convenient words to keep straight which bits we’re talking about.
The great trick that nucleic acids exploit is their ability to form double-chains, each half encoding the same ‘information’ in related ways. The DNA code letters, the four bases, come in two associated pairs, and the sequence of bases on one chain consists of the partners of the bases on the other chain. This makes possible the key feature of these pairings: each chain determines what happens in the other chain. If they split apart, and each chain acquires a new partner, by sticking on the complementary bases … lo and behold: originally we had one double-chain, and now we have two of them, each identical to the first. The molecule not only can replicate: it does, given enough unattached bases to play with. It would be hard to stop it.
RNA has other tricks. It can function as an enzyme, a biological catalyst; it can even be the catalyst for its own replication. (A catalyst is a molecule that promotes a chemical reaction without being used up: it gets involved, helps things along and then ducks back out.) And it can also catalyse other chemical reactions that are useful to life. It’s a universal fix-it molecule for living organisms. If it were possible to explain how RNA could appear in the absence of life, it would constitute a wonderfully useful step from non-living chemistry towards a primitive kind of life form. Unfortunately, it turned out to be very difficult to see how RNA could turn up in the primeval soup without any assistance. For many years, the RNA world theory was missing one of its most vital features.
This is no longer an obstacle. In recent years, many different solutions to this problem have been found, including several that work experimentally as well as theoretically. The chains of bases involved were initially fairly short – a chain of six is easy, but now there can be fifty or more bases in a chain. This is getting close to the length found in real biological enzymes, which usually have 100-250 bases. So there is real hope that long RNA chains must have been present in that ancient soup. More plausibly still, fatty membranes, which closely resemble cell membranes, have been synthesised in circumstances very similar to those that are thought to have existed on the primitive Earth, and RNA gets linked to these in useful ways. Moreover, it has recently been suggested that the RNA chains could repeatedly be broken apart – unzipped – by high temperatures in black smokers and reassembled at lower temperature in cycling convection currents. This is a lovely idea, exactly like the way DNA is multiplied in systems that analyse its sequence using the polymerase chain reaction, where alternating high and low temperatures break the chains apart and then permit them to build complementary chains, repeatedly doubling the number of copies. RNA could be replicated by this natural physico-chemical process.
For these reasons and many others, the RNA world has now become a respectable image for the earliest stages of life on Earth. It may not be what actually happened, but it provides a plausible scenario. And even if life did not arise in that way, the RNA world shows that there is no compulsion to invoke the supernatural. In the primitive seas, probably around smokers but perhaps on tidal beaches where pools were concentrated – and irradiated – in sunlight, and diluted when the tide came in, or under the influence of volcanoes or earthquakes, RNA strings were growing and being copied.
The copying process wasn’t always perfectly accurate, but that was a positive advantage, because it led, without any special interference, to diversity. If random variation of this kind could be coupled to some kind of selection, favouring sequences with specific features, then RNA could – had to – evolve. And selection wasn’t an issue; the big problem would have been to prevent it. As special sequences with particular properties appeared, competition between them for free nucleotides, and probably for interactions with particular fatty membranes, eliminated some sequences and encouraged others to proliferate. This led to longer chains with even more special properties.
Natural selection had begun … and the system was becoming alive.
In this view, not only does evolution by natural selection explain how life diversified: it is part and parcel of what brought it into being in the first place. Copying errors, if they occur,