Life on Earth

Although all those creatures are different,

they are in fact closely related to one another. They are all mammals.

But how have they become so varied?

And what is the ancestral form, the basic theme, on which they are all variations?

You can find a close approximation to that theme in the jungles of South-East Asia.

It's properly called a tupaia,

and it's certainly a mammal, with a hairy coat and warm blood.

But what kind? It looks very like a squirrel.

A close look at its anatomy reveals resemblances to a rabbit,

but it doesn't gnaw nuts and it doesn't nibble grass, it catches insects.

Its teeth are small, numerous and spiky, like a shrew's.

Indeed, its popular name is tree shrew,

but its large brain and those grasping hands

have suggested to some that it's related to monkeys.

It seems in fact to contain hints of many different mammals.

One thing, though, is clear.

It's very like the earliest of mammals,

that were living when the dinosaurs dominated the earth 100 million years ago.

The basic pattern on which there's been such a multitude of variations.

Some of those variations are so extreme that it's difficult to believe

there's any connection between them and the basic theme,

were it not for the evidence of fossils and the anatomy of the living animals.

The tree shrew's continuous activity and swift reactions are typical of a mammal.

A consequence of its ability to generate heat within its body

so that its chemistry works fast and provides it with abundant energy.

This talent probably developed a very long time ago indeed,

at a time when the dinosaurs dominated the earth.

For fossils of a creature remarkably similar to the living tree shrew

have been found in rocks that are 200 million years old.

Its numerous spiky teeth suggest that it ate insects,

and the shape of its limbs that it was a swift runner.

In fact, its lifestyle was not unlike a tree shrew.

And creatures like it survived alongside the dinosaurs throughout their reign,

probably scampering about at night

when the colder-blooded dinosaurs became torpid in the cold.

Then, 65 million years ago, the dinosaurs disappeared.

The forests and swamps of the world were suddenly empty of large creatures.

Primitive birds flapped through the sky,

but on the ground there were few creatures other than insects and other invertebrates

and those small warm-blooded primitive mammals that fed on them.

And here and there, in odd corners of the world,

their descendants still survive, little changed.

The tree shrew of Malaysia is one.

Here in the streams of the Pyrenees lives another little-known and very engaging one.

It's called a desman.

Like most of these primitive mammals, the desman has a stupendous appetite.

It eats two-thirds of its own body weight every day

and seems never to stop the hunt for more.

Its nose does most of the searching.

It scents the faintest changes in the taste of the water with its nostrils,

and feels its way around with all those whiskers.

Its feet are a combination of web and claw, for both swimming and clambering.

Its eyes are tiny, minute beads hidden in its long fur.

When at last it does find something good, it doesn't give up easily.

Its snorkel nose allows it to snatch a breath

with the minimum of interruption in the struggle.

Its hard-fought-for worm will now keep it going for another hour or so.

Another of these primitive survivals lives along the streams of North America.

It's not only an energetic swimmer, but a burrower as well.

It's possible the swimming way of life and the body design to go with it

led to a similar activity not in water, but underground.

What were paddles have become spades.

This is the star-nosed mole.

The odd fleshy flower on its nose is another highly sensitive smelling device.

It may have yet another way of investigating its surroundings.

Many of these little insect-eaters, such as shrews,

make squeaks so high-pitched that we can't hear them,

and the echo they produce helps the animals to find their way around.

Moles, like desmans and shrews, have formidable appetites

and have to eat every few hours.

Their tunnels are not simply passageways, but traps.

Worms and insects burrowing through the soil drop into them,

and the mole collects whatever turns up.

If its appetite is momentarily sated,

then it paralyses a surplus worm with sharp bites and stores it away in a special larder

before setting off again on its never-ending patrols.

Variations on the theme of the small insect-eater

began to appear soon after the dinosaurs vanished.

Creatures developed that specialised in feeding on one kind of insect: ants and termites.

This is another digger, the aardvark, from Africa.

And this is its South American equivalent, the giant anteater.

The essential equipment for a diet of ants and termites, it seems,

is an elongated snout for poking inside the nests

and a long sticky tongue for collecting the insects.

And the giant anteater has the most extreme version of both that exists.

Termites are easily crushed,

so the anteater has no need of teeth and has lost them all.

Termites' nests, however, can be as hard as cement,

and strong claws and muscular legs are needed to tear them open.

The anteater is very fussy about its food. In spite of its name it seldom eats ants.

Termites, like these, are a much more usual meal,

and even then it prefers some termites to others.

There are a dozen or so species of mammal round the world

that have specialised in living on ants and termites.

As a lifestyle it doesn't seem to require

a particularly quick intelligence or vivacious disposition.

And all these anteaters are relatively slow-moving creatures.

Because of that and their total lack of teeth,

they might seem to be easy meat for a hunter.

But the giant anteater's front legs are so strong that its hug is lethal,

and few creatures interfere with it.

The termite-eating specialist of Africa, the pangolin,

is much smaller and not nearly such a powerful digger.

It's developed a flexible armour of scales

and can curl itself up into a ball so that it's virtually impregnable.

Its muscular tail also acts as a counterbalance

so that the creature can trundle along with most of its weight on its back legs,

and its front legs at the ready for digging into termite mounds, like this one.

It's so confident of its defences that it takes little notice of any other creature around,

unless they molest it.

Smallest of all, the pygmy silky-furred anteater of South America.

It does seem to be defenceless and can't move fast enough

to escape even the clumsiest hunter.

But it keeps out of the way up in the branches, living almost entirely on tree ants.

This one has a baby on its back, and it may be either male or female,

for both parents take a share in carrying the load.

There's yet another kind of specialist anteater in South America,

intermediate between the giant and the pygmy: the tamandua.

It feeds mostly at night.

Its thick, bristly fur is supposed to protect the tamandua

from the bites of infuriated ants, swarming from their shattered nest.

But when you watch the animal feeding,

you can't help wondering just how effective that protection really is.

Ants and termites are among the most numerous insects, particularly in the tropics.

And the tamandua and its relatives around the world

have little difficulty in finding more than enough to eat.

And there are insects not only in water and in the soil and all over plants,

but in the air, and particularly at night.

It's difficult to realise just how many there are

until you put up a mercury vapour lamp in the tropics.

Here, within a few minutes, you've got all sorts of creatures.

Small moths, crickets, huge beetles, mantises,

big moths, insects of all kinds.

The insects first took to the air about 300 million years ago.

They had it to themselves for about 100 million years,

at least until the arrival of the reptiles.

Whether there were any night-flying, insect-hunting reptiles, we don't know,

but it seems unlikely because reptiles, being cold-blooded,

are usually active during the day.

And then about 150 million years ago, the birds developed.

But there's no reason to suppose

there were any more night-flying birds in the past

than there are today, and that's precious few.

So this great feast of insects

awaited any creature that could master the tricky technique of flying at night.

And one group of the mammals did.

The bats.

The majority of them are hunters of flying insects,

such as moths, mosquitoes or even beetles.

Caught on the wing and eaten at the roost as the bats hang upside down.

Bats began to fly a very long time ago.

These fossil bones of what is undoubtedly a bat are about 50 million years old.

The bat skeleton is very similar to the tree shrew's.

Seen here from above, and now side on.

But how did this flying variation arise?

It may be that the early insect-eaters sought their food up in the branches of trees,

as indeed some kinds of tree shrews do today.

And as they leapt about trying to snatch flying insects from the air,

some may have developed flaps of skin between their arms and the sides of their body

so that they could glide, as the living flying squirrels can today.

They then supported those flaps with their fingers and strengthened the arm muscles

until, eventually, they were able to flap their newly developed wings

and fly in search of their insect prey, and so became bats.

But living on insects has one great disadvantage.

In many parts of the world insects disappear almost totally during the winter.

What does an insect-eater do then?

It hibernates in any sheltered place it can find

where the temperature might remain a few degrees higher than elsewhere,

as it is inside this old Canadian mine.

These tiny lumps, as cold as stone, are living bats.

They fed voraciously during the summer, building up reserves of fat,

but now a profound change has taken place in their bodies.

Their heat has seeped away, and their body processes have slowed down

to almost, but not quite, a complete halt.

They must keep their body chemistry ticking over

just enough to generate sufficient heat to prevent them from freezing solid,

for that they can't survive.

Not all of them are successful.

Sometimes an individual cannot stave off the chill,

falls and is entombed in the ice.

You might think they huddle together to keep warm.

But careful measurements have shown those in groups get just as cold

as those hanging by themselves.

It may be that grouping protects them from another hazard,

the loss of moisture during breathing.

That does seem to be less for those in clusters.

Other creatures also take refuge in the mine,

the very ones which in summer are food for the bats: moths.

Both hunters and hunted shelter together from that overwhelming killer, cold.

In other parts of the world, as here in New Mexico,

bats solve the problem of lack of insect food by migrating.

From this cave they will fly south some 1,000km for the winter.

They have to, to find enough food,

for their populations are measured in millions,

and tons of insects are needed.

Caves like these contain the densest populations of mammals

to be found anywhere on earth.

How is it that all those bats flying at such a speed

can find their way around in the dark?

The answer is echolocation.