The Science of Discworld by Terry Pratchett
Author:Terry Pratchett
Language: eng
Format: mobi, epub, pdf
ISBN: 9780804168953
Publisher: Knopf Doubleday Publishing Group
Published: 2014-06-02T18:30:00+00:00
TWENTY-FOUR
DESPITE WHICH …
THAT BLUE IN THE ROUNDWORLD SEA isn’t a chemical – well, not in the usual ‘simple chemical’ sense of the word. It’s a mass of bacteria, called cyanobacteria. Another name for them is ‘blue-green algae’, which is wonderfully confusing. Modern so-called blue-green algae are usually red or brown, but the ancient ones probably were blue-green. And blue-green algae are really bacteria, whereas most other algae have cells with a nucleus and so are not bacteria. The blue-green colour comes from chlorophyll, but of a different kind from that in plants, together with yellow-orange chemicals called carotenoids.
Bacteria appeared on Earth at least 3.5 billion years ago, only a few hundred million years after the Earth cooled to the point at which living creatures could survive on it. We know this because of strange layered structures found in sedimentary rocks. The layers can be flat and bumpy, they can form huge branched pillars, or they can be highly convoluted like the leaves in a cabbage. Some deposits are half a mile thick and spread for hundreds of miles. Most date from 2 billion years ago, but those from Warrawoona in Australia are 3.5 billion years old.
To begin with, nobody knew what these deposits were. In the 1950s and 1960s they were revealed as traces of communities of bacteria, especially cyanobacteria.
Cyanobacteria collect together in shallow water to form huge, floating mats, like felt. They secrete a sticky gel as protection against ultraviolet light, and this causes sediment to stick to the mats. When the layer of sediment gets so thick that it blocks out the light, the bacteria form a new layer, and so on. When the layers fossilize they turn into stromatolites, which look rather like big cushions.
The wizards haven’t been expecting life. Roundworld runs on rules, but life doesn’t – or so they think. The wizards see a sharp discontinuity between life and non-life. This is the problem of expecting becomings to have boundaries – of imagining that it ought to be easy to class all objects into either the category ‘alive’ or the category ‘dead’. But that’s not possible, even ignoring the flow of time, in which ‘alive’ can become ‘dead’ – and vice versa. A ‘dead’ leaf is no longer part of a living tree, but it may well have a few revivable cells.
Mitochondria, now the part of a cell that generates its chemical energy, once used to be independent organisms. Is a virus alive? Without a bacterial host it can’t reproduce – but neither can DNA copy itself without a cell’s chemical machinery.
We used to build ‘simple’ chemical models of living processes, in the hope that a sufficiently complex network of chemistry could ‘take off’ – become self-referential, self-copying – by itself. There was the concept of the ‘primal soup’, lots of simple chemicals dissolved in the oceans, bumping into each other at random, and just occasionally forming something more complicated. It turns out that this isn’t quite the way to do it. You don’t have to work hard to make real-world chemistry complex: that’s the default.
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