The Rise of the Mammals Life on Earth

When specimens of this came from Australia in the 18th Century, people didn't believe their eyes.

They said it was a hoax - bits of different creatures

crudely sewn together - but it's no hoax, it's a platypus.

Yet in a way, those sceptics were right.

The platypus is an extraordinary mixture of different animals - part mammal & part reptile.

So it gives us some idea of how mammals developed.

At first sight, it looks a regular mammal & has dense soft fur,

a hallmark of mammals.

When you handle it, it's warm.

Its feet are webbed for swimming.

The strange bill isn't hard like a bird's beak, but soft, rubbery & very sensitive.

The platypus, with poor eyesight, uses it to find food - crayfish & other water creatures.

When it breeds, it does something that separates it from all other mammals except one.

In its nest in a burrow, it lays eggs.

This links it with reptiles

& entitles it to be regarded as the most primitive living mammal.

What makes it doubly paradoxical is that when the egg hatches

the baby's not left to find food itself like reptile babies, but is given food by the mother.

The platypus, like all mammals, has in its skin to help deal with overheating, sweat glands.

On the underside of the body these glands are especially big & produce a fatty sweat which is milk.

It oozes from the skin & the young suck it from tufts of hair.

There's no nipple so it hardly qualifies as a breast, a mamma,

which gives mammals their name.

Only one other mammal lacks a true breast - the echidna.

It, too, lives in Australia,

& it too, lays eggs - but the female doesn't deposit them in a nest, she carries them with her.

They have sticky shells & become glued to the hair on her underside in a groove across her stomach.

Each is no bigger than a pea,

& after 10 days, it hatches.

By now, glands near the groove are producing creamy milk.

The baby echidnas remain inside the groove for the next eight weeks,

steadily taking in milk and growing.

When the spines develop, they're uncomfortable passengers & the mother puts them in a den.

Eventually, they abandon milk & take to their adult diet.

Ants.

A long snout is, in evolutionary terms, a recent acquisition -

a tool for food gathering.

It houses a long sticky tongue with which the echidna flicks up its ants & termites.

The animal defends itself by digging downwards.

There's nothing visible but spines.

Platypus & echidna are oddities - we've no fossil evidence

to say where or when they developed,

but it's a good guess, because of another kind of echidna which lives not far away in New Guinea,

that the group originated in this part of the world.

It's certain it's these creatures from which modern mammals came

about 180 million years ago.

And we can trace the ancestry of mammals even farther than that.

Reptiles began 300 million years ago & with watertight skins & eggs

and so they survived in the driest country.

After 20 million years

a group of hunting reptiles evolved called pelycosaurs.

Reptiles can't generate heat in their bodies, so after a cold night

they're sluggish in the morning.

Pelycosaurs dealt with that

by developing sail-like fins on their backs

with which to catch the first rays of the sun, so they could get out hunting really early.

In places like this, it's easy to imagine some 12-foot species

like dimetrodon lying in the sun.

It's been calculated that with the fins,

they could raise their body temperature 6 degrees in an hour.

Without them, it would take nearly three hours.

Those fins were stop-gap devices only & later species did without them.

That's probably because they were able to generate heat internally -

and their teeth support that idea.

Dimetrodons' teeth, as those of most reptiles, were spikes which did no more than grip a victim.

But generating heat in the body requires a great deal of energy.

So a warm-blooded animal must eat more food than a reptile, & digest it fairly rapidly.

Changes in the teeth of successive generations of pelycosaurs suggest that's just what they did.

The spikes changed to tools for butchery.

Daggers appeared on the sides of the upper jaw

for slitting open the hide of prey.

Knives for slicing the meat, & grinders for crunching bones.

Most reptiles shed teeth as they become worn,

throughout their lives.

The teeth of these, not only became specialised but permanent.

The upper & lower ones meshed to give a highly efficient bite.

As they generated heat internally they also needed a coat of hair to conserve it - mammals had arrived.

The acquisition of warm blood brought more advantages to these creatures than speed of movement.

That you'd have seen had you been able to walk through forests of 180 million years ago at night.

The first true mammals appeared at a time when reptiles ruled the world.

But solar-powered animals had one major disability - at night when it was cool, they became sluggish.

It left the field to any creature that could be active at night - & mammals could do just that.

The ancient mammals were small, nocturnal insect hunters relying on smell to find food.

In fact, they were probably very similar to present day shrews & hedgehogs, though they laid eggs.

Warmth was the key to their survival & ultimate success.

Since they alone could hunt in the cool of the night they didn't have to face competition with reptiles.

So those primitive mammals were able to live right through the age of the dinosaurs

& be 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 & embryos.

Mammals developed three ways of dealing with that.

Primitive ones incubated eggs as the platypus does today.

Others developed better methods

The opossum that lives in North & South America is one - it doesn't lay eggs,

but gives birth 12 days after mating.

There may be 20 of them, the size of bees & the only well-formed features are the front legs.

With these, they haul themselves through hair on the mother's belly on the 1st journey of their lives.

At last they reach a pouch in the belly - inside are 13 nipples

& each fastens to one for milk.

If more than 13 are born, the last to reach the pouch will find no vacant nipple & die.

The Latin for pouch is marsupium, & this gives a name to those who reproduce in this way - marsupials.

This is the woolly opossum.

Its babies are sufficiently well-grown to have left the pouch,

but they still cling to their mother and return to the pouch for drinks

There are about 70 kinds of opossum in the New World & most live in South America.

Some are small as mice, others as big as domestic cats - there's even an amphibious one.

The yapok - it has webbed feet & eats fish.

When a mother goes for a swim, she closes the opening to her pouch to prevent the babies drowning.

But they need to breathe, so she only swims a few minutes at a time when she has young.

Mouse opossums are like the earliest marsupials.

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 larger one lives in the dank scrub of the High Andes - the rat opossum.

It's a ferocious hunter with fangs on its lower jaw with which it stabs its prey.

It, too, has an ancient ancestry.

It doesn't even have a pouch, but its young hang from the mother's teats.

Fossils resembling those primitive marsupials are found in America, dating back 60 million years.

That makes them the oldest marsupial fossils known, older than any found elsewhere on Earth.

So it's reasonable to assume marsupials originated here.

If that's so, how did they get to Australia, where they flourish in the greatest numbers today?

This tree may provide the answer -

it's growing in the bleak lands of Patagonia on the tip of S America.

It's a kind of beech related to the European beech & called the southern beech.

It's a tree with a long ancestry & was growing here

when marsupials first appeared & it seems likely they lived in forests like this.

It's only relatively recently that scientists have demonstrated beyond all doubt

that the continents aren't static but have drifted slowly over the globe for millions of years.

To go back to when marsupials appeared in South America, is to return to a time

when that continent wasn't joined with North America but fitted along the west coast of Africa.

Australia and Antarctica were also joined, and they lay beside the east coast of Africa.

Forests of southern beech grew in many parts of this land mass.

But as it split & drifted apart, so the pieces carried the forests & marsupials that lived in them.

The middle part drifted over the Pole & was covered in snow & ice,

so forests & inhabitants died out.

In the eastern fragments, they flourished -

for that was Australia.

Here in Australia, these ancient beautiful trees, the southern beech, still grow -

as they once did in Antarctica & still do in South America.

Evidence of the one-time unity of those three continents back in geological time.

With them in these forests grow other ancient plants -

tree-ferns & cycads. Living in holes in the trunks

& in leaves on the floor are small, warm-blooded, furry creatures

that bear their young the same way as American opossums - marsupials.

The Australian marsupials fared better than their American cousins for S America continued to drift.

Eventually it came into contact with N America & advanced mammals in that continent invaded south.

The S American marsupials couldn't face the competition & many became extinct.

Australia was different - this huge island continent has remained cut off from the rest of the world,

& here they have remained the dominant mammals.

They've branched out into many forms & a great number of them

are active at night.

Some are similar to S American opossums and, indeed, are known as possums.

There are mouse-size ones too & like the Americans, the female carries her young clinging to her.

This is the smallest marsupial of all - it looks like a mouse, but it's very different.

It doesn't gnaw seed, but hunts insects,

and will unhesitatingly tackle really big ones.

There are two-dozen kinds of this size & reproductive techniques are much as others in the group,

but because they're so small, the process is extremely difficult to observe.

By giving this expectant mother a nest floored by glass, we can film a birth for the first time.

30 days after mating, she licks the birth canal & minute, worm-like young,

smaller than a grain of rice, will emerge immediately after the 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 come from the birth canal.

There.

And it's gone.

The female may produce 6-8 young in a single batch.

When they arrive, the babies are so small, the mother seems almost unaware of their existence.

But they grow fast & soon are quite a burden.

Eventually, the pouch can't hold them & they hang beneath 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're 3-4 months old, they are independent & join the parents

hunting insects in the night.

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 food.

A marsupial mouse.

Bigger still, the size of a corgi,

the Tasmanian devil.

The quoll has found some carrion.

It doesn't stay long when the devil appears.

Here's a second.

Devils devour everything - skin, bones, the lot.

Not so long ago, there was an even bigger marsupial hunter -

The thylacine.

The last recorded living one died in a zoo in 1933 & today, the species may be extinct.

The resemblance between this & the wolf of the northern hemisphere is remarkably close.

The processes of evolution even on different stocks,

produce similar creatures for similar ways of life.

The thylacine & the wolf are both swift-running flesh-eaters,

so they look much the same.

There are other parallels between marsupials & other mammals/

The numbat has an elongated nose

& a long 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 a sugar-glider, and with good reason.

In both appearance & acrobatic skill, it's indistinguishable from the flying squirrel of N America.

Both have a wide flap of skin between their legs

that catches the air and enables them to guide great distances.

Evolution in Australia hasn't always produced such parallels to its products elsewhere.

This, for example, is the koala - it lives in trees, eats nothing but a few special leaves.

Sloths in S America do the same & are equally fussy about leaves - so are some monkeys in Africa.

But neither of them looks like the koala,

which has an Australian charm all of its own.

The wombat - a cousin of the koalas.

It lives entirely on the ground.

It, too, is a vegetarian, but is 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, come to that,

is exactly renowned for its darting intelligence

or speed of reaction under distressing circumstances.

Bandicoots look like rabbits, but the parallel isn't close.

Rabbits eat grass, bandicoots eat insects & meat.

The similarity between the ears & those of long-eared rabbits like the American jack rabbit

is due to the fact that they both live in deserts & use the ears for cooling their blood.

This Australian 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, the nearest would be the brush tongue of nectar-feeding bats.

When the super-continents broke up, Australia was largely covered by forests.

Those that remain

still contain primitive marsupials - this is a potoroo.

In it are the beginnings of features that characterise the most famous of all the marsupials -

the kangaroos.

For one thing, the potoroo has a tendency to hop.

For another, the young keep popping to the pouch for a drink after they can fend for themselves.

They have a preference for travelling that way even when quite large.

In more open woodlands, there are animals that developed these two tendencies further - wallabies.

There are two dozen kinds - this is a pademelon.

This odd wallaby has colonised the tropical island north of Australia - New Guinea.

Very few mammals live here. There's this, and a kangaroo.

Unbelievable though it seems for a creature designed for hopping, it's taken to the trees.

The tree kangaroo seems the clumsiest climber of all tree-living creatures.

The explanation is that in New Guinea,

it's the only mammal that gets up in the branches & with nothing competing with it for leaves,

it's had no need to become better adapted - it gets all it wants just as it is.

All in all, there are 150 kinds of marsupial in Australia & islands like New Guinea & Tasmania.

Not long ago, there were more.

You can see the most spectacular evidence of their existence in Australia's caves.

In 1969, two zoologists crawled down this narrow cavern in the hills of Naracoorte near Adelaide.

They were the first people ever to come this way & they hoped they might find a bone or two.

What they discovered a quarter of a mile farther on, exceeded their wildest imaginations.

They discovered the greatest, most important deposit of bones ever found in a cave in Australia.

It takes an hour & a half of crawling to reach this extraordinary gallery.

Ancient streams washed thousands of bones & left them - so fresh it might have been a few weeks back.

Nearly all belong to marsupials that have been extinct for thousands of years.

A skull of a giant kangaroo

that could browse to a height of 9 feet above the ground.

That's half as high again as any living kangaroo can do. It had a bulbous face

with big eyes & powerful high-crowned teeth,

to masticate tough leaves.

You could take this for the skull of a small rhino,

but it belonged to a giant wombat as big as an ox - its teeth say it chewed coarse vegetation.

The most extraordinary skull in the caves is this -

it belonged to a creature that was a kind of killer possum,

popularly known as a marsupial lion.

In life it was the size of a leopard, with legs like a koala,

except 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 were teeth elongated to form shearing blades

for slicing the flesh of its prey.

Maybe it jumped from trees onto those giant kangaroos. Who knows.

Despite the formidable armoury of teeth, all the marsupial lions became extinct 20,000 years ago.

As did the giant kangaroos - why?

Aboriginal man was in Australia by this time, but there's no evidence these creatures were overhunted.

No, the reason is there was a change in climate which became extra dry about this time.

That change in climate can be traced right back to a time 45 million years ago,

when the continent first split away from Antarctica.

After separating, Australia didn't stay still, but continued to drift towards the Equator.

It's still going in that direction today & as fast as ever - about 5 centimetres a year.

The effect on vegetation has been dramatic - lush forests changed to arid country, like this around me.

One group of marsupials were quick to respond to the change - some are just over there.

Out in open country, the small, wallaby-like marsupials, grew bigger.

They hopped farther & became kangaroos - the marsupial equivalent of deer & antelope.

With its huge hind legs & muscular counter-balance tail, red kangaroo, biggest of all marsupials,

can bound 27 feet & leap over things 10 feet high.

It's very hot, with temperatures as high as 45C, & kangaroos have a way of cooling themselves.

They plaster their forearms with saliva - as it evaporates,

it cools the blood running through 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 desert, food's always scarce

& 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 & tough on the teeth.

The problem of tooth wear is something facing grazing animals all over the world.

Antelope & deer solve it by having open roots to their teeth which grow throughout their lives.

Kangaroos have a different solution.

They have 4 pairs of molars on either side, but they move forward throughout their life.

As they're worn down in front so the 4th one comes into play.

The skull of an older animal - already the 1st molar has gone.

The 2nd is so worn down that it's useless, & had the animal not died, it would have been shed.

This process goes on throughout the animal's life.

By the time it's 20, if it hasn't died for another reason, it'd die from starvation with no teeth.

The red kangaroo has developed the marsupial reproductive method into a very efficient system indeed.

33 days after fertilisation of the egg, the young, just an embryo, is expelled from the womb.

The mother's cleaning up the birth fluids, not licking a path for her young as used to be thought.

Indeed, she gives the feeble, blind creature no help at all - it must find its way by itself.

The tiny baby - only one is born at a time - squirms its way to the pouch maybe for 5 minutes.

Its forelegs are well formed to help it move forward, but its hind legs are no more than buds.

The rim of the pouch - & safety.

The mother's teat is considerably bigger than the baby which weighs less than a gram.

Within a day of the young taking the teat, the mother produces another egg & will mate again.

That fertilised egg will wait undeveloped until, in 235 days, the first baby leaves the pouch.

Only then will the development of the next egg proceed.

This system of production is so efficient that every female

can reproduce 4 times in 3 years, & kangaroos have come to dominate the Australian countryside.

Why should kangaroos hop? One suggestion is that

as babies, they have to develop grasping forelimbs to haul themselves through the fur,

and this character, being fixed so early, can't then be changed into one suited to running.

Another explanation may be the position of the pouch.

If large babies are to be carried at speed, perhaps it's easier with a torso inclined upwards.

Whatever the reason, the kangaroo has brought the hop to a marvellous peak of power & grace.

Hopping at full stretch,

it can reach 50 kms an hour.

Not as fast as an antelope, but a fair speed nonetheless.

On the other hand,

when moving in a more leisured way, the style proves to be an economic one.

It demands far less energy than an antelope moving four-footedly at the same speed.

The milk supplied by the female to the young varies as time passes.

This well-grown youngster is not yet weaned, even though it nibbles grass now & then.

The milk from the teat it's used throughout its life

isn't the same as the liquid it drank when it first arrived in the pouch as a tiny worm.

The ingredients change to meet changing needs.

It'll continue taking milk after it's left the pouch for good.

The mother by then will have another baby in her pouch.

She'll be giving one kind from one nipple & 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 is chased, she'll often jettison her baby & so escape.

No pregnant antelope has that option.

A sustained drought, not uncommon, may make it difficult for her

to produce sufficient milk -

she may then discard the little babe without much trouble.

When the drought is over,

the egg in her womb is ready to start immediate development & the new baby's in her pouch in 33 days.

It's a commonly held belief, that marsupials are primitive, backward mammals,

with scarcely any improvement on the early egg-layers, the echidna & the platypus.

It was the view of Charles Darwin.

The fact is, today we recognise that many of them are extremely efficient organisms.

It's true, their basic method of reproduction appeared very early in the development of the mammal.

But many of them today have brought it to a high pitch of perfection.

No other creature can compare with female kangaroos,

which, throughout their maturity, continuously an almost without break

have 3 young at different stages of development.

One grazing & suckling,

one within the pouch,

one within the body itself, awaiting the best strategic moment

in which to be born.

The isolation of the marsupials brought about by drifting continents 45 million years ago

has given them a long, long time to weave variations

on the basic model - and some of those variations are very efficient creatures indeed.

While marsupials developed here, another mammal was coming to the fore in the northern hemisphere.

Like marsupials, its fossils dated back to dinosaurs - it was related to the American opossum.

Like it, it was small but differed in one respect & may have looked like this.

This solenodon, a relative of the shrews, is representative of them,

and they developed a third technique of reproduction.

It doesn't lay eggs like the platypus nor give birth to a worm like a kangaroo.

She retains her young in her body & nourishes it with a placenta -

a pad, rich in blood vessels, that's implanted on the wall of the womb & linked to the young by a cord.

It absorbs nutriments from the mother's blood & supplies them to the growing baby.

This innovation was bequeathed by the early insect-eaters of the north to all their descendants -

most of the mammals alive today - so none need give birth to babies until they're 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 a young placental mammal

can be ready for action on leaving its mother's body.

A wildebeest can run within minutes of its birth -

though it's a little groggy at first.

Sometimes it's important to keep suckling as short as possible.

Seals are vulnerable on the ice, the sooner pups can get to the safety of the sea, the better.

So their mothers provide a very rich milk.

In 3 weeks, they double their weight & can swim away

and lead independent lives.

Retaining the baby in a womb till it's fully formed

seems an obvious way to improve the care of young.

In fact, it causes problems in body chemistry.

For one thing, tissues in this pup

differ from those of its mother - they've elements from the father.

So that means it risks, in the womb, rejection by the mother's body,

just as a transplant does.

2nd, the young in the womb may be ejected if the mother produces another egg & comes on heat again.

That problem doesn't face a baby marsupial - for its short development

takes place within the mother's sexual cycle - but a placental mammal has a longer development.

It deals with that problem by producing from within the placenta a substance which suspends

the mother's egg production - it manufactures other substances.

These suppress antibodies that cause rejection & so allow the 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 & takes time to develop.

These seal pups were conceived almost a year ago.

Until a few days ago, when they were born,

they were kept in the safety of the mother's body

as she swam through the freezing polar seas.

No marsupial could be reared in such a way - marsupial babies in a pouch need to breathe air.

In fact, the placenta & the womb between them provide a degree of safety

and a continuity of sustenance unparalleled in the animal world.

Together they form a key to the success of placental mammals,

which have colonised all the world - including even these bleak,

inhospitable ice floes.

Subtitles by Red Bee Media Ltd