Most marsupials resemble 'parallel' placentals; a very curious case is the thylacine, otherwise known as the Tasmanian tiger or Tasmanian wolf, which is distinctly wolflike and has a striped rear. The thylacine was officially declared extinct in 1936, but there are persistent reports of occasional sightings, and suitable habitat still exists, so don't be surprised if the thylacine makes a comeback. National Park Ranger Charlie Beasley reported watching one for two minutes in Tasmania in 1995. Similar sightings have been reported from Queensland's Sunshine Coast since 1993: if these sightings are genuine, they are probably of thylacines whose recent ancestors escaped from zoos.
Why such a concentration of marsupials in Australia? The fossil record makes it clear that marsupials originated in the Americas -most probably North America, but that's not so certain. Placentals arose in what is now Asia, but was then linked to the other continents, so they spread into Europe and the Americas. Before placental mammals really got going in the Americas, marsupials migrated to Australia by way of Antarctica, which in those days wasn't the frozen wasteland it is now. Australia was already moving away from South America, but hadn't yet gone all that far, and neither had Antarctica, so presumably the migration involved 'island hopping', or taking advantage of land bridges that temporarily rose from the ocean. By 65 million years ago, oddly enough, the time that the dinosaurs died out, though that's probably not significant -Australia was well separated from the other continents, Antarctica included, and Australian evolution was pretty much on its own.
In the absence of serious competition, the marsupials thrived -just as ground birds did in New Zealand, and for the same reason. But back in the Americas and elsewhere, the superior placental mammals ousted the marsupials almost completely.
Until a few years ago it was assumed that the placentals never made it to Australia at all, except for the very late arrival of rodents and bats from South East Asia about 10 million years ago, and subsequent human introduction of species like dogs and rabbits. This theory was demolished when Mike Archer found a single fossil tooth at a place called Tingamarra. The tooth is from a placental mammal, and it is 55 million years old.
From the form of the tooth it is clear that this mammal had hooves.
Did a lot of placental mammals accompany the marsupials on their migration Down Under? Or was it just a few? Either way, why did the placentals die out and the marsupials thrive?
We have no idea.
Early 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. Humans stayed out of Australia until the Aborigines arrived 40,000-60,000 years ago. When European settlers turned up, from 1815 onwards, they very nearly wiped out numerous marsupial species.
The evolutionary history of the placenta! 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 changed. So in the mid-Eocene we find the earliest fossils of whales, such as the sixty-foot (20 m) long Basilosaurus, which had a pair of tiny legs at the base of its long tail. We've found fossils of its ancestors, and they really did look like small dogs.
The Mediterranean sea was dammed, Africa came into contact with Europe, and creatures previously confined to Africa spread into Europe, among them elephants, and apes. Horses evolved, as did true cats (such as the famous sabre-toothed tiger). By five million years ago, most of today's mammals were represented in recognizable form, and the climate had become similar to today's.
The scene was set for the evolution of humans.
Not that it had all been set up in order to lead to us, you appreciate. Our early ancestors just happened to be in a position to take advantage of the world as it then was. They did so.
We can trace the ancestry of modern mammals, indeed all living creatures that still exist today, by mapping out changes in their DNA. The rate at which DNA mutates, acquires random errors in its code, leads to a 'DNA clock' that can be used to estimate the timing of past events. When this technique was first discovered, it was widely hailed as a precise and therefore uncontroversial way to resolve difficult questions about which animals' ancestors were more closely related to what. It is now becoming clear that precision alone cannot provide definitive answers to such questions.