Life on Earth

If there's one place in the world where reptiles still rule,

it's here in the Galapagos Islands.

The ancestors of the reptiles, the amphibians, had wet, permeable skins.

As a consequence, they couldn't exist for long away from water.

Reptiles, however, like these marine iguanas, are not so restricted.

They can survive in places where amphibians would roast to death in minutes.

Out on the scorching lava fields, the iguanas lie unprotected

in the ferocious Equatorial sun.

They can do so because of the major innovation made by the first reptiles,

the nature of their skin.

It's not moist like a frog's, but tough, covered with scales,

and, most crucial of all, it's practically watertight.

This skin has enabled reptiles to colonise the hottest and driest places on earth.

And this is one of them. The Namib desert in south-western Africa.

The sand here gets so hot, it scorches the skin,

and even the sole of a reptile's foot can get burned,

so when the sun is too much, this little lizard gets relief by gymnastics.

The scales of the reptiles' skin are dead. Horny outgrowths, like our fingernails.

The reptiles use them for all kinds of purposes.

No bird would want to eat the Australian thorny devil

with scales like these.

Scales are used to protect the body against wear and tear.

Reptile legs stick out at the side rather than give support underneath.

Although this shingleback drags its belly along the ground,

its tough, heavy scales prevent damage.

The scales can be of different sizes.

Small where the skin needs to be flexible,

and large and robust, especially on the head, to reinforce the skull.

In some lizards, the horny skin and scales are fashioned into dramatic headgear.

Sometimes the adornments are designed not just to protect but to scare enemies,

as in the bearded dragon.

The Australian frilled lizard, like most of them, is a bluffer.

Its great ruff is no more than its scaly skin supported by bones from the throat.

In fact, it's a relatively harmless creature with no offensive weapons.

If its bluff doesn't work...

..it can run for it.

The horned iguana from the West Indies is one of the most heavily armoured of all lizards.

Big, powerful, it's the rhinoceros of the reptile world.

There's one group of lizards which, unusually for reptiles, is most active at night.

The geckos.

Some of their scales are the most complicated of all.

Geckos can run up vertical walls, even panes of glass.

The trick is done by scales on the soles of the feet.

Each scale is branched and carries hundreds of microscopic hairs,

invisible to the naked eye.

The electron microscope shows that each ends in a cluster of tiny hooks

that enables the gecko to hang on to virtually anything.

The reptile's skin is rich in pigment cells which can provide marvellous disguises.

The Madagascan gecko is coloured exactly like the bark it always sits on.

The chameleon can vary the shade of green in its skin

so that it becomes invisible among leaves.

The earless dragon from central Australia is obvious when it's feeding.

But when it's motionless, it's difficult to distinguish it from pebbles.

For the biggest scales of all, we go back to the Galapagos Islands

and look for their most famous inhabitant,

the giant tortoise.

Its scales are supported from beneath by massive, bony plates

so that the animal is as impregnable as a tank.

The mountain slopes where they live are dry for much of the year,

but their watertight skin keeps their liquid demands to a minimum.

But even so, they have to top up this water,

and this is one of the few waterholes in the crater.

These few are just wallowing in the mud,

but at this moment a shower has started, and it'll fill little puddles here in the mud,

and soon tortoises from all over the crater will stream down here to sip in those puddles.

Since the reptiles were the first backboned animals to live completely away from water,

they must have been the first creatures to develop real thirsts.

And they have vast capacities.

They can store several gallons of liquid in their bodies.

So when water is about, they make the best of it and fill their reserve tanks.

This may well be their last drink for months.

The tortoises are an extremely ancient group.

They appeared right at the dawn of the age of reptiles, some 180 million years ago.

They saw the dinosaurs come and vanish.

And they continued, as far as we can judge from their fossils,

almost unchanged right until today.

Their armoured shell may be unwieldy, but it's a most successful defence.

Inside it, nothing can reach them.

After a night of rain, the pools are full of water.

They're also full of wallowing tortoises.

Why, we just don't know,

although some say it's a way of keeping warm at night and cool by day.

Temperature control is something all reptiles must achieve,

and some on the Galapagos must cope with the scorching Equatorial sun

without the benefit of fresh water.

Marine iguanas live down on the hot, black lava rocks.

Although their skin is watertight, it's a very poor insulator.

That may seem to be a limitation in this heat, but they've turned it to their advantage.

We tend to think of reptiles as sluggish, cold-blooded creatures.

But that's a mistaken view.

Some of them, like these marine iguanas, for example,

can maintain a higher working body temperature than us.

In fact, the misleading term "cold-blooded"

simply means the animals can't generate their own body heat internally.

Instead, they get it from the sun by sunbathing.

What's more, they can control the amount of heat they absorb

to within very fine limits.

At the moment, it's early morning. The night has been relatively cold

and the iguanas are out on the rocks soaking up all the heat the sun provides.

For until they're warm, their body chemistry won't produce the power they need to be active.

But overheating can be as dangerous as chilling.

The iguanas can't cool themselves by sweating,

for the reptiles' skin hasn't got any sweat glands.

So when, around midday, the sun gets too warm for comfort,

they move down into clefts in the rocks and hang there in cool shadow.

By choosing their resting places with care,

they can keep their body temperature close to 37 degrees centigrade at all times.

When they've reached their working temperature, they go for a swim.

These reptiles feed on seaweed, and some of it they get by diving.

But the sun-warmed iguanas have a problem.

The sea is particularly cold here.

A current comes straight up from the Antarctic, and it's easy to get chilled and torpid.

So they overcome the difficulty by withdrawing warm blood into the centre of their body.

It simply delays the cooling process.

However, the marine iguanas must not stay out too long.

Should they become over-chilled, they will lose their energy

and no longer have the strength to cling to the rocks and resist the waves.

By mid-afternoon, they're all back on the sunbathing rocks,

eager to get warmed up again.

To get warm quickly, you need to expose

as much as possible of your surface to the sun and warm rocks.

So the most recently emerged iguanas slump out, spreadeagled,

just like exhausted human bathers after a cold swim.

It's vital for them to warm up because without warmth, they cannot digest their meals.

This is where they will all congregate as the day cools

to collect the last rays of the sinking sun.

One advantage of generating your own body heat,

as we and all mammals do, is that, when the sun goes down,

we can remain active and we can live in cold climates.

But the price we pay for those privileges is very high.

Something like 80% of the energy in the food we eat

goes to maintaining our body temperatures at around 38 degrees centigrade.

The reptiles' system, getting heat directly from the sun,

is much more economical.

A reptile can survive on 10% of the food a mammal of a similar size would require.

And so, although they can't live in the Arctic,

they can survive on low-calorie foods, like the seaweed these marine iguanas eat,

and live in great numbers in places where food is very scarce, such as deserts.

A chameleon, for example, can flourish in barren areas,

provided that every few days it catches one reasonably-sized insect.

The chameleon's talent for changing colour

serves not only to give it different disguises but to express its emotions.

It goes black with rage and becomes brightly coloured when courting,

as many reptiles do.

Anolis lizards display with extensible throat pouches,

each species with its own particular colour.

Nodding reinforces the effect. It's like waving a flag.

This is the rare green iguana from Fiji.

His courting colours are permanently on display, for only the male has black stripes.

He doesn't have a throat pouch, but, like many lizards,

uses head nodding to signal his status as a male in breeding fettle.

By comparison, the female is plainer and not nearly so demonstrative.

The Galapagos iguanas also nod.

For them, as for others, the gesture serves a double purpose:

not only to impress the females, but to warn off rival males.

Each species has its own rhythm.

The land iguanas have a slightly different language.

A nose-to-nose nodding session is enough to settle a territorial dispute between these males.

Eventually, all these displays lead to the desired consummation.

The reptiles were the first vertebrates for whom internal fertilisation was essential.

Their immediate ancestors, the amphibians, had no need for it.

They mated in water, and sperms shed from the body could swim to the eggs.

But out on dry land, the male reptiles had to find

some other way of ensuring that sperm met egg:

by placing it inside the female's body.

And it sometimes seems that that process for these antique creatures

is still a very clumsy and laborious business.

In due course, that produces the second of the great reptilian innovations.

Their waterproof skins had enabled them to live in the driest places on earth.

But that was of limited value if they had to retreat

to find open water in which to lay eggs, as the amphibians have to do.

The reptiles solved that problem by producing this.

The waterproof egg.

In effect, it's a tiny pond encapsulated in parchment and shell

in which their young can pass through what amounts to the tadpole stage.

The Fijian green iguana lays only two or three eggs,

burying them with care in the ground.

From all reptile eggs, the young clamber out, fully formed,

virtually exact miniatures of their parents and ready for immediate action.

These are baby skinks.

From an egg like this there once hatched one of the most spectacular reptiles of all.

For this is the fossilised egg of a dinosaur.

There can be no question of the success of these early reptiles.

They dominated the world for 130 million years.

During that time, they developed into all kinds of shapes and sizes.

Many took to the air, and some were the biggest flying animals to have existed,

with a wingspan as big as a small aeroplane.

Others even returned to the sea.

Some rowed themselves along with huge flippers,

while others sculled with their tails as dolphins do today.

And turtles grew to the size of a small boat.

From eggs hatched the dinosaur dynasty,

which was the most spectacular demonstration of reptilian success.

Deinonychus.

Tyrannosaurus rex.

The three-horned dinosaur, triceratops.

Stegosaurus, parasaurolophus.

And brachiosaurus, as big as a house.

These were among the most impressive animals ever to tread the earth.

And we know them from their bones.