The Science of Discworld Revised Edition

that are halfway between feathers and scales, even though their remote ancestors had scales all over. Probably … we can’t be sure from those very old fossils. Scales are, and probably were in those ancestors, patches of keratin very like fingernails; sometimes they overlap like roof tiles. Feathers are cylinders sitting in follicles, deep pits in the skin. About a millimetre up from their deep end, they have a ring of dividing cells, called the collar, which produce the cylinder by growing it outwards. As the products move up the follicle, they turn all the cells’ productivity into making keratin, the protein of horn, nails, hair and feathers. And the cylinder wall becomes horny in a strange pattern.
    The side of the feather facing backwards on the bird produces creases that pass around on both sides of the cylinder toward the front-facing side, diving into the follicle so that they are almost parallel with the length of the cylinder. They don’t quite meet, and the tissue between their deep ends will become the stalk of the feather. The other side splits open, and the barbs of the feather – between the creases – unfurl to make the feather vane. They are much longer than the cylinder’s circumference, so a narrow pin-feather can generate a feather with a broad vane.
    Not a bit like a scale. And far more complicated. Evolution had to work hard to come up with feathers.
    And they must have evolved for a good reason, because lots of dinosaurs had feathers in various versions. Some were like down-feathers, others more like paintbrushes or feather dusters. They could have evolved for warmth. Adult velociraptors and young tyrannosaurs may have been covered in down, like baby chicks. Palaeontologist Mark Norell says that ‘We have as much evidence that velociraptors had feathers as we do that Neanderthals had hair ’. But other scientists disagree.
    Perhaps feathers were sexual ornaments. More likely, we haven’t yet thought of whatever function they had.
    So much for stories that ‘some reptiles got feathers and became birds’.
    There’s a very general problem here, and it’s the problem the wizards are having all the time. The overall pattern of evolution, of the birds say, or of tyrannosaurs, looks very sensible. But at a deep level, deeper than the ‘common-sense story’ that the wizards are trying to use for Roundworld, we don’t
understand
it. We have stories to explain it, all right, but that isn’t what understanding means.
    In fact, we’re not altogether sure what it does mean, in a scientific sense. We know that apparently quite ordinary historical events ‘come to pieces’ when you try to analyse them from several directions. John F. Kennedy’s assassination is a perfect example: the bullets seem to have come from different directions, and there doesn’t appear to be one consistent story that means we can ‘understand’ what happened. We can describe the events, but the underlying causalities, like the physicists’ quantum theory and relativity theory, don’t match up.
    Evolution doesn’t just happen to one creature at a time. The entire ecosystem evolves, and as it does so, new tricks may become worth using, for a limited period of time and in a limited region of geography. A few of those tricks turn out to have useful effects that are quite different from the ‘reason’ why they evolved to begin with, and those effects may continue to be worth having long after the original reason for their appearance has ceased to apply.
    It’s not surprising, in that frame, that historical events as far back as the origin of feathers, or of birds, don’t make detailed sense either. That’s why we can’t imagine what it was like in the Late Cretaceous.
Walking with Dinosaurs
was beautifully made and based on up-to-date science, but ultimately it was unconvincing. The need to tell a story distorted what was actually known, and mixed it up with guesswork and wishful thinking. (You can’t be sure what colour an animal’s skin was when all you’ve got is fossil bones. And assuming it was a bit like something vaguely similar that’s alive today is cheating, not science.) Anything televisual was automatically preferred to some more mundane scenario. So we got a dinosaur soap, with everything over-dramatised.
    So we can’t imagine ourselves walking with dinosaurs. Neither, to be fair, can we imagine ourselves running from them. We simply can’t understand what it was like to be there, much