The Science of Discworld IV Judgement Da - By Terry Pratchett, Ian Stewart Page 0,52

the storm. We want our own special type of narrativium: stories that explain how such things get started, as well as how they work. This wish for simple stories makes us expect simple answers to questions about origins. However, science shows us that our love of stories misleads us. Origins are extremely tricky concepts.

The acorn and the oak have a superficially simple story, which we all understand: plant the acorn, water it, give it light, and it grows into the oak. However, that simple story cloaks a really difficult explanation of an immensely complex development: it is, in fact, much the same account as getting you from an egg. And there’s another complication: not only does the oak come from the acorn: the acorn’s origin is the oak. This is exactly like the chicken and egg cliché. The important question, though, is not ‘which came first?’. That’s a silly question, because they are both part of the repeating system. It’s clear that the chicken is only the egg’s way of making another egg. Before chickens, the same egg lineage used jungle fowl instead to make more eggs; long before that, it used little dinosaurs to make its eggs; and long before that, it used ancient amphibians.

The big problem with ‘turtles all the way down’ as an explanation is not the ludicrous mental image, amusing though that may be. Each turtle is indeed supported by the one beneath. The problem is how and why an entire infinite pile of turtles should exist. What matters in recursive systems is not which part came first, but the origin of the whole system. For eggs and their chickens, that story is mainly an evolutionary one, a sequence of developments that change progressively, so that now we have chickens when previously we had jungle fowl or dinosaurs. In this case, the origins of the system go all the way back to the first eggs, the first multicellular creatures that used embryonic development from eggs as part of their reproductive process. In that same way, the acorn is the modern version of a seed that used to produce early seed plants, and prior to that produced tree-ferns … all the way back to the origins of multicellular plants.

What we mean by ‘immensely complicated development’ also takes a bit of explaining. It’s clear that the acorn doesn’t become the oak tree, any more than the egg that generated you became you. The oak tree is mostly made from carbon dioxide extracted from the air, water from the soil and minerals, including nitrogen, also from the soil. In trees, those ingredients mainly make carbohydrates, cellulose and lignin, along with proteins for the working chemical machinery. The amount of material contributed by the acorn is minuscule. Similarly, almost all of the baby that (in a very restricted sense) became you, was built from a variety of chemicals obtained from your mother through the placenta. The tiny egg contributed very little by way of materials … but an awful lot by way of organisation. The egg functioned to recruit the chemicals that your mother provided, initiating and controlling the succession of stages – blastocyst, embryo, fetus – that led to your birth. Similarly, the acorn is already an embryo, and it has a very complex organisation, beautifully crafted to drive a root down into the soil, to extend leaves up into the air, and to start the business of becoming a tiny oak.

It’s that word ‘becoming’ that we all have trouble with. Jack, on a hospital ethics committee, once had to explain how an embryo → fetus → baby → becomes human. It’s not like switching on a light, he explained; it’s more like painting a picture, or writing a novel. There isn’t one paintbrush-stroke or one word that completes the task; it’s a gradual becoming. ‘That’s fine,’ a lay member of the committee replied, ‘but how far into a pregnancy is it before you have a human being, not just an egg?’ We seem to need to draw lines, even when nature fails to present us with tidily distinguished stages.

So let’s not start with complex development, like acorns and eggs, when thinking about origins. Let’s start with something genuinely simpler: a thunderstorm. Before the storm, there is a time of cool, clear skies, clouds moving with the wind, probably a weather front. What we don’t see, because it’s invisible, is the static electricity building up in the clouds. Clouds are masses of water droplets, billions of tiny spheres of