Science of Discworld III

fishes not wanting to be dinosaurs but just living their little fishy lives, the dinosaurs wiped out by a meteorite. The monkeys and apes, having seen what it was like to be at the peak of evolution, are now just slowly dying out – except in zoos, where they are kept to show us what our progenitors used to be like. Humans now occupy the top branch of the tree of life: since we are perfect, there’s nowhere for evolution to go any more, which is why it has stopped.
    If pressed for more detail, we dredge up various things we’ve learned, mostly from newspapers, about things called genes . Genes are made from a molecule called DNA, which takes the form of a double helix and contains a kind of code. The code specifies how to make that kind of organism, so human DNA contains the information needed to make a human, whereas cat DNA contains the information for a cat, and so on. Because the DNA helix is double, it can be split apart, and the separate parts can easily be copied, which is how living creatures reproduce. DNA is the molecule of life, and without it, life would not exist. Mutations are mistakes in the DNA copying process – typos in the messages of life.
    Your genes specify everything about you – whether you’ll be homosexual or heterosexual, what kinds of diseases you will be susceptible to, how long you will live … even what make of car you will prefer. Now that science has sequenced the human genome, the DNA sequence for a person, we know all of the information required to make a human, so we know everything there is to know about how human beings work.
    Some of us will be able to add that most DNA isn’t in the form of genes, but is just ‘junk’ left over from some distant part of our evolutionary history. The junk gets a free ride on the reproductiveroller-coaster, and it survives because it is ‘selfish’ and doesn’t care what happens to anything except itself.
    Here ends the folk view of evolution. We’ve parodied it a little, but not by as much as you might hope. The first part is a lie-to-children about natural selection; the second part is uncomfortably close to ‘neo-Darwinism’, which for most of the past 50 years has been the accepted intellectual heir to The Origin . Darwin told us what happens in evolution; neo-Darwinism tells us how it happens, and how it happens is DNA.
    There’s no question that DNA is central to life on Earth. But virtually every month, new discoveries are being made that profoundly change our view of evolution, genetics, and the growth and diversification of living creatures. This is a vast topic, and the best we can do here is to show you a few significant discoveries and explain why they are significant.
    Just as physics replaced Newton by Einstein, there has been a major revolution in the basic tenets of biology, so we now have a different, more universal view of what drives evolution. The ‘folk’ evolutionary viewpoint: ‘I’ve got this new mutation. I have become a new kind of creature. Is it going to do me any good?’ is not the way modern biologists think.
    There are many things wrong with our folk-evolution story. In fact we’ve deliberately constructed it so that every single detail is wrong. However, it’s not very different from many accounts in popular science books and television programmes. It assumes that primitive animals alive today are our ancestors, when they are our cousins. It assumes that we ‘came from’ apes, when of course the ape-like ancestor of man is the same creature as the man-like ancestor of modern apes. More seriously, it assumes that mutations in the genetic material, the changes that natural selection has to work on – indeed, to select among – are checked out as soon as they appear, andlabelled ‘bad’ (the organism dies, or at least fails to breed) or ‘good’ (the animal contributes its progeny to the future).
    Until the early 1960s, that was what most biologists thought too. Indeed, two very famous biologists, J.B.S. Haldane and Sir Ronald Fisher, produced important papers in the mid-1950s espousing just that view. In a population of about 1000 organisms, they believed, only about a third of the breeding population could be ‘lost’ to bad gene variants, or could be ousted by organisms carrying better versions, without the population moving towards extinction. They calculated that only about ten genes could have variants (known as ‘alleles’) that were increasing or decreasing as proportions of