"at the end of the 18th century, people refused to believe their eyes." "They said it was a hoax." "Bits and pieces of different creatures rather crudely sown together." "But it's no hoax, it's a platypus." "And yet, in a way those early sceptics were right." "The platypus is the most extraordinary mixture of different animals." "It's part mammal and part reptile." "And so it can give us some idea of how the first mammals developed." "At first sight the platypus looks like a regular mammal." "It has, after all, dense soft fur, which is one of the hallmarks of mammals." "And when you handle it, you can feel that its body is warm." "Its feet are webbed for better swimming, rather like an otter's." "The strange bill is not hard, like a bird's beak, but soft, rubbery and very sensitive." "The platypus, which has poor eyesight, uses it to find its food:" "crayfish and other water-living creatures." "But when it comes to breed, it does something that separates it from all other mammals except one." "In its nest, deep in a burrow, it lays eggs." "It's this that links the platypus with the reptiles, this that entitles it to be regarded as the most primitive living mammal." "But, and this is what makes it doubly paradoxical, when the egg hatches, the young platypus is not left to find food by itself, like reptile babies are, but is provided with food by the mother." "The platypus, like all mammals, has in its skin, as a mechanism for dealing with overheating, sweat glands." "On the underside of the body these sweat glands are specially big and produce a kind of fatty sweat, which is milk." "It simply oozes from the skin, and the young suck it from tufts of hair." "There is no nipple, so it hardly qualifies as a breast, or a mamma, its technical name, the feature that gives mammals their name." "Only one other mammal lacks a true breast, and this is it, the echidna." "It too, like the platypus, lives in Australia, and it too lays eggs, but the female doesn't deposit them in a nest, she carries them around with her." "They have sticky shells and become glued to the hair on her underside in a temporary groove that develops across her stomach." "Each is no bigger than a pea, and after ten days it hatches." "By now, glands on either side of the groove in the mother's stomach are producing rich, creamy milk." "The baby echidnas remain inside the groove for the next eight weeks, steadily taking in milk and growing." "When the spines develop, the youngsters must become pretty uncomfortable passengers, and their mother deposits them in a den." "Eventually, they abandon milk and take to their adult diet:" "ants." "The long snout, like the platypus' bill, is, in evolutionary terms, a recent acquisition, a specialised tool for food-gathering." "It houses a long, sticky tongue, with which the echidna flicks up its ants and termites." "The animal defends itself by the simple expedient of digging downwards, so that within a minute or so, there's nothing to be seen but spines." "The platypus and the echidna are isolated oddities." "We've virtually no fossil evidence to tell us where or when they developed." "It's a reasonable guess, because there's another kind of echidna which lives not far north of here in New Guinea, that the group did originate in this part of the world." "It's absolutely certain, however, that they do represent a group of primitive creatures from which more modern mammals developed about 180 million years ago." "And of course, we can trace back the ancestry of mammals even farther than that." "The reptiles had first developed about 300 million years ago." "They had watertight skins and shelled eggs, so they could survive in the driest country." "After some 20 million years, a group of swift-moving, hunting reptiles evolved called pelycosaurs." "Reptiles can't generate heat within their bodies, and that means that after a cold night, they are very sluggish and slow to get going in the morning." "Pelycosaurs dealt with that difficulty by developing great sail-like fins along their backs to catch the first rays of the morning sun so they could get out hunting really early." "In places like this it's easy to imagine some 12-foot species of pelycosaur, like dimetrodon, lying basking on the rocksin the early morning sun." "It's been calculated that with the aid of their fins, they could raise their body temperature some six degrees inside an hour, whereas without them, it would take nearly three hours." "But those fins were stopgap devices only." "Later species of pelycosaurs managed to do without them." "That's probably because they were able to generate heat internally, and their teeth provide evidence which supports that idea." "Dimetrodon's teeth, like those of most reptiles, were simple spikes which did little more than grip a victim." "But generating heat within the body requires a great deal of energy." "So a warm-blooded animal must eat much more food than a normal reptile and digest it fairly rapidly." "Changes in the teeth of successive generations of pelycosaurs suggest that that is just what they did." "The simple spikes changed into specialised tools for butchery." "Daggers appeared on either side of the upper jaw for slitting open the hide of the prey, knives for slicing the meat and grinders for crunching bones." "Most reptiles shed teeth as they become worn sporadically throughout their lives." "But the teeth of these creatures not only became specialised but permanent, and the upper and lower ones meshed accurately to give a highly efficient bite." "And since these creatures generated their heat internally, they would also have needed a coat of hair in order to conserve it." "The mammals had arrived." "The acquisition of warm blood brought more advantages to these little creatures than straightforward speed of movement." "As you would have seen had you been able to walk through the forests of 180 million years ago at night." "The first true mammals appeared at a time when the reptiles ruled the world." "But those great solar-powered animals had one major disability." "At night, when it got cool, they became sluggish." "That left the field open to any creature that could be active at night." "And the mammals, with their warm blood, could do just that." "The ancient mammals were small, nocturnal insect hunters that relied chiefly on their sense of smell to find their food." "In fact, they were probably very similar to present-day shrews and hedgehogs, although they may have laid eggs." "Warmth was the key to their survival and ultimate success." "Since they alone could hunt during the cool of the night, they didn't have to face competition with the reptiles." "Those primitive mammals were able to live right through the age of the dinosaurs and be ready poised to inherit the world when the reptiles finally declined." "The problem of keeping warm was one that didn't just face adults, it also faced eggs and embryos." "And the mammals developed three ways of dealing with that." "The primitive ones incubated their eggs, as the platypus does today." "Other mammals developed more efficient methods." "The opossum, that lives in North and South America, is one." "It doesn't lay eggs, but gives birth to its young only twelve and a half days after mating." "There may be as many as 20 of them." "They're only the size of bees, and their only well-formed features are their front legs." "With these tiny limbs they haul themselves through the hair of their mother's belly on the first and the most hazardous journey of their lives." "At last they reach a pouch in their mother's belly." "Inside are 13 nipples." "Each of the tiny infants fastens onto one and begins to take milk." "If more than 13 babies were born, the last to reach the pouch will find no vacant nipple and die." "The Latin for pouch is marsupium," "This gives the name to the whole group of mammals that reproduce in this way." "They're marsupials." "This is the woolly opossum." "Its babies are sufficiently well-grown enough to have left the pouch, but they still cling to their mother and return to the pouch for drinks." "There are about 70 different kinds of opossum in the New World." "Most of them live in South America." "Some are as small as mice, others as big as domestic cats." "There's even an amphibious one, the yapok, which has webbed hind feet and eats fish." "When the yapok mother goes for a swim, she can close the opening to her pouch to prevent her babies from drowning." "They still need to breathe, so she can only swim for a few minutes when she has young." "The mouse opossums are very like the earliest marsupials of all." "And fossils of similar creatures have been found in rocks that also contain the bones of dinosaurs." "They live now, as they must have done then, by feeding at night on worms, insects and small reptiles, like lizards." "A slightly larger one lives in the cool, dank scrub of the high Andes, the rat opossum." "It's a ferocious hunter with forward-pointing fangs in its lower jaw, with which it stabs its prey." "It too has a very ancient ancestry." "It doesn't even have a pouch, but its young hang to their mother's teats like a cluster of berries." "Fossils closely resembling those primitive marsupials have been found here in America dating back some 60 million years." "It makes them by far the oldest marsupial fossils known, much older than any that have been found elsewhere in the world." "So it seems reasonable to assume that the marsupials originated here in America." "If that's so, how did they get to Australia, where they flourish in the greatest numbers today?" "Well, this tree may provide evidence for the answer." "It is growing here in the bleak lands of Patagonia, on the southern tip of South America." "It's a kind of beech related to the European beech, and called, in fact, the southern beech." "It's a tree with a very long ancestry." "It was growing here when the marsupials first appeared, and it seems likely that they lived in forests not unlike this." "It's only relatively recently scientists have demonstrated beyond all doubt that the continents are not static, but have been drifting slowly over the surface of the globe for many millions of years." "To go back to the period when the first marsupials appeared in South America is to return to a time when that continent was not connected to North America, but fitted alongside the west coast of Africa." "Australia and Antarctica were also joined, and they lay beside the east coast of Africa." "The forests of southern beech grew over many parts of this great land mass." "But as it split and drifted apart, the separate pieces carried with them the beech forests and the marsupials that lived in them." "The middle section drifted over the pole and become covered in snow and ice, so that the forests and their inhabitants died out." "In the eastern fragment, however, they flourished, for that was Australia." "Here in Australia these ancient, beautiful trees, the southern beech, still grow,just as they once did in Antarctica and still do in South America." "Living evidence of the one-time unity of those three great continents back in geological time." "With them in these forests grow other ancient plants: tree ferns and cycads." "Living in holes in their trunks and scurrying around in the beech leaves on the floor, are small, warm-blooded, furry creatures that bear their young in the same way as the American opossums: marsupials." "The Australian marsupials fared much better than their American cousins, for South America continued to drift." "And eventually it came into contact with North America, and more advanced mammals from that continent invaded south." "The South American marsupials couldn't face the competition, and many of them became extinct." "But Australia was very different." "This huge island continent has remained cut off from the rest of the world, and here the marsupials have become and remained the dominant mammals." "They've branched out into many many different forms, and a great number of them are active at night." "Some are very similar to their South American cousins, the opossums, and, indeed, are known in Australia as possums." "There are mouse-sized ones here, too, and like their American relations, the female carries her young clinging to her." "This is the smallest marsupial of all." "It may look like a mouse, but it's very different." "It doesn't gnaw seeds, but hunts insects and will unhesitatingly tackle really big ones." "There are two dozen kinds of these mouse-sized marsupials." "Their reproductive techniques are the same as any others' in the group, but because they are so small the process is extremely difficult to observe." "By providing this expectant mother with a nest floored by glass, it was possible to film a birth for the first time." "30 days after mating she licks the opening of the birth canal." "Her minute, worm-like young, smaller than a grain of rice, will emerge from it immediately after the release of birth fluids, and within three seconds squirm across to the pouch a few millimetres in front." "First, the birth fluids pour out." "There's the first one, and there's the second." "Here's that crucial moment slowed down." "The opening to the pouch is that dark patch above the middle of the picture." "The young comes out of the birth canal... ..there." "And it's gone." "The female may produce six to eight young in a single batch." "When they first arrive in her pouch, the babies are so small their mother seems almost unaware of their existence." "But they grow fast, and within a week or two become a considerable burden." "Eventually the pouch can no longer hold them, and they hang beneath he like squirming pink grapes." "They don't let go of the teats until they're 56 days old, and they'll go on suckling milk sporadically for many days after that." "When they are three or four months old, they become independent and can join their parents hunting insects in the Australian night." "Other, larger marsupial hunters seek larger prey." "This is the quoll, as big as a cat." "It has a sensitive nose and acute ears to help it find its food..." "..a marsupial mouse." "Bigger still, the size of a corgi dog, the Tasmanian devil." "The quoll has found some carrion." "But it doesn't stay long when the devil appears." "Here's a second." "Like hyenas, devils devour pretty well everything: skin, bones, the lot." "Not so long ago, there was an even bigger marsupial hunter." "These are the jaws of the thylacine." "The last recorded living individual died in a zoo in 1933." "And today the species may well be extinct." "The resemblance between this marsupial and the wolf of the northern hemisphere, which does not rear its young in a pouch, is remarkably close." "The processes of evolution, even when working on different stocks, tend to produce similar creatures to fit similar ways of life." "The thylacine and the wolf are both swift-running flesh-eaters, and, consequently, they look very much the same." "There are several other such parallels between marsupials and other mammals." "The numbat has an elongated nose and a long sticky tongue, like a pangolin, so it's no surprise to find that, like a pangolin, it feeds on ants and termites." "One of the closest parallels of all appears in the eucalyptus trees of Australian forests at night." "This little marsupial is called a sugar glider, and with good reason." "In both its appearance and its acrobatic skill it's almost indistinguishable from the flying squirrel of North America." "Both have a wide flap of skin between their legs that catches the air and enables them to glide great distances." "Evolution in Australia has not always produced such close parallels to its products elsewhere." "This, for example, is the koala." "It lives in trees and eats nothing but the leaves of just a few particular kinds." "Sloths in South America do much the same and are equally fussy about their leaves, and so are some monkeys in Africa, but neither they nor the sloths look like the koala, which has an Australian charm all of its own." "The wombat is a much less specialised cousin of the koala's, and it lives entirely on the ground." "It too is a vegetarian, but is very much less selective about what it eats." "If you had to pick a northern hemisphere version of this creature, it might well be the marmot." "Both graze, and both dig burrows for themselves." "There are several kinds of wombat." "This is another, the hairy-nosed." "It too is a burrower, and neither it nor any wombat is exactly renowned for its darting intelligence or speed of reaction under distressing circumstances." "Bandicoots look at first sight rather like rabbits, but the parallel here is not really close at all." "Rabbits eat grass, bandicoots insects and meat." "The similarity between the ears and those of long-eared rabbits, like the American jackrabbit, is because both live in hot deserts and use their ears for cooling their blood." "This Australian, the honey possum, has no close equivalent at all elsewhere." "It lives on nectar, which it gathers with a tongue that has a brush at the end." "If you wanted to find a parallel for that, the nearest would be the brush tongue of nectar-feeding bats." "When the southern supercontinent broke up," "Australia was largely covered by forests, and those that remain still contain some very primitive marsupials." "This little creature is a potoroo, and in it you can see the beginnings of features that characterise the most famous of all the Australian marsupials, the kangaroos." "For one thing, the potoroo has a tendency to hop." "For another, its young retain the habit of popping back to the pouch for a drink of milk until long after they're able to fend for themselves." "They have a preference for travelling that way even when they're quite large." "In more open woodlands there are animals that have developed these two tendencies further: wallabies." "There are two dozen different kinds of them, and this one is known as a pademelon." "This odd wallaby has colonised the huge tropical island just north of Australia," "New Guinea." "Very few animals of any kind live here." "There's this creature and a kangaroo that, unbelievable it may seem for an animal clearly designed for hopping, has taken to trees." "The tree kangaroo seems just about the clumsiest climber of all tree-living creatures." "The explanation seems to be that here in New Guinea it's the only mammal that's got up into the branches." "With no other creature competing with it for leaves, it's had no need to become better adapted." "It can get all it wants just as it is." "All in all there are over 150 different kinds of marsupial in Australia and its offshore islands, like New Guinea and Tasmania." "But not so long ago, there were even more." "You can see the most spectacular evidence of their existence in" "Australia's caves." "In 1969 a couple of zoologists came crawling down this narrow cavern in the limestone hills of Naracoorte in South Australia, not far from Adelaide." "They were the first people ever to come this way, and they were hoping that they might find a bone or two." "What they discovered, another quarter of a mile farther on, exceeded their wildest imaginations." "They discovered the greatest and most important deposit of bones ever found in a cave in Australia." "It takes an hour and a half of crawling to reach this extraordinary gallery." "Ancient streams have washed down thousands of bones and left them here." "They look so fresh that they might have been deposited only a few weeks back, but nearly all belong to kinds of marsupial that have been extinct for thousands of years." "This is a skull of a giant kangaroo that could browse up to a height of about nine feet above the ground, half as high again as any living kangaroo can do." "It had a bulbous face with very big eyes and powerful high-crowned teeth, with which it could masticate really tough leaves." "You could take this for the skull of a small rhino, but in fact it belonged to a giant wombat as big as an ox." "Its teeth suggest that it chewed coarse vegetation." "The most extraordinary skull in these caves is this." "It belonged to a creature that was a kind of killer possum." "It's popularly known as a marsupial lion." "In life it was about the size of a leopard." "Its four legs were shaped rather like those of a koala, except that it had on its thumb a vicious, hooked claw, with which it ripped apart its prey." "But the most fantastic thing about it are its teeth." "In the back of its jaws it had these single teeth, elongated to form great shearing blades, with which, doubtless, it sliced through the flesh of its prey." "Maybe it jumped from trees onto those giant kangaroos." "Who knows?" "But despite the formidable armoury of teeth, all the marsupial lions became extinct between 20,000 and 18,000 years ago, as indeed did all the giant kangaroos." "Why?" "Well, aboriginal man had certainly reached Australia by this time, but there's no evidence that these creatures were overhunted." "No, the reason seems to be that there was a change in the climate, which became extra-dry about this time." "That change in Australia's climate can be traced right back to a time some 45 million years ago when the continent first split away from Antarctica." "Because after the separation, Australia didn't just stay still, but continued to drift slowly northwards towards the Equator." "Indeed, it's still moving in that direction even today, and as fast as it has ever done, which is about five centimetres a year." "The effect on the vegetation has been dramatic." "The lush, cool, wet forests changed into arid open country like this around me in central Australia." "And one group of marsupials were quick to respond to this change." "There are some of them just over there." "Out in this open country, the small wallaby-like marsupials grew bigger, hopped farther and faster and became kangaroos, the marsupial equivalent of deer and antelope." "With its huge hind legs and muscular counterbalancing tail, a red kangaroo, the biggest of all living marsupials, can bound 27 feet, about eight metres, and leap over obstacles ten feet, three metres high." "It's often extremely hot out here, with temperatures as high as 45 degrees centigrade." "And kangaroos have developed a special way of cooling themselves." "They plaster their forearms with saliva." "As this evaporates, it cools the blood running through special networks of capillaries just beneath the skin." "The kangaroos also take advantage of the best shade they can find during the hottest part of the day and scrape away the baking hot surface soil to make a cooler, more comfortable bed for themselves." "Out in the deser food is nearly always scarce, and kangaroos will eat even the tiniest morsel of greenery, searching through the dry branches with their front legs to find something edible." "The leaves of these bushes are very rough, and tough on the teeth." "And the problem of tooth wear is something which faces grazing animals all over the world." "Antelope and deer solve it by having open roots to their teeth, which grow throughout their lives." "Kangaroos have a different solution." "They have only four pairs of molars on either side, but they move steadily forwards throughout their life." "As they're worn down in the front, so this fourth one, in this young animal here, comes into play." "This is the skull of an older animal, and already the first molar has gone, and the second one is so worn down here that it's virtually useless, and had the animal not died, it would have been shed." "This process of moving forward goes on throughout the animal's life." "By the time it's 15 or 20, if it hasn't died for any other reason, it would die from starvation, because it would have lost all its teeth." "The red kangaroo has developed the marsupial reproductive technique into a very efficient system indeed." "33 days after the fertilised egg started its development, the little young, scarcely more than an embryo, is expelled from the womb." "The mother is cleaning up the birth fluids." "She's not licking a pathway through her fur for the young, as used to be thought." "Indeed, she gives the feeble, blind, little creature no assistance at all." "It has to find its way to the pouch entirely by itself." "The tiny baby, and only one is born at a time, valiantly squirms its way towards the pouch, a journey that may take it up to five minutes." "Its forelegs are sufficiently formed to help it move forward, but its hind legs are still no more than buds." "The rim of the pouch, and safety." "The mother's teat is considerably bigger than the little baby, which at this stage weighs less than a gramme." "Within a day of the young fastening on the teat, the mother produces another egg in the womb and will mate again, but that fertilised egg will wait there without developing until, in 235 days' time, the first baby is sufficiently well-grown to leave the pouch." "Only then will the development of the next egg proceed." "This system of continuous production is so efficient that every female can reproduce four times every three years." "And kangaroos have come to dominate the Australian countryside." "But why should kangaroos hop?" "One suggestion is that when they were newborn babies, they had to develop grasping forelimbs to haul themselves through the fur, and that this character, having been determined so early, cannot then be changed into one suited to running." "Another explanation may be the position of the pouch." "If large babies are to be carried in it at speed over rough ground, perhaps it's easier for mothers to do so with a torso somewhat inclined upwards." "Whatever the reason, the kangaroo has brought the hop to a marvellous peak of power and grace." "When hopping at full stretch, the kangaroo can reach speeds of up to 50km an hour, which is not as fast as the swiftest antelope, but a fair speed nonetheless." "On the other hand, when they're moving in a more leisured way, at about 25km an hour, the style proves to be a very economic one, demanding considerably less energy than an antelope moving four-footedly at the same sort of speed." "The milk, supplied by the females to their young, varies as time passes." "This well-grown youngster is not yet weaned, even though it nibbles grass every now and then." "The milk it takes from the single teat, which it has used throughout its young life, is by no means the same as the liquid it drank when it first arrived in the pouch as a tiny worm." "The ingredients have changed to match the youngster's changing needs." "It will continue to take milk for some time after it has left the pouch for good." "The mother, by then, will have another tiny baby in her pouch, so she will be supplying one kind of milk from one nipple, and a different mixture from another." "The rearing of young in a pouch has its hazards, particularly that early journey to get there, but in some ways it brings advantages to kangaroos." "If a female with a large youngster in her pouch is chased, she will often jettison her baby and so escape." "No heavily pregnant antelope has that option." "A sustained drought, not uncommon in the Australian desert, may make it difficult for her to produce sufficient milk for the young in her pouch." "She may then discard the little babe without much trouble." "When the drought is over, the fertilised egg within her womb is ready to start immediate development, and the new young can be in her pouch 33 days later." "It's a commonly held belief that marsupials are very primitive, backward mammals, scarcely an improvement, if any, on those early egg-layers, the echidna and the platypus." "It's a view, after all, that was held by Charles Darwin." "The fact of the matter is that today we recognise that many of them are extremely efficient organisms." "It's true, of course, that their basic method of reproduction appeared very early in the development of the mammals." "But many of the marsupials today have brought that to a high pitch of perfection." "As a result, no other creature can compare with these female kangaroos which throughout their maturity continuously, almost without break, have three young at different stages of development." "One grazing and coming back to suckle, one within the pouch and one within the body itself, awaiting the best strategic moment in which to be born." "No, the fact of the matter is that the accidental isolation of marsupials in Australia, brought about by the drifting of the continents some 45 million years ago, has given them a long, long time in which to weave variations" "on the basic model." "And some of those variations are very efficient creatures indeed." "While the marsupials were developing in Australia, another kind of mammal was coming to the fore in the Northern hemisphere." "Like the marsupials, its fossils dated back to the age of the dinosaurs." "It was related to the American opossum, like it, it was small and insect-eating, but it differed in one crucial respect, and may have looked something like this." "This solenodon, a relative of the shrews, is a fair representative of them." "And they developed a third technique of reproduction." "The female doesn't lay a shelled egg, like a platypus, nor give birth to a partially developed little worm, like a kangaroo." "She retains her young within her body and nourishes it with a placenta, a pad rich in blood vessels, that is implanted on the wall of the womb and linked to the young by a tubular cord." "It absorbs nutriments from the mother's blood and supplies them continuously to the growing baby." "This innovation was bequeathed by the early insect-eaters of the northern hemisphere to all their descendants, the majority of the mammals alive today." "So none need give birth to their babies until they're very well developed." "Baby rabbits develop within the mother for 28 days, twice as long as an opossum, a primitive marsupial of about the same size." "They don't open their eyes until several days after birth, but when necessary a young placental mammal can be ready for action almost as soon as it leaves its mother's body." "A young wildebeest can run within minutes of its birth, though it's a little groggy at first." "In some circumstances it's important to condense suckling to as short a time as possible." "These harp seals are very vulnerable out here on the ice." "The sooner the pups can get into the safety in the sea, the better." "So their mothers provide them with a very rich milk." "In three weeks they double their weight and can swim away and lead independent lives." "Retaining the baby within the womb until it's fully formed seems an obvious way for the mammals to improve the care of their young." "But in fact it causes considerable problems in body chemistry." "For one thing, the tissues of this little pup differ in a genetic way from those of its mother." "They contain elements from the father." "And so that means that it risks, when it's within the womb, rejection by the mother's body, just as a transplant does." "Secondly, the young in the womb may be ejected if the mother produces another egg and comes on heat again." "That problem doesn't face a baby marsupial, for in every case its short development takes place within the period of the mother's sexual cycle." "But a placental mammal, like this seal, has a much longer development, and it deals with the problem by producing, from within the placenta, a substance which actually suspends the mother's egg production." "It manufactures other biochemical substances which suppress production of the antibodies that cause the rejection of tissue,and so allows the developing young in the womb to remain there." "So the placenta has had to become a chemical factory of great complexity." "When the young is finally born, the placenta, too, is shed from the womb as the afterbirth." "The body of a mammal, whether it's our own or a seal's, is extremely complex and takes a very long time to fully develop." "These young seal pups around me were conceived almost a year ago, and, until a few days ago, were kept in the warmth and safety of their mother's body as she swam through the freezing polar seas." "No marsupial could be reared in such a way because marsupial babies in the pouch need to breathe air." "In fact, the placenta and the womb between them provide a degree of safety and a continuity of sustenance which is unparalleled in the animal world." "And together they form one of the keys to the success of the placental mammals, which have, in the end, colonised all parts of the world, including even these bleak, inhospitable ice flows."