Science of Discworld III

them on other aqueous planets when we explore our local region of the galaxy. Such evolutionary attractors, are called ‘universals’, in contrast to ‘parochials’: unlikely innovations that have happened only once in Earth’s history.
    The classic parochial is the curious suite of characters possessed by land vertebrates, because a particular species of Devonian fish succeeded in invading the land in our, real , history. Those fishes’ descendants were amphibians, reptiles, birds, and mammals – including us. Jointed limbs are a universal innovation. The limbs of spiders, hydraulically operated, differ in detail from the limbs of mammals, and were presumably acquired via a different ancestor, perhaps an earlier arthropod proto-spider. The mammalian internal skeleton, with one bone at the body end, then two, then a wrist or ankle, then five lines of bones for fingers or toes, was an independent evolution of the same universal trick.
    This highly unlikely combination now occurs in all land vertebrates (except most of the legless ones), because they are all descended from those fishes that came out of the water to colonise the land. Other parochials are feathers and teeth (of the kind that evolved from scales, which are what we have). And, especially, each of the special body-plans that characterise Earth’s animals and plants: mammal, insect, rotifer, trilobite, squid, conifer, orchid … None of these would appear again after a rerun of Earth’s evolutionary history, nor would we find exact replicas on other aqueous planets.
    We would expect much the same processes to occur, though, in a repeat run of Earth or on another similar world: an atmosphere far from chemical equilibrium as life forms pump up their chemistry using light; planktonic layers of the seas colonised by the larvae of sedentary animals; flying creatures of many kinds. Such ecosystems would also probably have ‘layers’, a hierarchical structure, fundamentally similar to the ecosystems that have emerged in so many different circumstances on Earth. So there would be ‘plantlike’ creatures, a productive majority of the biomass (like Earth’s grass ormarine algae). These would be browsed by tiny animals (mites, grasshoppers) and by larger animals (rabbits, antelopes), with a few very large creatures (elephants, whales). Comparable evolutionary histories would lead to the same dramatic scenarios, but performed by different actors.
    The central lesson is that although natural selection has a very varied base to work with (recombinations of ancient mutations, differently assorted in all those ‘waste’ progeny), clear large-scale themes emerge. Marine predators, such as sharks, dolphins, and ichthyosaurs all have much the same shape as barracuda, because hydrodynamic efficiency dictates that streamlining will catch you more prey, more cheaply. Very different lineages of planktonic larvae all have long spines or other extensions of the body to restrain the tendency to fall or rise because their density differs from that of seawater, and most of them pump ions in or out to adjust their densities too. As soon as creatures acquire blood systems, other creatures – leeches, fleas, mosquitoes – develop puncture tools to exploit them, and tiny parasites exploit both the blood as food and the bloodsuckers as postal systems. Examples are malaria, sleeping-sickness, and leishmaniasis in humans, and lots of other parasitic diseases in reptiles, fishes, and octopuses.
    Large-scale themes may be the obvious lesson, but the last examples reveal a more important one: organisms mostly form their own environments, and nearly all of the important context for organisms is other organisms.
    Human social history is like evolutionary history. We like to organise it into stories, but that’s not how it really works. History, too, can be convergent or divergent. It seems quite sensible to believe that small changes mostly get smeared out, or lost in the noise, so that big changes are needed to divert the course of history. But anyone familiar with chaos theory will also expect some tiny differences toset off divergent histories, drifting progressively further away from what might have happened otherwise.
    Changing history is a theme of time-travel stories, and the two issues come together in those stories called ‘worlds of if’.
    We have the strongest feeling that what we do, even what we decide, does change history. If I decide, now, not to go and meet