The Science of Discworld Revised Edition

marsupials probably lived in trees, to judge by their forepaws. Early placentals probably lived on the ground, especially in burrows. This difference in habitat allowed them to coexist for a long time. Marsupial extinctions in the Americas were helped along by humans, who found marsupials especially easy to kill.
    In Australia, about 40,000-50,000 years ago, there were sudden extinctions of
Genyornis
, the heaviest bird ever, and of the marsupial equivalent of a lion. Again, evidence suggests that humans were responsible , but the theory was hotly disputed because it was difficult to date the events accurately. And, we suspect, because many people wish to believe that all ‘primitive’ humans lived in exquisite harmony with their environment. In 2001 Linda Ayliffe and Richard Roberts used two accurate dating methods to find out when 45 species, found as fossils at 28 separate sites, disappeared. They
all
went extinct 46,000 years ago – just after the Aborigines, the first humans to reach Australia, arrived.
    Later arrivals were no better. When European settlers turned up, from 1815 onwards, they very nearly wiped out numerous marsupial species.
    The evolutionary history of the placental mammals is controversial and has not been mapped out in detail. An early branch of the family tree was the sloths, anteaters, and armadillos – all animals that
look
‘primitive’, even though there’s no earthly reason why they should, because today’s sloths, anteaters, and armadillos have evolved just as much as today’s everything else’s, having survived over the same period.
    Mammals really got going during the early Tertiary period, about 66 to 57 million years ago. The climate then was mild, with deciduous forests at both poles. It looks as if whatever killed the dinosaurs also changed the climate, so that in particular it was much more rainy than it had been during dinosaur times, and the rainfall was distributed more evenly throughout the year, instead of all coming at once in a rainy season. Tropical forests covered much of the planet, but they were mainly inhabited by tiny tree-dwelling mammals. No big carnivores, not even big plant-eaters … no leopards, no deer, no elephants. It took the mammals several million years to evolve bigger bodies. Possibly the forests were much denser than they had been when there were dinosaurs around, because there weren’t any big animals to trample paths through them. If so, there was less incentive for a big animal to evolve, because it wouldn’t be able to move easily through the forest.
    Once mammalian diversity started to get going, it exploded. There were tigerlike animals and hippolike animals and giant weasels . By modern standards, though, they were all a bit lumpish and cumbersome – nothing as graceful as the slim-boned creatures that came later, such as gazelles.
    By 32 million years ago, Antarctica had reverted to being an icecap, and the world was cooling. Mammalian evolution had settled down, and what changes did occur were relatively small. There were bear-dogs and giraffe-rhinoceroses and pigs the size of cows, llamas and camels and sylphlike deer, and a rabbit with hooves. By 23 million years ago, the climate was warming up again. Antarctica had separated from South America, making big changes to the flow of ocean currents: now cold water could go round and round the south pole indefinitely. The sea level fell as water got locked up in ice at the poles; with more land exposed and less ocean the climate became more extreme, because land temperatures can change more quickly than sea ones. Falling sea levels opened up land bridges between previously isolated continents; isolated ecologies started to mix up as animals migrated along the new connections. And round about this time, the evolution of some mammals took an unusual turn. A U-turn.
    They went back to the sea.
    The land animals had originally come out of the sea – despite the wizards’ best efforts to stop them. Now a few mammals decided they’d be better off going back there. The wizards consider such a tactic to be a spineless piece of backsliding, giving up and going back home. Even to us it looks like a retrograde step, almost counter-evolutionary: if it was such a good idea to come out of the oceans in the first place, how could it be worthwhile to go back again? But the evolutionary game is played against a changing background, and the oceans had changed. In particular, the available
food
had