The First Forests Life on Earth

The volcanoes of today are mere feeble flickerings

compared with those that dominated the world at the beginning of its history.

Then, enormous sheets of lava welled out of the craters,

titanic explosions blew whole mountains into fragments

and scattered them as dust and ash over the surface of the land.

That sort of activity continued for millions and millions of years,

and I'm talking about a period that was 4,500 million years ago.

The forces of erosion, frost and rain, snow and ice,

shattered the volcanic rocks into fragments.

Rivers carried them down piecemeal to the edges of the continents

and deposited them as sands and gravels and muds.

As the continents drifted over the globe and collided,

new mountain ranges were built up and, in their turn, worn down.

And throughout this immensity of time, the land remained sterile.

Nowhere was there even the smallest of animals

or the tiniest speck of green.

If you condense the whole history of life,

from its very beginnings until the present moment, into a year,

then it wasn't until about the end of September

that the first creatures of any size, jellyfish and so on,

appeared in the sea.

And it wasn't until the beginning of November that the first life,

a few patches of green, appeared on land.

Maybe at the edge of water, like this.

These first plants were simple algae

that had developed cell walls thick enough to enable them to survive

on the moist boulders and gravels.

Slowly, they spread over the lake beaches and sandspits,

pioneers of the great revolution

that was to lead to the greening of the earth.

Moving out of water for the plants had presented a number of problems.

One of the most serious was the question of support.

In water, algae like this can grow into long strands,

but robbed of the support of water,

none has a sufficiently rigid stem to allow it to grow upright.

So the first land plants had to remain lowly,

forming flat skins like liverworts or cushions like mosses.

All of them lived in wet, moist places and for a very good reason.

Their ancestors, the algae, had reproduced in two ways,

by budding and sexually,

and the sexual method involved the sex cells actually swimming

through water in order to find one another and fuse.

Well, mosses retain very much the same sort of method.

And it's this that keeps them tied to water.

So they can only live in places where at the very least,

it's wet during some time of the year,

so that sexual reproduction can take place.

Of course, in places like this, they are literally in their element.

Mosses and liverworts like this both produce two kinds of sex cells.

These outgrowths on the liverwort, only a few centimetres high,

develop tiny mobile sperms which actively swim.

These different growths contain larger static sex cells, the eggs.

Under the microscope, you can see the eggs at the base of tiny tubules

surrounded by a protective sheath of smaller cells.

When the outgrowths are ripe and conditions sufficiently wet,

fertilisation begins.

The wriggling sperm are released

and swim in the film of water that covers the plant.

The sperm appears as a milky fluid.

At the same time,

the female part of the liverwort that bears the egg cells

releases a special chemical that attracts the sperms.

Eventually, they reach the female organs.

Fertilisation occurs and the eggs develop,

repeatedly dividing to produce a capsule full of microscopic grains -

spores.

When they are ripe and the weather is dry, the capsules burst.

Each minute spore is capable of growing into a new liverwort plant.

Mosses also reproduce by these two alternating methods.

The sexual stage provides the variety of offspring necessary

for continued evolution.

The asexual spores can be carried on the wind

to distribute the plant over great distances.

The spore capsules of mosses are very varied in shape,

and they have the most ingenious ways

of making sure that they only release their contents

when the weather is suitably warm and dry.

Many species have detachable caps

which are blown off before the spores can be released.

And beneath, a perforated lid, like a pepper pot.

And the wind will now carry the microscopic spores for miles.

With such mechanisms as these,

the first plants colonised the moist places of the world,

and green carpets bordered the lakes and rivers.

Into these miniature jungles came the first land animals.

Millipedes, then as now, were vegetarians,

and they must have found plenty to eat among the mosses and liverworts.

The biggest of them today are only a few inches long,

but many ancient forms that pioneered life on land grew very much larger.

One, indeed, was as long as a cow.

Millipedes were descended from sea-living creatures

very distantly related to crustaceans such as shrimps.

From them, they inherited segmented bodies and an external skeleton

which gave them the necessary support

so they could move just as well in air, on land,

as their ancestors had done in the sea.

But breathing was another matter.

Their ancestors had extracted dissolved oxygen from water

with feathery gills alongside each leg.

But such things wouldn't work in air.

Instead, the first millipedes developed a system of branching tubes

within each segment, along which air diffuses to all parts of the body

so that the tissues can absorb oxygen directly.

These tubes open to the outside

through a tiny pore on the side of each segment.

But the amiable browsing millipedes didn't have the land to themselves for long.

Very soon after they had colonised it,

hunters came up from the sea to prey on them.

These hunters are still today active, mostly at night.

The scorpions.

They had evolved from a different group of segmented sea creatures,

but, again, they had an external skeleton which worked very effectively on land.

With powerful nipping claws and poisoned stings on their tails,

scorpions are well-armed and ferocious,

actively seeking out their prey wherever it may be hiding.

Another closely related group became mainly day hunters

in the miniature forests.

The spiders.

Although their sea-living ancestors had many pairs of legs,

spiders and scorpions have only four pairs. Better for speed.

And spiders have lost most signs of division in their bodies,

except for some very primitive ones that live in Southeast Asia.

Their abdomens show the last relics of that ancestral segmentation.

Early in their history, the spiders developed glands in the abdomen

with which they produce silk.

They use it in hunting, sometimes laying long trip lines,

sometimes constructing dense sheets.

And they manipulate the threads with modified limbs, the spinnerets.

By the time it's finished, any small creature trying to make its way here

will blunder into a silken trap.

And while it's still entangled, the spider will pounce on it.

Reproduction for all these land creatures presented new problems.

Without water to transport sperm to egg, there was nothing for it -

male and female had to get together.

For the millipede, this presented no real danger.

They are vegetarians,

so when individuals meet, neither risks being eaten by the other.

Their difficulties are entirely ones of manipulation.

The sex glands of both male and female

are at the base of the second pair of legs.

The male has reached forward with his seventh pair of legs

and collected from his second segment a little packet of sperm.

Now, if only he can get it into exactly the right position

alongside the female's pouch in HER second segment,

all will be well.

And there it goes.

The scorpion's sexual problems are much more complicated

and potentially dangerous.

They are hunters and have to make sure one doesn't regard the other

not as a mate but as a meal.

Courtship is necessary,

ritualised in a number of set movements.

First, those dangerous pincers have to be neutralised.

Now, with the pincers held out of action, more rituals follow.

The heads of male and female come close

and even touch.

Now a strange heaving back and forth,

which will eventually lead to the actual transfer of sperm.

The male's sex gland is on the underside of his body,

and from it, he has deposited a packet of sperm on the ground.

Now he has to tug the female into a position

where her sexual pouch is directly above it.

If this ritual is not performed correctly,

the scorpion's hunting instincts will not be pacified.

It's a delicate balance, and here it seems to be going wrong,

because this probing with the sting

is probably more to do with aggression than with mating.

And they break.

Spiders have the same kind of problem.

They, too, are hunters, and a male advancing on a female

has to make quite sure she knows who he is and what his intentions are.

The female jumping spider has sharp eyes, eight of them.

He signals with his front legs as though his life depended on it,

which indeed it does.

She signals back...

..and he is encouraged.

At close range,

the male begins to use tactile signals rather than visual ones.

He must constantly convince the female of his good intentions,

for he has to achieve a more intimate and direct contact with the female

than the male scorpion did.

He's prepared for this encounter by spinning a tiny web of silk

on which he's dropped some sperm from a gland under his abdomen.

And he's taken up the sperm in two special feelers, the palps.

Now he must reach over the female to pump sperm from one palp

into one of the female's sexual pouches.

It's rather like liquid being squeezed out of an eye dropper.

And there it goes.

Now the spider changes position to pass sperm from the other palp

into the female's other sexual opening.

The wolf spider is a larger and particularly aggressive species.

He too is courting a female.

His problem is especially dangerous here,

because the female lives in a burrow

from which she emerges only on hunting forays.

It's hardly surprising, therefore, that he approaches with the greatest caution.

At first, he uses a kind of semaphore.

If he doesn't keep this up, the female may mistake him for prey

and rush out and pounce on him.

Within the confines of the burrow, visual signals are difficult,

and so the male changes to delicate and sensitive strokings with his front legs.

At last, she receives him and he can take up his risky mating position,

reaching right round to the female's abdomen.

The early jungles, filled with such creatures,

were still only a few inches high,

no more than a thick, moist carpet draping the sandspits and boulders.

For plants like mosses and liverworts were still the only ones on land.

And this is just about as big as any moss in the world ever grows.

A series of isolated stems.

It has no real roots. It just absorbs what moisture it requires through its surface.

And it doesn't have true leaves. They're just simple scales.

And to see why it's so frail, one has to look inside the stem.

Sliced and examined under the electron microscope,

this is how it appears in section.

The cells are thin-walled with no rigidity to them,

unable to support a tall plant.

But that structure was soon to be strengthened.

In the course of time, some plants developed that WERE able to grow upright

and several feet tall.

And the fossilised remains of some of the earliest of them

have been found in the rocks of these bleak Welsh hillsides.

To find fossils,

you sometimes have to use violent methods.

And here are some.

They're just thin branching filaments,

but they'll show up even better

if I wet this slab.

They look like tiny moss filaments,

but when these flattened, 400-million-year-old stems are sectioned,

the electron microscope reveals quite different cells.

These have much thicker walls, forming tubes in the stem.

A plumbing system, up which the plant draws water.

And these new cells give the stem strength

and the ability to grow tall.

These very similar cells come not from a fossil plant

but from a living one, from this plant,

which grows on another Welsh hillside.

It may look superficially like a moss.

In fact, its common name is clubmoss,

but actually, it's fundamentally different.

By virtue of those tough, thick cells in its stem,

it's much more rigid than any moss.

Today, it only grows to that sort of height.

But in the past, it grew to the size of trees and formed great forests.

There were soon many kinds of plant with the new cell walls,

and some of them, the horsetails, are still common worldwide.

The highest, in South America, reaches three or four metres,

but 300 million years ago, they grew to 30 metres, 90-feet tall.

Then, as now, they developed a hard outer skin to prevent desiccation.

Under the microscope, you can see minute pores

through which the plant breathes,

taking in carbon dioxide and giving out oxygen.

And there was a third kind of plant that grew with the giant horsetails

and the clubmoss trees in those first forests -

tree ferns.

But height for the horsetail and the tree fern accentuated yet again

the problem of achieving sexual union

with a male cell that has to swim.

How could a microscopic cell swim from the top of THAT tree fern

to the top of that one? Impossible.

The structures that ARE up there produce spores,

reproductive cells that do not require fertilisation in order to develop,

just like those in the little capsules developed by mosses.

The ferns produce their spores from structures beneath the fronds.

Their shape and arrangement varies with each fern species.

Ferns, like mosses,

release their spores when the weather is dry,

and the wind can carry them far and wide.

Some fern spores are produced in cups at the end of curled strips,

one side of which is woody and the other thin-walled.

As these cups dry, they shrivel, pulling back the strip

until the tension is too much, the strip snaps back

and the spores are catapulted free.

The spores have tiny spines and ridges that help them catch the wind.

A few will fall on moist ground

and then germinate to produce a different kind of plant altogether.

This is the stage in the fern's life-cycle that bears the sex cells.

And this has had to remain small and close to the ground

in order that its sperm can swim from plant to plant.

When wet weather comes, the male organs release the sperm

which swim by thrashing their thread-like tails.

Hundreds of thousands are produced from the underside of the flat plant

and are carried away by the rainwater.

Eventually, some reach the female organs of the plant

and swim up the tubes that lead to the egg cells.

After fertilisation, a new growth develops from the egg,

sending up a tiny stalk.

These green shoots eventually grow tall and complete the cycle,

becoming, once more, a familiar spore-bearing fern.

Then, about 400 million years ago,

as the forests began to rise, new animals appeared.

These were descendants of the ancestral millipedes,

and several kinds still survive today.

This is a bristletail and it lives in soil worldwide.

And this, the silverfish, that now often lives in houses.

Faster than millipedes, they have fewer body segments

and even fewer legs - just three pairs.

They all feed on vegetable matter.

But as plants grew taller, so leaves and spores became more inaccessible.

And these little creatures doubtless

clambered up the stems and trunks after them.

The journey up must have been fairly easy,

but getting down again, sometimes over upward-pointing spikes,

may have been more laborious.

Maybe that was the reason for a dramatic development.

Some little creatures developed wings for flying from plant to plant.

Just how wings evolved we can't be certain,

but they may have first developed as tiny lobes on the back.

Dragonflies today develop their wings in just this way,

repeating millions of years of evolution in just one night.

The wings are stretched taut by blood pumping into the veins.

Later, the blood is drawn back into the body

and the gauzy wings slowly dry and harden.

Flight is the great achievement of the insects.

They were the first creatures to take to the air

and they were to have it almost to themselves for 100 million years.

Dragonflies were among the first flyers,

and they are still superb aeronauts.

They can reach speeds of 20 miles, 30 kilometres an hour.

They hunt in the air,

holding their legs crooked in front of them like a basket.

They even mate on the wing.

The females lay their eggs in water.

Their young, wingless larvae will grow up on the bottom of the pond,

breathing through feathery gills

and feeding on other small water-living creatures

until the time comes for them too to climb up a reed

and spread their wings.

The dragonflies' smaller relatives, damselflies, also haunt ponds.

The wings of these insects beat so rapidly

that only a slow-motion camera can show clearly how they fly.

This is the action slowed down 120 times.

The insect gets lift on the downbeat of the wing by twisting it,

so that the leading edge is inclined downwards.

But at the bottom of each stroke the wing is twisted back

so that it is effective on the upstroke as well.

It's an intricate set of mechanical movements

which man has never matched in the air.

Here, the insect is hovering.

The wings sweep alternately backwards and forwards,

again changing angle at the end of each sweep

in order to obtain lift on both strokes.

Man has achieved something similar with a helicopter,

whose blades rotate.

The insect can't rotate its wings,

but it's evolved a set of movements which are even more complex.

The principal navigational equipment of dragonflies and damselflies

are their superb eyes.

Because they're so dependent on them,

dragonflies normally fly only during the day.

Today's splendid species are among the biggest of insects,

but when the insects first had the air to themselves,

the dragonflies grew gigantic

and one appeared that had a wingspan of 70cm, over two feet.

The largest insect that has ever existed.

While all this was happening, some 300 million years ago,

the plants themselves were on the brink of an important advance.

This tiny sexual stage of the fern's life cycle

is obviously very vulnerable.

It can only live in moist conditions like these,

and down on the ground it's easily cropped by plant-eating animals.

It would obviously be much safer if this stage could take place

up in the top of the tree.

But that would require some way of transferring the sex cells

from tree to tree.

Well, they could be blown there by the wind.

But there was then, as there is now,

also a regular traffic in-between the tops of the trees.

Insects that go up there to seek the spores as food

and fly from one tree to another.

They could take them. And that's what happened.

New plants appeared in which the sexual generation remained fixed

to the asexual tree stage.

And one of the first of them was a plant like this,

a cycad.

Cycads bear two kinds of cones,

each of which represent, in effect, part of the tiny sexual stage

that once grew down on the ground.

The male cones produce pollen,

the grains of which germinate to produce the male cells,

and the female cones contain the large egg cells.

Insects help to transport the pollen from the male cone to the female,

and there it produces a tube down which swims the sperm.

At its tip, within the female cone, a drop of water appears,

and in that the sperm swims,

re-enacting the journeys made through the primordial seas

by the sperm cells of their algal ancestors.

Only after several days does it fuse with the egg.

This cycad leaf is about 200 million years old.

That's to say it was fossilised around the end of November

in the Life On Earth year.

And at that time a new and revolutionary plant had appeared

that was growing alongside these cycads.

It was the conifer, and this is one of its trunks.

It's not wood, as you might think, but solid stone.

I'm in the middle of one of the most spectacular deposits

of plant fossils in the whole world.

The Petrified Forest in Arizona.

These conifers grew to over 200 feet tall

and they stood in thick, dense, dark forests alongside the swamps

where the cycads grew.

And when the trunks fell, they often dropped into a river

which swept them down here

so that they formed great logjams around here.

And then the river muds and sands and silts buried them.

And the silts eventually formed mudstones like those over there.

When the mudstones eroded away, as they have done here,

they re-exposed these trunks that have been turned to stone.

Conifers are built on very similar lines to the cycads,

except that they have both the male and the female cone on the same tree.

These are the male cones,

and they use wind to transport their pollen.

But to ensure that such a haphazard method of fertilisation is successful,

they have to produce pollen in huge quantities.

One cone may produce several million grains,

and there are many thousands of cones on an average-sized tree.

The female cones are fewer in number and grow on the same branches.

They're small globes in conspicuous positions on the tips of shoots,

where they have a good chance of receiving pollen.

Pollen falling on the female cone

is only the beginning of a very long process.

It takes a whole year for the grains to grow down to the eggs,

and at the end of that year the cone looks like that.

But even that's not the end of things.

During the next year, the cone grows still more,

it develops wrappings around the fertilised eggs

and then it dries out and opens up.

Out drop small, neatly packaged brown objects.

Seeds.

They contain the first kind of plant eggs to have been fertilised

without the help of water.

Ancient though the conifers' technique of reproduction is,

it has proved a huge success.

Today, about a third of the forests in the world are formed by conifers.

Firs, larches, cedars, pines. They're all members of this group.

The biggest living organism of any kind is a conifer,

the giant sequoia of California

that grows to 112 metres - 367 feet high.

Some have a diameter of 12 metres, 40 feet.

Conifers have a special way of healing wounds to their trunks.

They seal them with resin.

When it first flows, it's runny, but it soon forms a sticky lump

which not only covers the wound but incidentally acts as an insect trap.

Lumps of resin from the ancient coniferous forests survive as amber,

and in them are insects,

as perfect now as the day when they blundered into the resin

100 million years ago.

From fossils like these, we know that the insects by that time

had developed into an enormous variety of forms

that swarmed through the trees and over the ground,

feeding on every part of the plants.

Pollen and fruit, leaves and wood, root and branch,

just as they do today.

Bugs stab stems with stiletto-like mouthparts to reach the sap.

There are over 3,000 species of aphids alone,

tapping this ready source of food in plants all over the world.

All they have to do is to pierce the plant vessels.

They don't even need to suck, such is the pressure of the sap within the stem.

Locusts and grasshoppers chew the leaves.

Beetles munch through cuticles and even manage to digest wood.

Some insects not only eat plants,

but in order to hide while doing so

they've come to look like plants, like leaves and sticks.

Hunters from the ground pursue the insects up into the trees.

Spiders.

But lying in ambush on trunks and on leaves has its limitations.

Most insects fly.

Spiders never developed wings,

so they were unable to pursue their prey into the air.

Instead, they set traps for them.

The silk that they had spread in sheets and trip lines on the ground

they now wove into nets,

setting them across the insect flyways.

With these elegant and varied constructions,

spiders began to take a heavy toll of flying insects

and today spiders are one of the most effective predators

on the insect populations.

The insects developed their flying skills in many different ways.

The two pairs of wings used by the dragonflies and their relatives

were also used by other insects. This is a lacewing.

But this design was modified by other insects.

The caddis-fly, not needing the speed of a dragonfly to catch prey,

overlapped its two pairs of wings, producing a unified surface area.

On the other hand, bees must have compact wings

which can be neatly folded back when visiting flowers or in the hive.

To get the right lift, their smaller wings must beat faster.

They look as though they only have one pair of wings,

but in fact they have two.

They're hitched together to form what is virtually a single surface

by a line of hooks along the front edge of the back wing.

Other insects spend more time among dense foliage.

The front wings of this bug have thickened bases to them

which strengthen them and protect the rear ones when folded.

Beetles have gone one stage further.

Many burrow through litter and dense vegetation,

and their front wings have become converted into protective covers.

In order to lift the heavy body during flying,

the operational wings have to be large.

If they're to be protected when not in use, they have to be folded,

and the trick is done with spring-loaded joints

in the veins of the wings.

Once in the air, the wing covers have to be held up out of the way.

But they may also help a little in flight, acting as stabilisers,

preventing rolling and yawing.

Like many insects, this beetle increases lift

by clapping its wings together at the top of the upstroke,

thereby improving airflow over the wings.

The chafer is the heavyweight of the insect fliers.

Its wings beat comparatively slowly, about 40 times a second.

And it's the least agile of insects in the air,

ponderous and unable easily to bank and swerve.

It holds its wing covers out of the way along its back

and balances itself with outstretched legs.

Its wing structure is tremendously strong,

in order to support a heavy insect,

and yet flexible enough to change its angle on each stroke

and even fold back on itself when the insect stops flying.

Even that feat is overshadowed

by the achievement of the most skilled aeronauts of all,

the flies.

This one, the hoverfly, is perhaps the champion.

It uses only one pair of wings, the front ones,

which it keeps in perfect condition with frequent cleaning.

It can hang absolutely stationary in the air,

and does so even when it mates.

It can compensate for any sudden current of wind to hold its position.

It can fly backwards and dart off at great speed in any direction.

And to perform these manoeuvres

it beats its wings at the astonishing speed of 175 beats a second.

A normal slow-motion camera still shows the wings as a blur.

They control flight with a device which can be seen clearly in another fly,

the crane-fly, or daddy-long-legs.

Those two objects, like drumsticks, swinging up and down,

are their back pair of wings after millions of years of evolution.

They're jointed to the body just as the rear wings are,

and they act like gyroscopes.

By beating very fast, and here they're slowed down 120 times,

they give the fly stability in the air.

For, like gyroscopes in the automatic controls of an aeroplane,

they enable the fly to be aware of the attitude of its body in the air

and to detect when there's been any change in the flight path.

Houseflies also have these "drumsticks",

though they're much smaller.

It's these that enable flies to perform such extraordinary

and tantalising aerobatics.

And the same organs perform similar functions for the hoverfly,

giving it that superb flight control.

The design of the insect body is particularly suited not to great size

but to miniaturisation.

The hoverfly is one of the most intricately constructed insects of all.

A marvel of microscopic machinery

that's built up from an egg in a few days

and is often crushed beneath a thumb.

The main developments of the insects

took place at a comparatively early stage in the history of life on earth.

At the time when these petrified forest trees were alive,

200 million years ago,

every single main type of insect that we know today

was already in existence.

Here, for example, is a piece of petrified wood,

and before it was turned to stone some beetle had bored holes into it,

just as beetles bore into dead wood today.

And now the stage was set for a revolution,

and one in which the insects were to play a crucial part.

Charles Darwin called its history "an abominable mystery".

Even today, we've only got a sketchy idea of just what happened.

But some of the plants developed flowers.

The woodlands and the lakes bloomed

and colour came to the earth.

Flowers became beautiful,

not to delight the eye of man, but to attract insects.

This led to some of the most intimate of all the relationships

that have evolved between plants and insects - pollination.

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