The Science of Discworld Revised Edition

different from the DNA of a modern cockroach. Your genes have to run very fast in order for your body to stand still.
    The general picture of evolution that theorists have homed in on resembles a branching tree, with time rising like the sap from the trunk at the bottom, four billion years in the past, to the tips of the topmost twigs, the present. Each bough, branch, or twig represents a species, and all branches point upwards. This ‘Tree of Life’ picture is faithful to one key feature of evolution – once a branch has split, it doesn’t join up again. Species diverge, but they can’t merge. 1
    However, the tree image is misleading in several respects. There is, for instance, no relation between the thickness of a branch and the size of the corresponding population – the thick trunk at the bottom may represent fewer organisms, or less total organic mass, than the twig at the top. (Think about the human twig …) The way branches split may also be misleading: it implies a kind of long-term continuity of species, even when new ones appear, because on a tree the new branches grow gradually out of the old ones. Darwin thought that speciation – the formation of new species – is generally gradual, but he may have been wrong. The theory of ‘punctuated equilibrium’ of Stephen Jay Gould and Niles Eldredge maintains the contrary: speciation is sudden. In fact there are excellent mathematical reasons for expecting speciation to have elements of both – sometimes sudden, sometimes gradual.
    Another problem with the Tree of Life image is that many of its branches are missing – many species go unrepresented in the fossil record. The most misleading feature of all is the way humans get placed right at the top. For psychological reasons we equate height with importance (as in the phrase ‘your royal highness’), and we rather like the idea that we’re the most important creature on the planet. However, the height of a species in the Tree of Life indicates when it flourished, so every modern organism, be it a cockroach, a bee, a tapeworm, or a cow, is just as exalted as we are.
    Gould, in
Wonderful Life
, objected to the ‘tree’ image for other reasons, and he based his objections on a remarkable series of fossils preserved in a layer of rock known as the Burgess Shale. These fossils, which date from the start of the Cambrian era, 2 are the remains of soft-bodied creatures living on mud-banks at the base of an algal reef, which became trapped under a mudslide. Very few fossils of soft-bodied creatures exist, because normally only the harder parts survive fossilization. (Some good deposits are now known in China, too.) However, the significance of the Burgess Shale fossils went unrecognized from their discovery by Charles Walcott in 1909, until Harry Whittington took a closer look at them in 1971. The organisms were all squashed flat, and it was virtually impossible to recognize what shape they’d been while alive. Then Simon Conway Morris teased the squished layers apart, and reconstructed the original forms using a computer – and the strange secret of the Burgess Shale was revealed to the world.
    Until that point, palaeontologists had classified the Burgess Shale organisms into various conventional types – worms, arthropods, whatever. But now it became clear that most of those assignments were mistaken. We knew, for example, just four conventional types of arthropod: trilobites (now extinct), chelicerates (spiders, scorpions), crustaceans (crabs, shrimp), and uniramians (insects and others). The Burgess Shale contains representatives of all of these – but it also contains
twenty
other radically different types. In that one mudslide, preserved in layers of shale like pressed flowers in the pages of a book, we find more diversity than in the whole of life today.
    Musing on this amazing discovery, Gould realized that most branches of the Tree of Life that grew from the Burgess beasts must have ‘snapped off’ by way of extinction. Long ago, 20 of those 24 arthropod body plans disappeared from the face of the Earth. The Grim Reaper was pruning the Tree of Life, and being heavy-handed with the shears. So Gould suggested that a better image than a tree would be something like scrubland. Here and there ‘bushes’ of species sprouted from the primal ground level. Most, however, ceased to grow, and were pruned to a standstill hundreds of millions of years ago. Other bushes grew to tall shrubs before