The Science of Discworld IV

diversity in the animals of the Burgess Shale, and the differences of opinion between Gould and Morris. Among these animals from the Cambrian explosion, several were early chordates, ancestral to our own group of animals, including today’s fishes, amphibians, reptiles, birds and mammals, as well as a diversity of modern oddities like sea squirts and lampreys. The Burgess Shale fossil
Pikaia
is the best-known early chordate, but there are others in similar Australian and Chinese fossil beds.
    The early chordates produced a great adaptive radiation, firstly of jawless armoured fishes, then of a substantial number of jawed forms, including sharks, rays and bony fishes. Some of the latter, in the Devonian period, came out onto the land as early amphibians. These aquatic forms are/were the rocket-ship phase of chordate existence. The amphibians, and their diverse reptilian descendants, such as dinosaurs, birds and mammal-like reptiles that included our ancestors, constitute the next step up, the chordate space bolas. The third stage was achieved separately, and rather differently, by birds and mammals. Birds specialised in warm-bloodedness and efficient lung ventilation for flight, so that they had to provide food for their young, caring for them in nests until they could adopt the very demanding lifestyle of their parents. Mammals became turbo-powered by maintaining a stable high body temperature, and invaded many more habitats than birds, from burrowing and swimming to flying. Which they now do nearly as well as birds, but without flow-through lung ventilation. From a wide-screen chordate viewpoint, mammalian design is their space elevator.
    Within that last step, we can find a similar series of invasions of the adjacent possible, in which terrestrial ecosystems were themselves changed by the presence of large land animals. Grasslandsuch as savannah and steppes, arctic birch, lichen and moss tundra are all maintained by continuing interactions with large herbivorous mammals. Vast numbers of small rodents – mice, rats, voles, lemmings, hamsters – live in and under these grasslands. They eat more of the vegetation than their larger cousins do, and they contribute more to those ecosystems. Some interactions between mammals and their environment are familiar: rabbits making warrens, badgers excavating setts, deer ringing trees. We have to visit zoos to see the full adaptive radiation, including those strange rodents of the South American pampas: pacas, capybaras and cavies (guinea-pigs). And bats. And porpoises, dolphins, toothed whales and filter-feeding baleen whales. And all of the primates, including us. So mammals, like insects among the invertebrates, are the big terrestrial success story.
    In terms of our space-exploration analogy, the mammal-like reptiles of four hundred million years ago, and today’s monotremes (egg-laying oddities like the echidna and the duck-billed platypus) are the rocket-ships. The marsupial mammals – kangaroos, potoroos and opossums – are the space bolas. The placental mammals – most of today’s mammals, including cows, pigs, cats, dogs, hippos, elephants, monkeys, apes and humans – are the space elevator.
    Any evolutionary series can be presented as a ladder of emergent properties, new ways of being that obey new rules and have effectively discarded the old constraints. This vision is as appropriate to mammals as it is to writing tools or radio receivers. It is a general property of our self-complexifying planet in its self-complexifying universe. As time passes, more different things happen in more ways, with new rules and new functions.
    That vision, of the multifarious universe knotting itself into patterns that themselves build upon previous patterns, is almost perfectly opposite to the twentieth-century view of ever-increasing entropy leading to heat death. Can this self-complication continue infinitely? We don’t know, but it is as sensible a view as its opposite,and there is considerable evidence for it. Does that mean that
anything
not possible now will necessarily be possible in future? Of course not. At each step upwards, there is selection among possibilities.
    This selection process is what mathematicians call symmetry-breaking: more possibilities seem to be available beforehand than are actualised at the next stage, yet paradoxically there are more possibilities afterwards than before. If advancement is the rule, and it seems to be, then contingency and