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.
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 suggest 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 how 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 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.
The issue of interpretation — what do
es this result mean? — can still be controversial, even if the result itself can be made precise. For example, S. Blair Hedges and Sudhir Kumar have applied the DNA clock to 658 genes in 207 species of modern vertebrates: rhinos, elephants, rabbits, and so on. Their results suggest that many of these lineages were around at least 100 million years ago, coexisting with the dinosaurs, though no doubt the early elephant and rhino ancestors were rather small. The fossil record agrees that there were mammals then, but not those. The molecular biologists claim that the fossil record must be misleading; palaeontologists are convinced that the DNA clock sometimes ticks faster and sometimes ticks slower. The debate continues — but for what it's worth, our money is on the palaeontologists.
One big surprise about mammal DNA is how much of it there is. You might expect a sophisticated creature like a mammal to be 'hard to build' and therefore require more DNA, just as the blueprint for a jumbo jet has to be more complicated than that for a kite.
Not so.
Mammals have less DNA — shorter genomes — than many apparently simpler animals, for example frogs and newts.
There's a good reason for this apparent paradox, and it illuminates the difference between DNA and a blueprint. DNA is more like a recipe — and a recipe that makes a lot of assumptions about what else you have in your kitchen, so that none of that needs to be spelled out in the recipe book. In essence, the kitchen for mammals has a really well controlled oven, capable of ensuring nice, even cooking temperature, so a whole lot of tricks about what to do if the temperature changes need not be mentioned.* In the frog kitchen, on the other hand, the temperature goes up and down depending on the time of day and the weather, so the recipe has to deal with all contingencies, requiring more DNA code. By 'kitchen' here we mean the environment in which the embryonic animal has to develop. For a frog, the kitchen is a pond. For a mammal, the kitchen is mother.
Mammals evolved good temperature control — unlike the reptiles, they are warm-blooded, but what matters is not so much being warm, as being controllable. Frog DNA is full of genes for making lots of different enzymes, together with instructions along the lines of 'use enzyme A if the temperature is lower than 6°C, use B if the temperature is between 7°C and 11°C, use C if the temperature is between 12°C and 15°C ...' Mammal DNA just says 'Use enzyme X', knowing that mother will take care of temperature variations. Frog DNA is a rocket: mammal DNA is a space elevator.
How did this change take place? Perhaps when mammals first evolved, their DNA gained extra instructions, but after temperature control evolved, a lot of the DNA became redundant, and it either got dumped or got subverted to other uses. On the other hand, we have no idea what the DNA of early mammals actually looked like — maybe it was all shorter in those days, maybe today's frogs and newts have much more extensive recipes than ancient ones. But on balance it seems more likely that mammals just eliminated a lot of surplus instructions.
Modern technology uses the same trick. Because the machinery that makes today's consumer goods is extremely precise and accurate, those goods can be simpler than they were in the past. A soft drinks can, for example, is little more than a piece of aluminium that has been formed into a cylinder, with another flat bit on top to act as a lid, a weak line for the tab to tear along, and a ring (or nowadays a lever) attached to the tab. It replaces the bottle, which consisted of two or more bits of moulded glass 'welded' together, a metal cap, and a slice of cork. The simplicity of the can comes at a price: very careful control of the forming process.
There are many scientists who insist that an organism's DNA determines everything about it — even though it manifestly does not — and they argue that the mother's temperature-control system is included in her DNA recipe. This may well be true, but even if it is, 'this organism's' DNA has somehow migrated to another organism (mother, not her offspring). As soon as two generations are involved in implementing the genetic blueprint, a gap opens up into which things can be inserted that are not genetic at all. We've already mentioned several, for example prions in the reproduction of yeast.
Our mammalian ancestry may even be responsible for one of the more bizarre modern myths, persistent tales of people being abducted by aliens. Ufologists allege that one American in twenty now claims to have undergone such an experience (but they would, wouldn't they?). If true, this figure is a remarkable and not very happy comment either on the critical faculties of that great nation or on the habits of an unknown spacefaring species.
As it happens, the figure is bogus. It originates in a Roper poll of 1994, which revealed that one American in fifty had undergone such an experience. But as Joel Best pointed out in his book Damned Lies and Statistics in 2001, the number of people who claimed to have been abducted by aliens was actually zero. The pollsters, worried a direct question about aliens would put people off, used five 'symptoms' of abduction instead. Anyone who scored sufficiently highly on those symptoms was deemed to have undergone an abduction experience.
The questions were things like 'Have you ever woken up paralysed with the sense of some strange presence in the room?' This sensation is typical of 'sleep paralysis', the most obvious rational explanation of abduction experiences, which we describe shortly. So really the Roper poll was a survey about sleep paralysis. Only the researchers thought that it had anything to do with alien abduction. The subjects had more sense.
Be that as it may, a lot of people are convinced that strange aliens, usually with big black eyes and pear-shaped heads like the ones in Close Encounters of the Third Kind, landed a UFO near them, loaded them on board, and took them for a flight round the solar system while carrying out weird experiments, often of a sexual nature, on them. After which they were calmly returned to the very spot from which they had been abducted, as if absolutely nothing had happened.
The first thing to say is that without doubt many of these experiences are false. Ian once did a radio broadcast which included a woman who had undergone a convincing experience of being abducted — except that she knew she hadn't really been, because her family told her she'd been asleep beside the fire the whole time. Jack once met a woman who claimed that the aliens abducted her and took away her baby. So he asked a question that nobody else had thought to ask, the woman included: 'Were you pregnant?'
'No.'
The point is that to the victims, the experience felt real. Even though logic told them it couldn't have happened, they either didn't apply the logic, or they did but still remembered the experience vividly. We deduce that the human mind sometimes has vivid memories that do not correspond to real events. Of course we must also observe that just because some alien abductions aren't real, that doesn't imply they all aren't. However, if we can find a sensible mechanism for otherwise reasonable people believing that they really were carted off in a UFO, then the burden of proof shifts dramatically and evidence of abduction stronger than sincere expressions of belief becomes necessary.
Reports of alien abductions are not new. Back in the Middle Ages, however, they would have been either flights on witches' broomsticks or encounters with fabulous creatures like the succubus, a demon in a woman's body who allegedly had sex with men while they slept. The witches of Discworld employ broomsticks for transport only. The sex bit doesn't appeal to them at all — except for Nanny Ogg, of course.
Folk tales of succubi and their like can be found worldwide. In Newfoundland people tell of an ancient hag sitting on their chests at night, and in Vietnam they speak of the 'grey ghost'. What seems to be going on is some common mental pattern, overlaid with cultural influences. That's why abductions by witches riding broomsticks have gone out of vogue, but abductions by aliens riding UFOs are flavour-of-the-decade.
Susan Blackmore thinks that all of these experiences are, and were, caused by sleep paralysis. This is a feature of the mind that prevents sleeping people from moving their limbs as they would if they were acting out their dreams. Such a 'mental switch' is important for any animal that dreams: you don't really want to go slee
pwalking out of your cosy burrow and straight down a predator's throat. Plenty of mammals dream — most of us have seen a cat or dog asleep with its legs twitching, and the evidence from recordings of the brain's electrical activity is that the animals are engaged in something that closely resembles the brain activity of a dreaming human. We can't be sure whether cats have visual dreams like we do, but sleep and dreaming take place in primitive parts of the brain, so they probably go back a long way in our evolutionary history. At any rate, if the sleep paralysis system malfunctions, people who are partially awake may undergo sleep paralysis. Experiments show that in such cases they typically get a strong impression that 'somebody is there'.
This feature of the human mind may go back to the time, just after the meteorite hit, when the nocturnal mammals suddenly awoke in a world without dinosaurs. Their senses of hearing and sight, previously separate from each other because they had evolved at very different periods and in very different circumstances, would have become linked together. When their ears heard something strange, their visual sense would kick in and make them feel that they could see what was causing it. We inherited this tendency, but we interpret it in terms of the current culture: bogeymen, witches, maybe even dragons a few centuries ago, aliens with big black eyes today. The sexual link is straightforward, too: dreams about sex are very common anyway.
Oh, yes, one more thing: since we've all watched Close Encounters, we know exactly what an alien must look like ... just as everyone used to know that witches soared through the air on a broomstick. So our visual system knows what shape it should give to whatever it sees when we get that funny feeling that something is haunting us. And flying saucers have come on nicely, too, from being the rivet-studded things that were all the rage in galactic circles in the early Fifties.
The Science of Discworld Page 34