The Science of Discworld IV

years’ time. There may be something left.’
    The Dean was not one to take no for an answer, and tried again. ‘Or, of course, innocence will prevail!’
    ‘Yes,’ said Rincewind gloomily. ‘It might. And
I
might prefer horses, but I suspect the automobiles are breeding faster …’

FOURTEEN
----
A BETTER MOUSETRAP

    Rincewind has a soft spot for horses – which unfortunately leave soft spots for everyone else. Even so, he prefers horses to automobiles. You don’t have to
make
a horse – they make themselves from previous horses.
    Cars are made individually, by people. They are designed to serve a purpose, which was present in the designer’s mind before the cars were made, and indeed
caused
it to be made. Without people, you could leave the Earth on its own for a billion years and it wouldn’t produce a car. But it did produce a horse, without human intervention, in a rather shorter period.
    Scientists believe that horses evolved. The proof includes an iconic series of fossils, showing exactly
how
they evolved, between 54 million and a million years ago. The sequence begins with a horse-like mammal a mere 0.4 metres long. This genus was originally given the poetic name
Eohippus
(‘dawn horse’), but has now been renamed
Hyracotherium
because of the rules of taxonomy, which in this case managed to deliver a silly result. fn1 It moves on to
Mesohippus
, 35 millionyears old and 0.6 metres long; then
Merychippus
, 15 million years old and 1 metre long; then
Pliohippus
, 8 million years old and 1.3 metres long; and finally (so far)
Equus
, essentially the same as the modern horse, 1 million years old and 1.6 metres long.
    Taxonomists can track, in great detail, the sequence of changes that occurred in this lineage of ancient horse ancestors; for example, in the animal’s teeth and hooves. They can also track the timing of these changes, because rocks can be dated. So now evidence from geology can be thrown into the mix. It would take only one fossil species in the wrong stratum of rock to cast doubt on the evolutionary story. The succession of rocks, their ages as determined by a variety of different methods, the evolutionary sequences of fossils, and the DNA of horses and modern relatives – all agree, to a remarkable extent.
    There is similar evidence that humans evolved from ape-like ancestors, but the story is not as neat and tidy, with many potentially coexisting species. These ancestral hominins evolved from other mammals, which evolved from reptiles, which evolved from amphibians, which evolved from fish. fn2 Rincewind knows how land animals evolved: he was there. Roundworld’s inhabitants weren’t, which is one reason why they argue about what happened.
    Both William Paley, in his
Natural Theology
of 1802, and modern creationists, believe that horses and humans were designed by God, and that these creatures’ modern forms are exactly those given to them at the moment of their creation. The hypothesis of intelligent design attempts to infer the existence of an unspecified cosmic designer (we all know who, but it would be unscientific to say so …) from the existence of complex structures in living organisms. Darwin argued that design in this sense is neither necessary, nor plausible: instead, living creatures evolved. Almost all biologists agree. Neo-Darwinism underpins those ideas with genes.
    Evolved, or designed?
    Maybe the difference isn’t as great as most of us think.
    When design is presented as an alternative to evolution, there is an unstated assumption that the two are very different. Design is a conscious process carried out by a designer who knows what result he, she, or it is aiming at, and whose purpose is to achieve it. Evolution selects, from a lot of random variants, changes that lead to some kind of improvement in survival prospects; then it makes lots of copies of the successes. It has no aims and no purpose. It is not ‘blind chance’, a description that creationists often use, forgetting (we are being charitable) the crucial element of selection. But the process of evolution is exploratory, not goal-seeking.
    On closer examination, however, design and evolution are much more similar than most of us imagine. Technology appears to be designed, but mostly it evolves. Improved technology is selected because it works better, and it then displaces earlier technology. This process is analogous to the way that natural selection causes organisms to evolve, so it is reasonable to