The Science of Discworld IV

not perfectly. So the theory of the origin of life turned into a kind of primeval pizza, with molecules dotted around on a surface, rather than a primeval soup sloshing around in pools or the open sea. In 1999 we liked this idea because it was different from heredity-first systems: we couldn’t see why they would necessarily replicate – what was in it for them. Moreover, Wächtershäuser was a lawyer as well as a biochemist, and it’s unusual to get good original scientific ideas from a lawyer.
    However, since then a different idea, the RNA world, has really taken off. RNA and DNA are both nucleic acids, so named because they are found in the nuclei of cells. There are many other kinds ofnucleic acid; some are much simpler than DNA and RNA, and some are much more complicated. Both are long chains formed by joining together four smaller molecular units, called nucleotides. Nucleotides are combinations of bases, which in turn are specific molecules that look like complicated amino acids, linked together by sugars and phosphate. Does that help? We thought not. You can look up all 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 workexperimentally 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