MAMMALS ARE ANIMALS whose females suckle their young upon modified sweat glands. They thus embody milk and maternity: they are nurture encapsulated. Most mammals are hairy, and even those that are not had hairy ancestors. And such creatures bearing fur or pelt, and feeding their young upon a specially secreted, nutritious fluid, had spread over all the lands of the Earth in Tertiary times. They were accompanied by the birds, which have thrived alongside mammals in dynamic counterpoint ever since.
After the demise of the ruling reptiles, warm-blooded mammals were released from more than 100 million years of inconspicuousness. The group had appeared at almost the same time as the dinosaurian reptiles, having been derived from one of the numerous mammal-like reptiles, probably at some time in the Triassic. Like poor cousins of a great family, they had to live on the edge of the estate, making ends meet, until a failure of succession allowed them the freedom of the grounds. We can imagine them quivering with be-whiskered sensitivity under the cycad groves as thunderous sauropods stamped past; they were awaiting nightfall, then to emerge to scuttle among the leavings from the reptilian tycoon’s repast. There were good livings to be made at small size, especially in the roles of scavengers and insectivores. Insects were abundant in the Mesozoic forests, as amber beads confirm. Small, insectivorous mammals such as shrews are still common in many places, which is evidence that insects are a predictable food source as well as a nutritious one. I have lain very still in the rough grass on the edge of a field and watched a tiny pygmy shrew dispatch a moth with speed and precision. The shrew emerged from obscure concealment among dense herbiage, its long nose twitching and cautious. It seemed to catch the moth in its paws, severed off the wings in a trice with a couple of nips, and then proceeded to eat it with indulgence. I swear it even made a succulent, chomping noise, like a greedy schoolboy making the most of an apple. Then the creature was gone—bent upon eating its body weight in insects within a twelve-hour period, which is the price that must be exacted for having warm blood. It was not difficult to imagine a similar scene being enacted in the Jurassic, out of sight of large carnivores, and beneath the notice of mighty herbivores. One such creature is Megazostrodon, a delicate Jurassic mammal from South Africa that looks like a shrew. Not surprisingly, the early fossil record of these small and fragile insectivores is patchy in the extreme: the discovery of an entire skeleton of a new early mammal is always big news, almost in inverse proportion to the size of the fossil. The commonest evidence of the early history of mammals available to the palaeontologist is tiny teeth alone. It is likely that mammals were commoner from the first than their sparse fossil record suggests.
When the terrestrial dinosaurs had died away all the ecological niches that they had occupied fell vacant; they had been at the top of the food chain for a long time. The Palaeocene period, the earliest division of the Tertiary, was a unique time. It was as if a selection of vast feasts were laid out—not merely fatter grasshoppers or tastier grubs. If Nature truly abhors a vacuum, this is when there was a rush to fill each vacancy in the living world with a practitioner skilled in an appropriate trade: grazers and hunters and every other type of vertebrate activity imaginable. Even if several of the fundamental mammal designs had first appeared during the Cretaceous, it was after the demise of the dinosaurs that they went crazy, in a kind of stampede of creation of new designs that turned small, even literally downtrodden animals into the greatest range of shapes that has ever graced the Earth. For it was the mammals and the birds that took advantage of the opportunities that presented themselves. The other survivors that crossed through the great death at the end of the Cretaceous did not—neither the lizards, nor the crocodiles, nor the turtles. What birds and mammals share is a homeothermic metabolism (that is, they are both warm-blooded). Both are accordingly insulated—by feathers and fur, respectively. It must have been this property that favoured them in the repopulation of the biological firmament. This may be because the climate had become generally more seasonal, which warm-blooded animals can cope with successfully. There is good evidence for the growth of cool polar regions during the Tertiary. It is curious to reflect that humans, the bald apes that now rule or misrule over vulnerable continents, have lost the very feature that once gave its kind the edge over the rest of creation.
The rapidity of what is usually called the “radiation” of the mammals can be seen written clearly in rock sections. What were previously very rare fossils become common ones; large species appear, which are usually detected in the field by tapping out their robust limb bones. Shrew-sized animals have dog-sized companions within a mere 3 million years of the extinction of the dinosaurs. Professor S.J. Stanley has shown that in the early Tertiary the rate of evolutionary change in the mammals was many times faster than it was in clams and snails, which were prospering and changing in the sea at exactly the same time. For example, during comparable time intervals many more kinds of new species, genera and even families of mammals appeared as fossils when compared with the molluscs. This may have been because the seashells were not diversifying into “empty” ecological space like the mammals. The marine world was not a tabula rasa upon which any efficient design could be successfully graven. As we have seen many times, the sea has always been a haven of continuity, a place where revolutions are muffled.
To the student of the geological past, mammals have one advantage over their reptilian predecessors—they have particularly distinctive teeth. My mammal colleagues can wax eloquent on the discovery of a solitary tooth. Teeth are composed of various cusps, which are the bits that break off when you eat the wrong kind of muesli. I have known mammal workers to get excited by a single cusp perched on the tip of a pin! Since much of the evidence for the presence of mammals in Antarctica hung on such a cusp for several years, this is perhaps understandable. Mammologists can identify animals closely from even a single tooth. My colleague Andrew Currant, an expert on Ice Age mammals, once had the misfortune to swallow the very small tooth of a very important arctic vole. Quite what he was doing with the tooth in his mouth he is not keen to explain, although apparently it had something to do with getting rid of some old glue. He was able to recover the tooth after the passage of several hours, which is the best proof I can muster of the regard in which mammologists hold dental characteristics.
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THERE ARE MANY KINDS of mammals living at the present day, each with a history worth consideration. I can describe in detail only a handful—there are just too many—but I shall attempt to outline some of the mechanisms that underlay their differentiation. Consider their variety. Herbivores vary from ponderous cows, faster horses, through gazelles as elegant and supple as young ballerinas, to massive pachyderms—elephant, rhinoceros and hippopotamus, which are the ecological doubles of the brontosaurs. Odder herbivores must not be forgotten: camels, giraffes and kangaroos, tree sloths, the giant South American rodents—capybaras—and the improbable giant panda, connoisseur of bamboos. Then there is the smaller tribe of rabbits and hares, capable of living everywhere from high Arctic to subtropical desert. Small rodents of a hundred species gorge on seeds and nuts and much besides, nibblers and gnawers all. Pigs are rooters, a talent turned to advantage by the French peasants who use them to snuffle out the world’s most delicious and expensive foodstuff, the truffle. Vegetarians are preyed upon by those that are not. Some carnivores are small and vicious, like weasels and stoats—which, it will be recalled, were described by gruff Mr. Badger in The Wind in the Willows as definitely not the right kind of people. (Mole, of course, was, and is, a very efficient subterranean carnivore, while Ratty is vegetarian, but not above taking the odd caddis fly. Ratty is no rat at all, but a water vole, at least in zoological terms, although linguistically he might have prior claim to be the true and only rat, since “rat” is an Anglo-Saxon word, predating the arrival of the brown rat and ship rat in Britain.) The lion (Panthera leo) is indubitably king of the African predators, and no nomenclatural quibbling, even if the tiger is the more ferocious beast, and both are aristocrats among numerous cats. Dogs, hyenas, foxes, bears, gluttons add to the roll call of the Order Carnivora. Weird mammals have forsaken succulent meat for hordes of ants: aardvarks and pangolins and anteaters with elongate snouts and sticky tongues. The more conventional insectivores are the shrews, but there are also hedgehogs—with hairs modified into spines. The most curious insectivores of all are bats—pipistrelles and barbastrelles and horseshoes—animals that can fold their wings like vellum umbrellas and see in the dark by emitting high-pitched squeaks. They include fruit-eaters, too, and a bloody vampire; the smallest one (Craseonycteris) hardly weighs more than a bumble bee. As a cast of characters, do not the mammals already strain credibility?
But there are still more. There are those that returned to the water. Lazy dugongs and manatees are the herbivores; seals and sea lions the carnivores, with the faces of dogs but bodies slick and dappled and rounded, and breath worse than a camel’s. The crown of them all are the cetaceans—porpoises and dolphins and whales—who acknowledge their mammalian roots by teat and milk alone, while their bodies scull, glide or wallow through the ocean more comfortably than any submarine, and as gracefully as any salmon. Yet they include giants that would outweigh the largest of the dinosaurs, and put to shame the most outrageous inventions of the reptiles. I wonder if the blue whale were known only from fossils whether any palaeontologist would believe his arithmetic: 170 tons of blubber, and all built on the nourishment gained by filtering planktonic shrimps through slats of baleen. A second evolutionary line led to the toothed whales, the giant of which is the sperm whale, a vast, blunt-headed animal capable of diving to enormous depths in pursuit of squid. Whales have become the symbol of conservationists because they defy probability, and they symbolize the fecundity and inventiveness of life. With whales, you do not need to argue the case that the loss of a species diminishes us all, something which seems more debatable in the case of a subtly different species of vole. The gross carnage of whales is historically familiar, and celebrated in the novel that most closely associates the physical and the metaphysical, Moby Dick. On my first visit to Spitsbergen I saw the debris of ancient butchery near Smeerenburg—drifts of ribs, strewn around the boreal waste like so many white poles stranded from vanished encampments. What 65 million years of evolution had accomplished a harpoon could undo in a bloody afternoon.
Yet whales, too, began with legs. If one needed proof that bones never lie, the fossils that record the history of the whale probably give the best example of evolutionary transition, and are as good an example of the contribution of palaeontology to mammalian history as any. The Eocene Basilosaurus has a name that sounds as if it should grace a reptile—sauruses almost to a beast—and it was originally thought to be one. Yet when its bones were fully known it proved to be a mammal, and a primitive whale twenty metres long to boot. Its head is relatively short, and its body correspondingly long, when compared with living whales. It still retained a relic of the hind limbs, and they have fingers and arms, but shrunken and useless, a kind of evolutionary signpost which pointed the way to an ancestral animal that could walk on all fours. Other recent discoveries from the Eocene rocks further filled in the apparently insurmountable gap between the great whale and a normal quadruped. Ambulocetus (the name attempts to describe its salient feature—“walking whale”) is known so far from a partial skeleton—and it has legs, arranged after the fashion of a sea lion, so they were still functional. Probably the oldest member of the whale group is Pakicetus, known only from the back of the skull, and part of its lower jaw. But even this fragment shows that the ear region was poorly modified for deep diving, compared with the specialized structures in other whales, which can tolerate considerable water pressure. Extraordinarily, it was found with freshwater snails and other fossils which preferred to be away from the sea, which may indicate that the whales were born among mammals that gambolled after fish and shellfish in the shallow rivers and estuaries of the early Tertiary, and only later moved into the ocean, of which they became the masters. There is a modern equivalent among the carnivores: Lutra lutra, the otter, which can bridge the marine and freshwater worlds like no other creature except the salmon—which is possibly its favourite food. I have spotted them in trout-rich rivers in South Wales; I have seen their close relatives exuberantly chasing through the kelp groves of California. The story of the genesis of whales is particularly eloquent, and it may stand for so much of mammal evolution. An animal with bizarre shape really can be shown to have had ancient relatives with combinations of characteristics that link them with more conventional animals. It also shows how fragmentary our knowledge is of some of the crucial linking species, and this is something that recurs with many kinds of animals, not least our own species. I do not propose to describe many other stories of the kind: “first this species, then that, then the other,” as they soon degenerate into mere catalogues of Latin names. The most reiterated of these genealogies is probably that of the horse (Equus), and I have become thoroughly bewildered over the years by how complicated that story has grown since the first, small, five-toed Eocene horse-like animal (Hyracotherium) appeared in the fossil record. At the last count there were more than twenty generic names dotted about on the tree of relationships, and to describe it would resemble the beginning of one of those Old Testament books wherein Zebediah begat Obadiah, and Obadiah begat Numquat and Aliquot, and Aliquot Mizpah and Mephaniah etc., etc., and so on for many generations. But there are a dozen narratives of this kind that could be told, and many palaeontologists spend their working lives with teeth and a few bony scraps filling in missing paragraphs or, on occasion, writing a new chapter. Very rarely, the almost impossible is discovered—like a walking whale. It requires a singular kind of devotion, at times almost religious in its self-effacing fervour, to keep plugging away at ungrateful bones.
THE EARLIEST TIMES of many kinds of mammals are poorly known. Placental mammals, those having a womb in which to nourish their babies to some degree of maturity before birth, are considered to be comparatively advanced. The most primitive living mammals are the monotremes, which retain the characteristic of laying eggs from a reptilian ancestor. As with other examples of survivors mentioned in this book, they are Australasian natives. The duck-billed platypus is an animal so extraordinary that when it was first described it was assumed to be a hoax. When it was finally proved that an aquatic mammal with a duck-like bill that laid eggs really existed, it merely turned a sceptic’s dream into a zoologist’s nightmare. Its commoner cousin, the echidna, looks rather like a hedgehog. Although the monotremes must have existed all the way back to the early days of the Jurassic, there are very few fossils to prove it. Maybe they were always rather rare. Marsupials, on the other hand, give birth to tiny little babies that crawl their way through moistened fur to a special pouch (the marsupium), in which they can suckle and grow. They, too, are abundant in Australia, but not entirely confined to that continent; there are South American opossums which have spread northwards into what is now the United States in comparatively recent times. Much of marsupial history is also poorly known. Their early fossils occur well down in the Cretaceous period, but at this time they appear to have been exclusively North American. Somewhat later in the Cretaceous they are quite well known from South America, and one scenario has them populating Australia from there. In the early Tertiary they were practically global. But by whatever route they reached the antipodes, they prospered when they got there.
The history of the mammals is intimately linked to that of the continents. It will be remembered that during the Permian the continents were gathered together into the great supercontinent, Pangaea. For the first and last time in history it was possible for an animal (if it was tolerant of hardship) to wander at will over continental parts of the globe. There would have been barriers—deserts, rivers, mountains—but no great oceans to halt migration. The subsequent story of terrestrial life is entwined with the break-up of that mighty continent, driven by the great engine of plate tectonics. Narrow seas grew into wider oceans. The Atlantic Ocean, north and south, cleaved apart the Americas from Eurasia and Africa. This process was well underway in the Cretaceous, but there were still possible connections, for it was neither a simple, nor a clean split. North and South America then had a kind of sparring history in the Tertiary, sometimes engaging, but more often apart, separated through what is now Panama. Africa had similar interactions with southern Europe, and thence to the rest of Asia. The Tethys, that ancient sea, continually redefined its shape and extent. In the Mediterranean region the shifting relationships of land, continents and seas are as complex and tortuous as the relationships in the dynasty of the Borgias. Antarctica set on its own course southwards, a course of doom for some, but the making of many species of penguins and seals. India left its billet against Africa and drifted away, to collide with Asia, and to ruckle up the high Himalaya, leaving Madagascar behind, stranded near Africa. The tumid west coast of Madagascar is surely the most obvious testament to continental movement; one almost feels compelled to cut it out of an atlas and push it back into the hole from whence it was launched.
Each continent carried with it a cargo of animals and plants, and when they were isolated from their common origin they evolved in isolation. This was a wonderful contribution to the richness of the natural world, for in this way five or six times as many species (or higher ranks of life as time passed) could be supported. Separation breeds diversity. This is why there are so many different kinds of lemurs on Madagascar, or why there are no native cats in Australia, and why llamas look different from pronghorns. Because the number of livelihoods that an animal may have are limited, there were different animals doing similar things on the several nascent continents. Grazers, carnivores, insectivores, canopy-dwellers, all evolved more than once from different ancestral species. And for every continent there were a hundred oceanic islands on which life could also be seeded. So the passing of Pangaea was the creation of the modern world, and a requiem for the vanished supercontinent soon became a celebration mass for the modern world.
The logical way to describe what happened to mammals, therefore, is continent by continent, and I have picked just a handful of examples from each.
The creative effect of isolation is best illustrated by the most solitary continent. Australia launched itself early from disintegrating Pangaea, taking a lonely path which would doubtless be described as rugged independence by the present incumbents of that continent. It took with it a cargo of marsupials; and they remained marsupials while in most of the rest of the world the placental mammals eventually became dominant. There was a huge continent for the marsupials to experiment upon. They are often considered rather a dim kind of mammal, and it is true that philosophical discussion of any kind is wasted upon a koala. But they are by no means the evolutionary dullards that common prejudice might imply. The kangaroo and wallaby, for example, are wonderfully adapted creatures. The female kangaroo is always pregnant, but during hard times development of the foetus is suppressed; during good times the joey develops and takes up residence in the pouch, which is possibly the last word in maternal care. Similar contraptions are sold in childcare shops, made of washable fabric. The lolloping, bounding bipedal gait of the kangaroo is energetically efficient; kangaroos can make good flesh from hard tucker. When they have to live alongside sheep or cattle, both advanced placental herbivores, it is the kangaroo that survives hard times in better shape. Their distinctive design is unique among large plant-eaters. In central Australia you will be grinding along in a Toyota a hundred and fifty kilometres from a town and hours from standing water when suddenly a huge grey or red kangaroo will bound across your path with a kind of earnest nonchalance which is matched by no other mammal. On the contrary, other marsupial designs are extraordinarily similar to those found in mammals with wombs instead of pouches, and this is invariably because they are performing similar ecological roles. There are marsupial mice that look like any other mice, and there was until very recently the Tasmanian wolf (Thylacinus), which looked for all the world like a dog. The last one died in a zoo in 1936. I have a photograph of it apparently hugely yawning, but I am told that this was a threatening posture similar to that used by its relative, the Tasmanian Devil. Such a wide gape reminds one of the agonized screams in the paintings of Francis Bacon, and seems oddly appropriate for an animal which is the last of its kind. Without stretching the imagination too far one can find marsupial raccoons, rats, moles and squirrels. Only a million years ago there was a much wider range of marsupials that roamed the Australian plains. They included a large herbivorous quadruped, Diprotodon, a curious animal that has a passing resemblance, but hardly more, to an African pachyderm. It was hunted by Thylacoleo, which had a plausible likeness to the king of the beasts, though undoubtedly another marsupial. These remarkable animals lived on into comparatively recent times, and some workers claim that man himself may have hastened their end.
Until quite recently the earlier history of marsupials in Australia was largely a mystery. Kangaroos and their like, koalas, Thylacoleo, and the devils and wolves all appeared to have had only a sparse history from the rocks. Then some wonderful fossils were discovered at Riversleigh in northern Queensland that suddenly filled in a story that had hitherto been obscure. These fossils were preserved in limestone, and could be etched out to reveal whole jaws. One of the glories of palaeontology is that there are always surprises. An intelligent guess might have anticipated a set of ancestral kangaroos or possums from Riversleigh. Instead, Riversleigh abounded with many kinds of completely new marsupials, the like of which had never been dreamed of. There were mystery marsupials so arcane that they were known to their discoverers as “thingodonts;” they are now scientifically christened with the more prosaic name of Yolkaparidon, although still nobody knows what other marsupial they are related to, nor how they lived. There were even giant relatives of the Platypus.
Riversleigh is a remote station which lies close to the Gregory river. This is a narrow waterway which winds through the outback, supporting a thin, green ribbon of lush vegetation among the vast plains of gum trees and Spinifex bushes that cover so much of the interior of the continent. In Oligocene and Miocene times the whole landscape was as rich as Amazonia, green and moist, with trees and ferns in profusion. This is the vanished landscape that is recalled in the Riversleigh fossil beds. The forest grew upon an ancient limestone terrain, and as so often happens in such country there were pools and fissures etched by the abundant rains. It was in such pools that animal skulls and bones were preserved, covered with a protective layer of tufa deposited from the limy waters; they are still preserved in the same way in limestone karst country today. Even now, the waters of the Gregory river are so charged with lime that tufa builds natural dams and terraces, over which clear water cascades.
Australia was isolated in the Miocene, having by then been long severed from Gondwana, and so marsupials provided the raw material for a forest full of mammals. Only bats flew in from foreign parts, and it is as well that they did, for it was the identification of a fossil bat genus with one from as far away as France which allowed the amazing fossil finds to be dated. Year after year from the first discoveries in 1976 successive expeditions found new locations for fossils in this spacious land. They proved to be of more than one age. Many of the fossil finds have not yet received a full scientific description—and Latin name—to mark their official existence: they are in a kind of creative limbo, awaiting definition. Almost the whole mammal story of Australia proved to be recorded in the rocks as the investigators moved from one side to another: the fossils demonstrated a gradual drying-out of the climate, which pushed rain forest to a few peripheral sites. There was a growth in variety, and a subsequent decline of large, four-footed herbivores like Diprotodon; and not one, but several marsupial “lions” and “dogs” fought over the flesh of these large animals. In the streams that ran through the Riversleigh forests turtles were more abundant and varied than anywhere else in the world, a belated reptilian blossoming; and crocodiles like Baru were lurking in the swamps, just as they do today in the north around the Gulf of Carpentaria. Kangaroos were a whole tribe of different animals, not just the familiar large hoppers, but also relatives of the rat kangaroos, and potoroos, and, most extraordinary of all, there were meat-eating kangaroos that hunted their peaceable neighbours. We must never assume that what we know from the living world is what pertained in the past, for history can ambush our perceptions. It may be asked how the astonished scientists knew that the carnivorous animal in front of them was a kangaroo. They knew the bones of all kangaroos so well that the features of the skull betrayed their common ancestry—but equally the specialized teeth of a carnivore are unambiguous. The trees of the Riversleigh forests were full of pygmy possums, and many other curious possums besides, some odd and enigmatic (and given odd and enigmatic names like ektopodontids) and known from nowhere else. What emerges is a picture of a forest that was livelier and more varied than anything that remains in the drier climate of Australia today. It was the triumph of marsupials. Far from being poor relations of the rest of the mammals, they were gloriously diverse. Finding the Riversleigh fossils was equivalent to the discovery of the Inca ruins in the Andes, for assumptions about the possibilities of the past were challenged by physical evidence. Just as the hardy peoples that cultivate potatoes in the shadow of the teetering Andean palaces seem to be a pale reflection of a glorious South American civilization, so the marsupials that survive now appear to be but a sample from a vanished cornucopia of life. As the newspapers in Queensland reporting the Riversleigh discoveries put it, “History was rewritten,” and, for once, the cliché was nothing less than the truth. The caution is that it may be rewritten again, that some other corner of Australia will disclose yet another lost episode in the history of life. The sad thing is that some of the most extraordinary marsupials survived until comparatively recently. Would life not have been richer if a walk in the remaining rain forest patches in northern New South Wales still carried the risk of an encounter with a marsupial lion?
The Riversleigh case is the most startling recent example that I know of the discovery of an unexpected fauna. There have been similar cases before. In the last century, the effects of the Tertiary isolation of South America became known to an inquisitive public. We have become accustomed to the strangeness of the animals now, because reconstructions of life on ancient pampas have long been one of the obligatory dioramas which accompany standard histories of our planet. But when they were first known the Tertiary and Pleistocene fossil animals of Argentina caused a sensation. For long periods after the dissolution of Pangaea, South America was almost as much an island as Australia. As a consequence, the animals that evolved there in isolation were as various and striking as those I have just described from our present island continent. Charles Darwin encountered their remains on the voyage of HMS Beagle. He considered Toxodon, which he described on his visit to Patagonia, as “perhaps one of the strangest animals ever discovered.” Well, so he might—after all, it was a massively built creature three metres in length, which may have had a trunk like an elephant but retained something of the rodent about it, and is thought to have grazed upon water plants like a hippopotamus, using its spatula-like snout. Just as in Australia, there were a whole series of extraordinary South American natives. Many of them belong to two groups called notoungulates and litopterns, which include horse-like, camel-like, rabbit-like, deer-like and even rhino-like animals. Several hundred different kinds of these animals are known—they were no flash-in-the-evolutionary-pans. These were womb-bearing mammals, which bore their young not only alive, but nourished by the bountiful placenta within the body. Most of the South Americans were thus more closely related to the mammals that dominate the rest of the world—and to ourselves—than are the majority of Australians. But among them there were other, curious animals that seem to confirm the notion that isolated animals become slow, bumpkin-esque creatures. The sloths still slothfully dangle from rain-forest trees, stuffing leaves and favouring the observer with a stare which is quite evidently dim. These animals even have greenish algae growing in their fur, an appropriate tribute to their vegetating existence. The giant sloth, Megatherium, was a huge relative of these creatures—large enough to stand up against a tree and pull down edible fruits and leaves. The skeleton of one of these magnificent sloths stands outside the entrance to the Department of Palaeontology in the Natural History Museum—not, I trust, an ironical comment on the work that goes on there. It is commonly confused with a dinosaur by those who mistake size for zoological affinity. A relative of this sloth evidently survived into historical times, for its dried pelt, still with a covering of hair, has been recovered from a vast cave eaten out from a conglomerate formation, the Cueva del Mylodon, near Puerto Natales in Patagonia. The animal evidently lived in such caves, for its dung occurs there too, in heaps, and it can still be burned. Would it not be wonderful to drink a cup of tea brewed on giant sloth’s dung! The same species of algae have been recovered from the fossil fur as still live upon the tree sloths. The last of these giants were probably alive only 10,000 years ago. As with the extinct marsupials, there are those who believe that man himself may have hunted the lugubrious giants to extinction.
There were other curiosities in Tertiary South America. Rodents as large as bears were the evolutionary acme of the great tribe of rats, dormice and lemmings. They were named dinomyids—literally “terrible mice,” a wonderful contradiction in terms. On the ancient pampas they were serious grazers. There was a carnivorous marsupial “dog” called Borhyaena. The glyptodont was hardly less extraordinary, and represents the mammals’ closest imitation of the heavily armoured vegetarian dinosaurs such as Ankylosaurus. Nothing closely similar survives, unless it is its living relative, the armadillo. For Glyptodon is a three-metre tank of an animal, with a bony covering forming a great domed shell on its back as massive as that of a giant tortoise, and probably quite as impenetrable. It could scarcely have been a swift mover, but then I doubt that it needed to concern itself with running away from mammalian enemies. When the isthmus of Panama formed, finally connecting the two Americas, it was among the animals that trundled northwards, and survived and prospered for some considerable time. The armadillo was one of its companions in the northward trek, and that animal still flourishes among the mesquite bushes of the southern United States. The glyptodonts died out as the result of a climate change, perhaps, or maybe it was another victim of that uniquely human combination of ingenuity in hunting and killing, and a propensity to destroy beyond recall.
The Panama link ended the isolation of South America; there was complete connection between the northern and southern continents about 3 million years ago. The thin isthmus served as a corridor, and animals moved both ways along it. At least as many mammals successfully moved north as moved south. The horses and deer and sundry carnivores (pumas, bears, dogs) that soon occupied the forests and plains of South America were recruited from the northern hemisphere, where they had had a long and very complicated previous history. Elephants, too, were present among the southward invaders. Armadillos, porcupines and cavies were part of the trade the other way. Many of the other South Americans survived well for a while in Central America and some moved further north, but, as with the glyptodonts, they perished under mysterious circumstances.
There was an altogether opposite effect in the sea. Seashell and coral species were common to the Pacific and Atlantic Oceans through much of the Tertiary across what is now Panama. They were inhabitants of a single sea. When the isthmus prised the two oceans apart, so the animals gradually acquired a personality appropriate to one side of the divide or the other; the gene pool was divided in two. Different and new species appeared that were confined to the Pacific or to the Caribbean, respectively. Specialists can often tell at a glance from which side of the divide a shell originated. This is only now being thrown into reverse by the connection afforded by the Panama Canal, doubtless aided by the steamers that pass through it. So the paradox is that the very land bridge that served to unite the mammals served equally to part the marine fauna.
IT WAS COMPARATIVELY EASY to describe salient features of the occupancy of Australia and South America by mammals after the dinosaurs’ demise. The histories of these continents have a coherence guaranteed by their comparative insularity. Those migrations just described that brought South America’s isolation to an end are bewilderingly and continuously part of the mammal story over the rest of the world. North America and Europe, Africa and Asia, India and Arabia—all have histories which interact time after time. The story is like that of a military campaign in which the various forces surge this way and that; there are flanking movements, and peace treaties, which are then broken on a whim. The movements of each battalion are fascinating to the military historian, even as they are extraordinarily difficult to reconstruct with certainly. The non-historian might be intrigued by a human detail, or the outcome of one or another battle, but cannot be expected to hold in his head the hundred thousand events which are a true description of history. So it is with Tertiary biological history. There were hundreds of different mammals in Eurasia and Africa, and each one has some story to tell. An inventory of these animals would take up the rest of this book, and I am not going to provide it. Almost any summary is a simplification which would arouse the ire of a specialist.
There are some animals so extraordinary that they cannot be omitted without losing some of the most improbable and unpredictable episodes. For example, in the early Tertiary of North America there were giant birds called Diatryma. These birds had lost the capacity to fly, and, because they had taken up the trade of hunter, they had massive bills capable of shredding many of the early mammal herbivores. Looking at these animals it is easy to believe that the dinosaurs did not really breathe their last at the K-T boundary. They lived on as voracious ground eagles striding about on massive, muscular legs. Had these monstrous birds survived, how different children’s stories would have been. Red Riding Hood would have dreaded a monstrous ostrich, and the king of the beasts would have hatched from a royal egg.
Then there are some generalizations which are worth making. Mammal history divides into two halves: the earlier half is the more exotic, populated by many strange beasts that no longer survive; the latter half includes, if not actual members of the living fauna, a selection of animals that are clearly related to species we can now go and examine in a zoo, if not in our back yard. The first half embraces the Palaeocene, Eocene and Oligocene; the latter half much of the Miocene, Pliocene and Pleistocene periods. There were a number of ancient exotics which survived at least until the last, Pleistocene ice age, and even overlapped with man himself, as did the giant sloth. Even so, there are sufficient species related to the living faunas and floras in the earlier phase to allow a different kind of reasoning for interpreting vanished environments from any we have met before (I exclude algal mats): this is a direct comparison with living equivalents. So a fossil palm can be easily assigned to a living family of palms, and their ecological preferences can be directly investigated where they still live today. This makes the assumption—not necessarily a sound one—that their habits have remained unchanged for maybe 50 million years. None the less, this comparative method gives a new precision to scenarios of Tertiary life.
Perhaps the best way to approach European mammal history is to describe another site where the record of life is truly exceptional. This site is in Germany, near Frankfurt, at Messel. Set in the midst of a tract of secondary woodland there lies a vast pit, a kilometre across, now partly filled with water, in which there are a series of dark oil shales so rich in the remains of animals and plants that the deposit has been characterized as a fossil ecosystem, preserved in its entirety. Oil and paraffin were formerly extracted by distilling the shales, and this fossil fuel was itself derived from decay of the profusion of life that lived in and around Messel. For this was the site of an Eocene lake, 50 million years ago, in which soft sediments accumulated to preserve the fossils. The lake was surrounded by a subtropical forest humming with life. From time to time slurries of muddy sediment swept across the lake, entombing animals, plant fragments and insects alike. The rapid burial preserved them all in wonderful detail. Imagine a delicate bat, Palaeochiropteryx, as fragile as a paper kite, with every bone laid out upon a dark slab, as if it had been waiting its turn as an extra in a Dracula movie. Then there are Jewel beetles (Buprestidae), which in the living fauna shine with iridescent greens and blues as precious as emerald: and so they do in the Messel specimens, a dance of colours preserved so perfectly as to mock time. There are giant ants and termites, drowned in the lake during their nuptial flight, which was probably very short—a single day in remote history ensnared for ever upon a rock surface; even their wings and antennae are perfectly preserved. There may have been blooms of algal plankton in the lake which removed oxygen that would otherwise have promoted decay; the anaerobic conditions which resulted safely carried the preserved carcases of these soft-bodied animals through to the present day. A spider, born aloft over the water on a silken thread, met the same fate. Normally, these creatures would have been snapped up by fierce fish like garpikes and bowfins, which abounded in the Messel lake. Similar fish still survive in the waterways of North America, and ichthyologists are convinced that they are among the most primitive of the living bony (teleost) fishes; they were evidently commoner and more widespread in the past. There is one fossil eel.
Many amphibians turn up in the dark oil shales at Messel: there are toads and salamanders and frogs. The frogs appear to be preserved in mid hop, legs tucked in and bowed ready for action—there is even a tadpole or two to show that the lake could nurture the young just like lakes today. Freshwater turtles abounded, especially Trinoyx, and there were no less than six kinds of crocodile, of which the commonest is Diplocynodon (so much for the setback for these reptiles at the K-T boundary!). There are some preparations of these animals that have been extracted entirely from their rocky matrix, bone by bone, so that they look as if they might be freshly imported from the upper reaches of the Parana river. Lizards and snakes were terrestrial animals, then, as now, and by no means as common as turtles in the freshwater sediments. They are of great importance to herpetologists as they are so well preserved. The lizard fossils include early representatives of many living groups: skinks, monitors and iguanas, as well as the legless varieties that do their best to pretend that they are snakes. Not that there is a lack of true snakes; the Messel evidence confirms that this is one group of reptiles that really did have its main evolutionary burst after the extinction of the dinosaurs, although, like the marsupials, their origins were earlier. The fossils preserve every rib, as numerous as the legs of millipedes: there are boas several metres long, fossil stranglers and squeezers, but none of the fanged, venomous snakes which are most numerous in species today; they seem to have been a comparatively modern, Miocene invention.
There were birds. It may be easily imagined that birds are hard to fossilize, having an unpropitious combination of characteristics: they were both delicate and tasty. None the less, some survived as skeletons, even with feathers, and a curious and interesting assemblage they were, too, absolutely vital to our knowledge of the history of the most popular item of contemporary wildlife (there are more pairs of binoculars in Great Britain than there are dogs). There were a few types of birds which can now be matched only by South American seriemas, odd survivors from a formerly common group. But there are also recognizable flamingos, owls, plovers, nightjars, swifts and rollers. Some of the fossils are so delicate that they have to be studied by X-rays. The anatomy of the fossil flamingo solved a controversy that had riven the ornithological community for years. It proved that the flamingo relatives lay with the avocet rather than with the ducks and storks; the fossil had not yet acquired the weird bill of living flamingos, although the rest of the anatomy, including the legs, was already typical. The avocet is the most elegant of wading birds, and has acquired familiarity in Britain as the emblem of the Royal Society for the Protection of Birds. Apart from its distant relative, the majority of Messel birds probably lived in the bushes and trees surrounding the lake. We can be sure that the still, humid air was punctured by the shrill or sonorous cries of birds.
As for the mammals, there was a variety of delicate species that would not have been preserved anywhere else. The marsupials were there in the shape of an inconspicuous opossum; in the Eocene they were still widely distributed. Insectivores included an extraordinary long-tailed, shrew-like animal called Leptictidium, which probably ran half upright on its long hind legs. There were relatives of the hedgehog, complete with spines; but one of them probably (or should it be improbably?) hopped like a rabbit. I have mentioned the bats, preserved complete with their wing membranes. There are three main kinds, and they are commoner as fossils here than anywhere else in the world. They still have their stomach contents preserved intact, which indicates that they died suddenly. It has even been suggested that they were overcome by poisonous fumes emanating from the lake. You can occasionally see vile-smelling and toxic black bubbles glooping up from stagnant pools in wet jungles today, so perhaps it is not stretching probabilities to imagine a stricken bat hitting the water and unable to relaunch itself into the air, struggling fitfully, and then drowning and sinking downwards, to be preserved in the rank mud.
There was a lemur-like animal lurking in the bushes, a small creature with forward-facing eyes, which might not be thought too remarkable, except that the lemurs are primates, and thus primitive members of the group that includes both apes and humankind. The face of this diminutive lemur is the face of the future. A naturalist descendant of this animal hiding among the bushes would have had little trouble in recognizing several species of rodents, for the Messel animals had the characteristic incisors in upper and lower jaws, teeth that need to nibble continuously to hone their gnawing edges. The Eocene species were rather large, as rodents go; some of them were as big as a small cat. Ailuravus has stomach contents preserved, which are invariably full of macerated leaves, so it was probably arboreal, scuttling and weaving rapidly through the branches, nibbling frantically. It might have had cause to leap as fast as it could to escape the attention of predators, for there were inevitably some of these in attendance, although they are very rare as fossils. They have been studied by Dr. Springhorn, who provides another example of the marriage of name and specialization—what better name for a mammal worker than a cross between springbok and pronghorn? One of the hunters was a very early example of the true carnivore line, somewhere close to the common ancestor of lion, bear and seal. Another is an altogether stranger creature, a creodont, belonging to an extinct group that has a different arrangement of its cutting and rending (carnassial) teeth, a dental feature which at once betrays carnivorous habits. Neither probably had much success in chasing pangolins across the forest floor. The Messel pangolin is hardly different from its living counterpart, with its curious but impregnable armour of overlapping, triangular scales, and a mouth specially adapted for a diet of ants and termites. It can curl into a ball to defy even the most determined attacker. There can be no more striking demonstration of the differences in rates of change which happened during the long history of mammals—50 million years ago rodents were near the beginning of their history, but the pangolin had already done much of its evolutionary work. The other anteater is equally suggestive, because it was undoubtedly related to the South American anteater that still roams the pampas from one ants’ nest to another. It is an edentate—a mammal with a reduced dentition—a group which also includes sloths; edentates have always been regarded as quintessential to the South American continent in its island phase, and this muddling migrant was an enigma. It surely proves that there was more contact between South America and the rest of the world than had been thought.
Finally, there were hoofed mammals (ungulates), the great group of grazers that now provide us with food and milk, and steeds and wool—the very nub of most tribal societies. The Messel deposits have yielded the most perfect small horses, Propalaeotherium, with two species, the size of a terrier and an alsatian respectively. That these exquisite creatures are fossil horses is clearly shown by the anatomy of the head bones, yet they still retain several toes in their “hooves,” an ancient feature retained from still more distant ancestors. There were even pregnant mares, preserving the foetus inside the body, down to its last fine bone, surely the ultimate proof of the ascendancy of the womb. These small horses shyly picked their way through the undergrowth, plucking at leaves, only stirred by fear into sudden flight. Later species, of which there are many, became dwellers on the plains, and were progressively built for speed, culminating in forms with one toe—the hoof. The tapir and rhino are distant relatives of horses, and they, too, have early relatives in the Messel fauna. The other major types of grazers are those with cloven hoofs, like cows, goats, camels, sheep, pigs and deer, which between them provide most of the human race with nourishment, and a smaller tribe with sport. Two rather undistinguished-looking animals that foraged on the forest floor were close to the ancestral deer. The most splendid ungulates appeared elsewhere, and in other faunas. This account of Messel inevitably begins to sound like an inventory—but what an inventory! This was a world we could recognize. Rich and varied in its ecology, it was populated by many animals which were only starting their evolutionary history, but which had already acquired the habits that their descendants have today.
Amid this diversity, there were glimmerings of difference in intelligence. The hunter must outsmart the hunted. Warm-blooded physiology hotted up the stakes. More food had to be consumed to feed the metabolic fires; reptilian opportunism had to be supplanted by stratagem, which is part instinct, part experience. Intelligent hunters must be outwitted by delicate nerves, subtle senses. The twitch of a leaf would set a bird into its alarm call and provoke it to flight, or send a small mammal scuttling away down a specially constructed tunnel. Sense built upon sense. Consider the cat—how acute its vision, precise its attack, discreet its stalking, persistent its lust for blood. I have seen feral cats in the middle of the outback in Australia destroying the marsupials created by 60 million years of isolation with their unsheathed claws and nocturnal virtuosity. In the archives of the Natural History Museum in London there are drawings of marsupials which no longer survive in the wild. The books were made in the early days of Australian naturalists, who must have come across the animals in their surveys. Their only record is now bound in leather in the care of the museum archivist. I have examined some of these drawings intently, as one might scan the face of a lost relative, and could not see in their wide-eyed expressions any obvious deficiency; but a cat or a fox evidently knew their vulnerability for what it was. Some cats (and many dogs) established social systems, the better to ensnare their victims and nourish their offspring. Social communication demanded mutual understanding, and that in turn probably placed a premium on greater intelligence. Since intelligence is reflected in elaboration of neural pathways in the brain, this requires more cortical tissue—more of what Hercule Poirot always irritatingly described as the “little grey cells.” Larger brains (relative to total body size) are reflected in brain cases, and this, too, can be discovered from fossils.
We attribute virtue to intelligence, and tacitly condemn many species to a role of stupid, bit-part players. Sheep are legendarily near the bottom of the league, quivering dullards animated by nervousness alone, dunces of the mammal class, dolts and dimwits. It is grudgingly acknowledged that these allegedly obtuse animals can survive in places and conditions where sparkling wits are useless, but somehow the poor sheep acquires no credit for this performance. I suppose, on the contrary, that the sheep is quite as intelligent as it needs to be, and if the wolf is brighter (which it is), this is only in proportion to the ingenuity it must exercise to catch the sheep. If, like those Australian feral cats, it were in an altogether different class, the poor sheep would not long withstand the brilliance of the onslaught—but, within a few generations, unrestrained appetite would have destroyed the larder, and malnourished wolves would then quickly die out. Intelligence is also a matter of context.
Carnivores today lack one of their most distinctive historic designs: the sabre-tooths. They survived until comparatively recently. The last of them, Smilodon, is known from the Pleistocene tar pits of Rancho La Brea. The tar pits are in the middle of downtown Los Angeles in California. Within a few blocks there are the granite-skinned, gleaming facades of banks. It has been a long time since there was a ranch there. The natural tar seeps were once patchy water pools, and any grazing animal that lost its footing might be trapped in the bitumen, never to escape. It is likely that Smilodon was drawn there by easy pickings, and then itself became a victim. Its exhumed skeleton, perfect in every detail, can be examined in the adjacent museum, which is easily recognized among the thicket of high blocks by its modest proportions. There is something ironic about the relics of vanished carnivores secreted in the midst of this chrome-and-glass shrine to business competition, and when the great San Andreas quake finally brings it all tumbling down, I like to imagine the shades of Smilodon prowling among the carcases of the greediest animal of all, and then wildness will return again to the Pacific coast. Smilodon’s sabre-like canine teeth were fifteen centimetres long, and the cat was capable of opening its mouth into a great gape, a precursor to stabbing. The teeth were probably able to penetrate the tough hides of the rhinoceroses and elephants that also inhabited North America (and Europe) at the time. The sabre-toothed cat is reputed to have been able to bite out a chunk of flesh before leaving the victim to bleed to death. It is an extraordinary fact that similar fangs were evolved not once, but on several occasions in Tertiary times, and among very distantly related animals—they even appeared among the South American marsupials. This kind of parallel (correctly termed “convergent”) evolution is familiar to palaeontologists. Nature is not profligate with useful designs.
In those television series in which actors in spaceships explore paradoxical corners of the space-time continuum it is odd that most of the aliens they encounter look remarkably like Homo sapiens, apart from their often being greenish. I have occasionally wondered whether these episodes might have been scripted by a palaeontologist, who has deduced that the optimum shape for an intelligent being is upright with arms, legs, eyes and clean teeth. Otherwise, there really seems no explanation for the high degree of convergence, which is arguably a similar case to that of the sabre-tooths. A little more reflection reveals the improbability of my intergalactic scenario. History is not just a matter of this chapter, but of the whole book. The sabre-tooth arrived at its design as much because of events in the Devonian, when land tetrapods first acquired legs and fingers, as because of events since the extinction of the dinosaurs; design is a consequence of a thousand prior circumstances. The chances of the worthies of Star Trek encountering a matching history on some distant planet—especially a history which could produce a blonde with lips—is statistically remote. Even the simplest planetary difference would redirect every detail. To take a simple case, a larger planet with higher gravitation would have implications for the size and musculature of any animals that evolved—and who knows if the answer to intelligent locomotion might not be the evolution of a cerebral worm with a hundred wheels? I conclude that the predominance of humanoids in space fiction is because it is difficult to generate drama with life forms resembling custard.
A less flippant question is whether the possibilities of design have been fully explored on our seas and continents. For example, are there ways of being a predator which could be produced by our mammalian anatomy, but which have never been tried? Is it easier to imitate than to innovate? The answer to this might be, as they say on Star Trek, “affirmative.” To consider one example, the possibilities of venom have not been fully explored in mammals, although so effective in one order of reptiles—the fanged snakes (a lizard, the Gila monster, is also venomous). One could imagine that a poison-fanged cat would be invulnerable. There is no reason, in principle, why a venomous hunting mammal should not have evolved, given the fact that poison glands have evolved on so many occasions in the animal kingdom as a whole. Instead, the cat’s adaptations were towards refinement of the senses, combined with intelligence and claws. Doubtless that is enough. The converse question is whether there is some ecological role that has been unexplored, some wholesale opportunity missed. I stated early on in this book that there is but a finite range of ecological roles, although there are equally many living players willing to act them out. This seems to be as true in the age of mammals as in the age of trilobites. After some thought I have identified one niche which never seems to have been occupied. High in the atmosphere there is a stream of air which transports insects and spiders, like some plankton of the ether. Could an aerial “whale” have evolved to harvest this stratospheric protein: a light, flying animal with a wide feeding gape, an animal that could cast a shadow across the sky? There may be several respectable, mechanical reasons why such a creature could never exist, or maybe it is that, like Icarus, creatures of the earth were not meant to soar too close to the Sun.
On the contrary, it is difficult to think of anything of which an insect might be capable which some species or another has not succeeded in doing. Feeding on living flesh? Mimicking a bird dropping? Or a dead leaf? Or another, nastier insect? Living on nothing but paper in the complete absence of water? There are insect species which are dab hands at all of these activities. Insects steadily continued to proliferate through the Tertiary as never before, even as the flowering plants prospered and diversified. Butterflies added their gaudiness to the forest glades. Bees perfected the art of pollination. The sights, smells and sounds of a summer afternoon grew from the growing interdependence of plant and pollinator. Honey appeared. So the biblical paradise-on-Earth, the promised land “flowing with milk and honey” could not have flourished prior to the Tertiary, since milk and honey only became abundant in the Tertiary. In millions of rotting logs, or dungheaps, or dark places in caves, the phalanx of beetles was doubling and redoubling. We know little of the process, but much of the effects of the success of the Coleoptera. Beetles are nowadays so rich in species that we will never know or name them all. When asked what characteristic God might display, J. B. S. Haldane famously remarked, “An inordinate fondness for beetles”—a phrase which might well serve as a motto for all coleopterists.
WE KNOW THAT as the continents moved slowly to their present positions, and while the oceans opened, volcanic islands welled up from the mantle and broke the ocean’s surface. They, too, would have been populated by insects, and there is no doubt that special beetles would have evolved on each island. The island may then have sunk beneath the waves, and, no question, carried their endemic beetle species with them to extinction. Island endemic species are famously vulnerable. The most timely example is a death on 31 January 1996, when the last specimen of Partula turgida, which was kept in London Zoo, was declared “demised.” This is (I should say was) a tree snail, rather than a beetle, one of more than 100 species endemic to the volcanic Society Islands, near Hawaii, which arose as a response to their remoteness and isolation. Introductions of other slicker, foreign snails are held to be responsible for their subsequent fate. There was a French collector, M. Thirioux, who acquired and “preserved” the last two specimens of an exotic lizard species from Rodriguez Island, in the Pacific Ocean, only to die himself of a heart attack later the same day, thus neatly ensuring the simultaneous demise both of a species and his own person.
It is not surprising that the fossil record of ephemeral islands is poor. In the Mediterranean region half a dozen ancient islands have been recognized, dating from Miocene times. They, too, produced bizarre endemics. My favourite is a giant hedgehog, Deinogalerix, five times longer than the hedgehogs in my garden, which was probably the terror of all the smaller mammals on its island home. Once the island was no longer isolated from the mainland, and thus the biological history of Europe, the inelegant monster no longer filled the bill, and it became extinct. There was a giant dormouse on Malta in the Pleistocene which was the size of a squirrel. There are some who link the myth of the one-eyed giant, Cyclops, with an extinct Mediterranean island elephant, fossils of which show the large, median nasal opening (surely a gaping “eye”) which is a distinguishing character of these pachyderms. Odysseus and Theseus voyaged among the Aegean islands, which were the abode of several incomparable monsters. One can imagine an astonished mariner coming across a fossil in a cavern, or weathered out of a limestone bluff, and deducing the form of the creature that left it behind. The hero then scurried off, wide-eyed with hyperbole, and a legend was in the process of being born. In all truth, there really were islands which must only be imagined where unmourned monsters once thrived, now consumed beneath plate margins, or plunged beneath the sea into an early oblivion. We may people these islands with what phantasmagoria we please.
THERE IS NOTHING more superficially ordinary than grass. It is there to be lain upon, to be fed to our animals and cursed roundly every Sunday in summer. It is a flowering plant, although its wind-pollinated flowers are scarcely conspicuous—notwithstanding that hay fever is a conspicuous human reaction to its prolific pollen. However, grass is a special herb. The appearance of grasses in the Tertiary was of crucial importance to the modern mammal fauna, for many of the animals that figure prominently in human history feed, predominantly, upon grass. It has a remarkable property: its leaves grow from concealed bases—not from the tips of shoots, as is the case with most plants. So grass can be cropped—its leaves endlessly nibbled—without compromising its generative heart. Grass makes meadows, which virtually nothing else does.
“All flesh is grass,” so the Book of Isaiah tells us, and indeed much of it is. Even the flesh of wolves is (in fairy tales at least) the flesh of sheep, and hence grass transformed. Its place in the economy of things can be compared with the endless “soup” of algal plankton that forms the basis of many food-chains in the oceans. Grasslands finally took over large tracts of the world during the Miocene, a time which was so often a watershed at the origin of the modern world—the great grasslands, savannah, prairie and pampas, date back to this period. The ultimate control on the spread of grass may have been climatic, especially the amelioration of the tropical belt produced by the growth of the Antarctic ice sheets. These became established when the Antarctic continent had drifted fully to its southern polar position. In tandem with the expansion of grasses, ruminant animals—those that chew the cud—were best able to exploit the new feeding opportunities. The several “stomachs” of ruminants like cows, camels and deer means that the process of gathering food can be separated from the longer process of digesting it. The pulp is brought back up into the mouth for a second chomp at leisure, and this creates an efficient method of converting grass to flesh. The rise of such grazers may even have discouraged plants without the special, regenerative growth habit of grasses. In rich herbivore communities, like that of the African savannah, grass-grazers lived (and still live) side by side with animals that nibble shoots from trees and shrubs, or even process whole branches, as do elephants.
These magisterial mammals were once much more various and widespread than they are today. The earliest Eocene elephants were about the size of a small pig. As their tusks grew in length from one species to another, so, presumably, did the trunk, which took over the job of stuffing the mouth with the great quantity of food an elephant needs. It really is the most extraordinary structure—without parallel for muscular flexibility in the animal kingdom—apart from the octopus’s arm. By the Miocene, there were many different kinds of elephant, of which my favourite has to be the splendidly named Gomphotherium, a massive elephant with four tusks rather than two, which were carried on the head like two pairs of ungainly tongs. The tusks originated from both lower and upper jaws. There was another elephant, Deinotherium, in which only the lower jaw was so favoured, its one pair of tusks curving down like the tines of some kind of primitive agricultural instrument; it looks improbable, like one of Dr. Dolittle’s inventions. The rest of the anatomy of both Gompho and Deino looks conventionally elephantine. Modern elephants, of course, have only the upper pair of tusks developed. Their extinct relatives roamed the world in the Pleistocene Ice Age. The woolly mammoth is the one always pictured in reconstructions of life in the Ice Age. Vast, with tusks as elegant as Brancusi sculptures, the shaggy animals fed in the tundra and forests which fringed the great ice sheet that covered so much of the northern hemisphere 40,000 years ago. We know its anatomy in extraordinary detail because deep-frozen specimens have been recovered from several sites in Siberia. They were engulfed by bogs and then frozen into the permafrost. Their meat is so fresh that it has been eaten by modern dogs; the fossil hair is coarse and brown, and concealed within it there are even parasites. Surely, if any animal could be revivified it should be this one, for even its DNA is preserved—in pieces. A straight-tusked, mastodont relative of this creature was one of the victims caught in tar pits at Rancho La Brea, but this species is now bones alone. The great, grinding teeth of elephants turn up quite regularly from the terrace deposits of the river Thames, even in the middle of London. Each tooth is ribbed like a bony washboard, but as massive as a kerbstone. When I was a child I had an encyclopaedia which pictured London not so many thousands of years ago, somehow cleverly superimposing Nelson’s Column in Trafalgar Square with faded elephants (and somewhere in the background rhinoceros and hippo, too) so that in the artist’s eye the extinct animals were as real as the Column, and dwarfed the famous lions at its foot. This picture has stayed with me as a memento of how even the solidity of paving stones and the pomp of monuments are little more than a phase in our history, and may yet vanish and be forgotten as so many other scenes have passed away before.
The Ice Age of the Pleistocene period is the third one I have described in this history; in geological terms, we have only just emerged from it. The waxing and waning of ice sheets during the last 1.5 million years forced great migrations upon animals. A number of species died out for ever at the onset of global refrigeration. But it made opportunities for others. Cold-lovers, like mammoths, cave bears and woolly rhinoceros, prospered at times when the ice advanced, while warmth-loving animals, like hippos and elephants, supplanted them in the northern hemisphere during warm phases (interglacials) between glacial pulses; some of the interglacials were even warmer than the climate is today. The vegetation changed in harmony, and obvious changes in the types of pollen preserved in sediments supply a kind of thermometer for the past. The Pleistocene was so close to the present that we can be quite certain that the plants will not have changed their habits since they contributed their fossils. There were several major cold phases, and more have been recognized in the last few decades. Even the four major advances of the ice sheets on land in the last million years are now known to have been interrupted by many more minor pulses, short-lived ameliorations of the climate known as “interstadials.” At its greatest, an ice cap extended through the centre of North America beyond the Great Lakes, and in Europe covered much of England and Germany and Russia. The ice left its signature on scratched rocks, and deposited moraines, just as I described from Oman as the legacy of the ancient Gondwana glaciation of the Carboniferous and Permian periods. The signs are more blatant in Scotland or Wisconsin, less glossed over by the passage of tens of millions of years. You can still see glaciated valleys, or pluck scratched pebbles from the glacial drift, as if the ice had retreated only yesterday.
It is not difficult to visualize herds of reindeer and mammoths moving in around the tundra as warmth-loving mammals drifted southwards, prompted by their subtle instincts of a change for the worse. Some giant mammals evolved specifically to cope with cold, because large size is efficient in a cold climate for conserving heat. There were giant bears, and Irish elk, with antlers to match. There is a whole room in the basement of the Natural History Museum in London hung with these elk antlers, like the abandoned trophies of some megalomaniac big game hunter. Not all the interesting animals are giants; the arctic vole also has its story to tell, as its appearance signifies a cold pulse in the climatic history. The history of ice advance and retreat has become very complex, and ice age climate modelling is becoming a major field of investigation in its own right. The least ambiguous record of climatic events is probably preserved in sediment cores recovered from the deep sea, where the gentle rain of microscopic fossils continued unabated even as ice sheets grew and shrunk on land. Various species of planktonic animals moved back and forth, north or south, in sympathy with the fluctuations in the climate. Hence a core taken from the sea floor shows a diary of climate change that is more easily read than the shirting glacial deposits left on land, where a younger glacial pulse may have polished away the record of an older one. Limy fossil shells have even retained a climatic signature in the elements that make up their skeletons. Modern equipment can measure isotopes of oxygen precisely enough to read the fluctuations in temperature directly. This is how the new, and very complex curves for waxing and waning ice sheets were computed. Nick Shackleton at Cambridge University was one of the first to do these calculations. He is one of a dynasty of geological Shackletons. His father, Robert, is famed for his indestructibility as well as his acumen. At the age of eighty-five he led an expedition across the high plateau of Tibet, which would test the stamina of anyone not brought up on rancid yak’s butter. Both Shackletons are related to Shackleton the explorer, and it is oddly appropriate that Nick should be doing his research into the vagaries of climate which tested his great-great-uncle.
Our fellow mammals are so closely bound to human history that it is especially hard to consider their evolution in a detached way. Mammals have been our companions and our food, and in many societies they have also been the barometer of wealth; a man may be judged by the number of cattle he has, in the same way that corporate man takes note of the model of his neighbour’s company car. We talk to dogs and cats as to uncomplaining friends. It is scarcely surprising that any account of mammals has a tendency to anthropomorphize. Beatrix Potter’s animals still look like the creatures they are supposed to be, give or take a few clothes. But the descendant of her mice is called Wiffly the Mouse and lives in a chintzy drawing room and never seems to do mousy things like eat undesirable creatures: home baking’s more the thing. Mickey Mouse takes the process a stage further, having lost most of the protuberant nose, and with the face shortened to make him—well, virtually human. The same thing has happened to bears; the early teddy bears had long noses like the dogs to which they are related in the mammal Order Carnivora. They have got shorter and shorter, until now the teddy bear is, essentially, an orangey, furry human baby. Whatever the rights and wrongs of hunting it, the baby seal is such a popular icon because of its big eyes and round face, and general babyness. We breed the snouts out of dogs and treasure cats with huge, moon-like faces, like babies freshly placated. Koalas are the marsupials that small girls want to take home from the zoo. Ugly animals are those who either resemble unappealing—indeed very ugly—humans, or those that do not resemble us at all. Some of the baboons appal by their uncanny resemblance to Uncle George after a few drinks. Of all the animal kingdom, the mole rat is probably the least appealing aesthetically, however interesting its social life, looking as it does like a bloated, toothed maggot.
The tendency to anthropomorphize has produced varieties of pseudo-evolution. Rudyard Kipling did it best with his Just So Stories, amusingly mythic but emotionally plausible accounts of how peculiar animals got their peculiarities. It is genuinely difficult to find an entirely adaptive explanation of how the leopard got its spots; nor are fossils likely to help us, since the one thing that is never preserved is spots. There are some narratives which are revealed by the truth of bones: I have mentioned whales with limbs, and growing elephant trunks, and I might have explained how much is now known of the early history of giraffes prior to development of the neck. Tribal societies often intertwined the stories of the origin of their favourite or revered mammals or birds with stories of their own gods. These tales serve to bind the people with the animals they hunt. The kind of ritual dancing that many North American native peoples employed is as much an expression of reverence for the hunted animal as it is part of a sympathetic magic to ensure the success of the hunt. Explanations can do more than simply square with the facts of descent. I am sure that my blandishments about the breast being a modified sweat gland, or about the history of the limbed whales, would be treated with amused astonishment by peoples who know how the gods made the animals. And surely the story I have related in this chapter is as astonishing as any tale of creation told to young hunters around the fire, even as yelping mammals called to one another in the distance, and the undergrowth rustled with the scurrying of tiny, furry creatures in search of the night’s supply of insects.
67. One of the last of the South American endemics. The superbly preserved pelt of a sloth, Mylodon, which was probably still alive 13,000 years ago
68. An Eocene bat from Messel—a perfect preservation of a delicate mammal
69. An X-ray photograph of the same species of Messel bat Palaeochiropteryx, showing the finest details of the skeleton
70. The extinct elephant Zygolphodon from Greece, with its Cyclops-like nasal opening
71. The mammoth found in 1793 in the delta of the Lena River, Siberia, in the company of a woolly curator
72. Leg of mammoth complete with wool, mummified by permafrost, from the Indigirka River (found 1972)
73. Giant single-celled foraminifera, Nummulites gizehensis, abounded in the warm seas of the early Tertiary. They are the size of small coins. This limestone is famous for providing the building material of the Egyptian pyramids.
74. Lady Smith Woodward’s tablecloth. The wife of the Keeper of Palaeontology in the British Museum embroidered the signatures of all her teatime guests upon her tablecloth.
75. The “father” of continental drift, Alfred Wegener, in field gear
76. Dr. Louis B. Leakey— a pioneer in the quest for human origins in Africa—studying Australopithecus boisei, one of the “robust” australopithecines
77. Footprints proving upright gait in hominids more than three and a half million years ago—the famous Laetoli (Tanzania) discovery of 1976
78. The story in the skulls. The conventional evolutionary alignment from primitive to advanced (and, more or less, old to young); clockwise from left: chimpanzee; Australopithecus africanus; H. erectus Zhoukoudian, China; H. sapiens “Cro-magnon;” H. neanderthalensis
79. Above, a comparatively crude and ancient early Palaeolithic stone tool from Olduvai Gorge; left, flint hand axes of Upper Palaeolithic type (Thames Valley, England), showing more sophisticated manufacture
80. Neanderthal man—an image of the “ape-man” in atavistic, Yahoo-like form published in the Illustrated London News in 1909. He recalls the evil Mr. Hyde in the silent film versions of R. L. Stevenson’s The Strange Case of Dr. Jekyll and Mr. Hyde.
81. An illustration of Yahoos from a nineteenth-century edition of Gulliver’s Travels by Jonathan Swift, c. 1840, illustrated by J. J. Grandville
82. A Neanderthal skeleton, buried on death, indicating their reverence for the dead (Kebara, Israel; about 60,000 years old)
83. House of mammoth bones, about 20,000 years old. The construction had been completely buried beneath a covering of loess and was reconstructed by archaeologists. Mezhirich River, Ukraine
84. Excavating a cave for remains of the history of Homo neanderthalensis. Palaeolithic Vanguard Cave in Gibraltar being excavated in 1996 by Tracey Elliott (left) and Lucy Gibbons, who rest on one time plane while successively older ones show as bands in the rock layers beneath their knees
85. Artefact as art: an exquisitely wrought late Palaeolithic stone tool in its original haft of antler—surely the product of advanced sensibilities. Late Palaeolithic, Switzerland
86. Temples to the laws of chance: ranks of one-armed bandits in the gambling casinos of Las Vegas
87. Emmer wheat, one of the ancestors of modern cereals
88. Isolation produced a list of endemic insect species on the Hawaiian Islands in a few million years. This example is an Antlion, Eidoleaon.