The Science of Discworld IV

story. That’s what ‘explain’ means to
Pan narrans
. However, the stories scientists tell about the origin of life are generally difficult and complex, especially when it comes to filling in details. What happened probably can’t be turned into a story. Even if we could go back and see what happened, what we observed might not make a great deal of sense.
    Nevertheless, we can seek stories that provide useful insights.
    Most scientific thinking about the origin of life considers two phases: pre-biotic and biotic. Often the problem is simplified further, to inorganic chemistry before life appeared, and organic chemistry afterwards. These are the two great branches of chemistry. The latter concerns itself with the massive and complex molecules that can be formed using lots of atoms of carbon, and the former concerns itself with everything else. And life, as it now exists on Roundworld, makes essential and ubiquitous use of organic chemistry. However, there is no good reason to imagine that the origin of life fits neatly into this convenient but rather arbitrary pair of categories. Organic molecules almost certainly existed before there were organisms to use them. Sotrying to understand the origin of life as some kind of sudden switch from inorganic chemistry to organic chemistry is a mistake, confusing two different distinctions.
    Yes, there was a time before there was life, and a time when life was beginning. But there wasn’t a sudden origin like turning a light on. There was a period, perhaps quite a long one – hundreds of millions of years – of what has been called mesobiosis. This is chemistry, both inorganic and organic,
becoming
life: the journey, not the starting point or the destination.
    A large number of alternative pathways by which life might have originated have been proposed. In the 1980s Jack counted thirty-five plausible theories, and there must be hundreds by now. It is sobering to realise that we may never know which pathway actually happened. Indeed, this is quite likely. The pathway that occurred could well have been one of thousands that we haven’t yet thought of. For some of us, an account which starts in chemistry and finishes in simple biochemistry is sufficient; others will want to see bacterial-grade life produced artificially in the laboratory before being convinced that the sequence of steps can work. Yet others will want to see an artificial elephant, synthesised from chemicals in bottles, and would still insist that someone cheated.
    Many of you will be convinced that life is so different from the non-living, even from the freshly dead, that no account of a more or less continuous series of steps can be plausible. In part, this conviction arises from our neurophysiology: we use different areas of our brains for thinking about living or inorganic entities, mice or stones. So it is difficult for us to construct thought-chains leading from stones to mice, or even from school chemistry to ‘germs’. Instead, we come up with concepts like the soul, which makes a clear distinction between our thinking about a living person, and the very different way we think about a dead body.
    We’ll summarise some of the plausible accounts of life’s origins, so that you may enjoy the various ideas on offer and the different waysof thinking about the problem that they illustrate. We have written about the origins of life several times in the
Science of Discworld
series, so we will try to make this account a little different. The virus story at the end of this section, for instance, is quite new. It was sitting quietly behind the scenes around 2000, but in 2009 a review paper by Harald Brüssow opened it up for discussion. To put it in context, we need to look at some of the earlier proposals.
    The most important early experiment was that of Stanley Miller, working in Harold Urey’s laboratory in the 1950s. He imitated the effects of lightning on a reasonable approximation to Earth’s early atmosphere: ammonia, carbon dioxide, methane and water vapour. Initially, he got several noxious gases, like cyanide and formaldehyde, both notable poisons; this encouraged him, because ‘poison’ is not an inherent property – it describes an effect on living organisms. Most gases don’t get involved with life at all. Further runs of the experiment produced amino acids, some of the most important chemicals for life, because they aggregate into proteins. He came up with a variety of other small