When the dinosaurs died out, the mammals had a field day. Released from dinosaurian thrall, they could occupy environmental niches that, only a few million years before, would merely have presented a dinosaur with an easy meal. It seems likely that the current diversity of mammals has a lot to do with the suddenness with which they came into their kingdom, for a while, almost any lifestyle was good enough to make a living. However, it would be wrong to imagine that the mammals came into existence to fill the gaps left by the vanished dinosaurs. Mammals had coexisted with dinosaurs for at least 150 million years.
Harry Jerison has suggested that before the dinosaurs became really dominant, many mammals were able to make their living in daylight, and they evolved good eyesight to do so. As the dinosaurs became a bigger and bigger problem, the mammals adopted a lower profile, mostly staying hidden undergound during the day. If you're a nocturnal animal, you rely on a really good sense of hearing, so evolutionary pressures then equipped the mammals with excellent ears, including those three little bones. However, they retained their eyesight. So when the mammals again dared to venture out into the daylight, they had good eyesight and good hearing. The combination gave them a substantial advantage over most remaining competitors.
Mammals evolved from an order of Triassic reptiles known as therapsids, small, quick-moving hunters, mostly, though some were herbivores. Compared to other reptiles, the therapsids were not especially impressive, but their low-profile lifestyle led, in stages, to the distinctive features of mammals. A diaphragm leads to more efficient breathing, useful if you need to run fast. It also lets the young animals continue to breathe while mother is feeding them her milk, changes to animals 'co-evolve' as a whole suite of cooperative attributes, not one at a time. Hair keeps you warm, and the warmer you are, the faster all your bodily parts can move ... and so on.
All this makes it difficult to decide when the mammal-like reptilian ancestors of the therapsids became reptile-like mammals ... but, as we've said, humans have problems with becomings. There was no such point: instead, there was a mostly gradual, but occasionally bumpy, transition[50]. The earliest fossils that can definitely be identified as mammals come from 210 million years ago, creatures rejoicing in the name 'morganucodontids'. These were shrews, probably nocturnal, probably insect-eaters, probably egg-layers. Darwin's detractors objected to having apes as their ancestors: heaven knows what they would have thought about bug-eating egg-laying shrews. But there's good news too, if you're of that turn of mind, because morganucodontids were brainy. Not especially brainy for a shrew, but brainy compared to the reptiles from which they evolved. Admittedly, this was largely because the therapsids were as thick as two short ... er, slices of tree-fern, but it was a start.
How do we know that these early shrews were true mammals? One of the bits of an animal that survives as a fossil far more often than any other bit is the tooth. This is why palaeontologists use teeth, above all else, to identify species of long-dead animals. There are plenty of species for which the sole evidence is a tooth or two. Fortunately, you can tell a lot about an animal by its teeth. On the whole, the bigger the tooth is, the bigger the animal, an elephant's tooth today is a lot bigger than an entire mouse, so whatever animal it came from, it couldn't be mouse-sized. If you can find a jawbone, a whole array of teeth, all the better. The shape of a tooth tells us a lot about what the animal ate, grinding teeth are for plants, slicing teeth are for meat. The arrangement of teeth in a jawbone tells us a lot more. The morganucodontids made a major breakthrough in tooth design: teeth that interlocked when the jaws were brought together, very effective at cutting bits off meat or insects. They also paid a heavy price for their teeth, one that we still pay today. Reptiles continually produce new teeth: as old ones wear down, they get replaced. We produce just two set of teeth: milk teeth as children and the real thing as adults. When our adult teeth wear out, the only replacements available are artificial. Blame the morganucodontids for this: if you want to take advantage of precisely interlocking teeth, you have to maintain that precision, which is impractical if you keep discarding teeth and growing new ones. So they grew only two sets of teeth, and we have to do likewise.
From this we can deduce more. With only two sets of teeth, the morganucodontids had to have some special trick for feeding their young, something different from the reptiles with their continuous succession of teeth. There isn't room for a full set of adult teeth in a baby shrew, and if teeth only come in two stages, you can't add the odd one every so often as the jaw grows bigger. The easy solution is to have babies with no teeth at all, to start with. But what can they then eat? Something nutritious and easily digested, milk. So we think that milk-production evolved before those high-precision interlocking teeth. This is one reason why the morganucodontids are definitely placed among the mammals.
Amazing what you can learn from a few teeth.
As they prospered and diversified, mammals evolved into two main types: placental mammals, where the mother carries the young in her uterus, and marsupials, where she carries them in a pouch. The marsupial that springs most readily to mind is the kangaroo, possibly because it springs most readily to almost anything, as for example in The Last Continent:
'And ... what's kangaroo for "You are needed for a quest of the utmost importance"?' said Rincewind, with guileful innocence.
'You know, it's funny you should ask that...'
The sandals barely moved. Rincewind rose from them like a man leaving the starting blocks, and when he landed his feet were already making running movements in the air.
After a while the kangaroo came alongside and accompanied him in a series of easy bounds.
'Why are you running away without even listening to what I have to say?'
I've had long experience of being me,' panted Rincewind. 'I know what's going to happen. I'm going to be dragged into things that shouldn't concern me. And you're just a hallucination caused by rich food on an empty stomach, so don't try to stop me!'
'Stop you?' said the kangaroo. 'When you're heading in the right direction?'
Australia alone has over a hundred species of marsupials, in fact most native Australian mammals are marsupials. Another seventy or so are found in the same general region, Tasmania, New Guinea, Timor, Sulawesi, various smaller neighbouring islands. The rest are opossums and some diminutive ratlike creatures, mainly in South America, though ranging into Central America and for one species of opossum right up into Canada.
It looks as though placental mammals generally win out against marsupials, but the difference isn't so great, and if there aren't any competing placental mammals then marsupials do very well indeed. There are even some close parallels between marsupials and pla-centals, a good example is the koala 'bear', which isn't a true bear but looks like an unusually cuddly one.
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 Ant
arctica, 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.
The issue of interpretation, what does 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[51]. 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 ha
s 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.
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