The Science of Discworld IV Judgement Da - By Terry Pratchett, Ian Stewart Page 0,80

also changes its future by the changes it causes in the other programmes.

To what extent are those changes predictable or accidental? There is a difference here between two modern viewpoints, one associated with the palaeontologist Simon Conway Morris in Life’s Solution: Inevitable Humans in a Lonely Universe, and the other with the late Stephen Jay Gould in Wonderful Life. This difference is crucial to the issue of design in evolution.

Gould made great play of the variety of the animals represented by the fossils in the Burgess Shale, deposited at the start of the Cambrian period about 570 million years ago. These fossils had been described by previous biologists, but Morris reworked and reconstructed them. He classified these fossils into a wide range of morphological types; in fact, many more basic kinds of animal design (‘phyla’) than had been assigned previously. Gould used this wide range of body designs, only a few of which have descendants among present-day creatures, to argue that life can do almost anything by way of morphology, even in its fundamental or basic structure, and that the organisms that now exist are accidental survivals from the much vaster range that existed at the start of the Cambrian.

Morris, however, has come to believe the opposite, namely: because some of the many themes have converged to produce similar beasts, some specific designs must be winners, no matter how they are realised. Therefore any wide array of different body structures will necessarily evolve to generate much the same spectrum that we observe today, automatically selected because those are the body-plans that work best. The fossil record contains many cases of this kind of convergence:fn3 ichthyosaurs and dolphins have evolved to look like sharks and other carnivorous fishes, because that’s the shape that’s most efficient for a fishy predator. In short, Morris believes that if we were to find living creatures on a similar planet to Earth, or if we were to run Earthly evolution again, then much the same range of animal designs would appear. Aliens on a world like ours would be much like us, even if their biochemistry were totally different.

In contrast, Gould believed, as we do,fn4 that in such a rerun the resulting spectrum of life forms would not resemble the current ones at all. Different designs, fundamentally different body forms, would be just as likely as the ones that happen to exist now. The current body-plans are just a contingent, accidental collection that happened to survive. Aliens, even the highest ones, would most likely be very different in design from us, whatever world they evolved on. Including a reboot of ours.

The old view of the role of genes in Darwinian evolution emphasised mutations: random changes to DNA sequences. However, at least in sexual species, the main source of genetic variability is actually recombination: mix-and-match shuffling of gene variants from the parents. New mutations are not needed to innovate; new combinations of existing genes are sufficient. The diversity of available gene variations can be traced back to much older mutations, but you don’t need a mutation now to change an organism.

All biologists now agree that the body-plans of organisms are not built up piece by piece, mutation by mutation, but have been selected by recombination. Instead of mutations to new genetic variants, we find recombinations of many ancient mutations. These are sorted from kits of compatible parts in every generation, not put together higgledy-piggledy and expected to work. If, as seems plausible, only a few developmental trajectories can lead to larvae that can feed and grow into working adults, compared to the huge number that can’t, then it is to be expected that the successful designs are all separated, without intermediate forms bridging the gaps. ‘Missing links’ need not be missing – or links – because continuous variation is not required in a discontinuous process.

By looking at so-called r-strategists, animals like plaice and oysters whose larvae comprise only a few developmentally competent ones among a majority that aren’t, we can see how this is achieved today. What it does not tell us – what distinguishes the Morris and Gould views – is whether the successful designs are out there in some Platonic organism-space, waiting to be found, or whether the organisms have all invented their own, unpredictable, designs as they went along. Morris, a Christian, believes the former: the appearance of design is the revelation of transcendental attractors in God’s design-space of possible organisms. We, however, believe that there are so many possible ways of being a