Charles Darwin and the Tree of Life Page #5

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Anti-evolutionists maintain

that the eye would only work

if it was complete in all its details.

Darwin, on the other hand,

argued that the eye had developed

becoming increasingly complex

over a long period of time.

That would only work

if each stage of development

was an improvement on the previous one.

And today, we know enough

about the animal kingdom

to know that is indeed the case.

Some very simple animals have nothing

more than light-sensitive spots

that enable them to tell the difference

between light and dark.

But if a patch of such spots formed

even the shallowest of pits,

one edge of the pit would throw a shadow

and so reveal the direction of light.

If the pit got deeper

and started to close,

then light would form a blurred image.

Mucus secreted by the cells

would bend the light and focus it.

If this mucus hardened,

it would form a proper lens

and transmit a brighter

and clearer image.

All these different

fully functional stages

at different levels of complexity

are found in living animals today.

This single-celled creature has

one of those light-sensitive spots.

Flatworms have a small pit

containing light spots

so they can detect the shadow

of a predator.

A snail's blurry vision is good enough

to enable it to find its way to food.

And the octopus has an eye

with a proper lens

and can see as much detail as we can.

So the structure of the human eye

does not demand the assistance

of a supernatural designer.

It can have evolved gradually,

with each stage

bringing a real advantage

as Darwin's theory demands.

Natural selection, of course, requires

that an animal's characteristics

are handed from one generation

to the next.

It's obvious that children

resemble their parents.

Anyone knows that.

But when you come to think of it,

how does that come about?

In Darwin's time,

nobody had the faintest idea

about the mechanism or the rules

that govern that process.

Except, perhaps, for one man,

who was working in the city of Brno

in what is now the Czech Republic

at exactly the same time that Darwin

was writing his book in Kent.

That man's name was Gregor Mendel.

He discovered the laws of inheritance

by breeding thousands of pea plants

and observing how they changed

from one generation to the next.

He found that while many characteristics

were passed down directly

from one generation to another,

others could actually skip a generation.

How could that happen?

Mendel explained this by suggesting

that each plant, each organism,

contained within it

factors which were responsible

for creating

those particular characteristics.

Today we call those things genes.

But nobody had any idea how they worked

until a hundred years

after Mendel's time.

And then the answer

was discovered in Cambridge.

In 1953,

here in the Cavendish Laboratories,

two young researchers Francis Crick

and James Watson

were building models like this.

It was their way of thinking about

and investigating the structure

of a complex molecule that is found

in the genes of all animals, DNA.

The crucial bit are these chains

which encircle the rod.

And here is a second and entwined.

This is the double helix.

The workings of the DNA molecule

are now understood in such detail

that we can demonstrate something

that is truly astounding.

A gene taken from one animal

can function in another.

The gene that causes a jellyfish

to be luminous, for example,

transplanted into a mouse,

will make that mouse luminous.

The genetic code

can also reveal relationships.

Even our law courts accept

that DNA fingerprinting

can establish whether a man

is the father of a particular child.

And it can also reveal

whether one kind of animal

is related to another.

It proves, for example, that kangaroos,

ground-living animals

that run with great leaps

are closely related to koalas,

that have taken to climbing trees.

That insect-eating shrews

have cousins that took to the air

in search of insects, bats.

And that one branch of the elephant

family, way back in geological history,

took to the water and became sea cows.

So, 150 years after the publication

of Darwin's revolutionary book,

modern genetics has confirmed

its fundamental truth.

All life is related.

And it enables us to construct

with confidence the complex tree

that represents the history of life.

It began in the sea,

some 3,000 million years ago.

Complex chemical molecules

began to clump together

to form microscopic blobs, cells.

These were the seeds from which

the tree of life developed.

They were able to split,

replicating themselves as bacteria do.

And as time passed,

they diversified into different groups.

Some remained attached to one another,

so that they formed chains.

We know them today as algae.

Others formed hollow balls,

which collapsed upon themselves,

creating a body with an internal cavity.

They were the first

multi-celled organisms.

Sponges are their direct descendants.

As more variations appeared, the tree

of life grew and became more diverse.

Some organisms became more mobile

and developed a mouth

that opened into a gut.

Others had bodies stiffened

by an internal rod.

They, understandably, developed

sense organs around their front end.

A related group had bodies that were

divided into segments,

with little projections on either side

that helped them to move around

on the sea floor.

Some of these segmented creatures

developed hard, protective skins

which gave their body some rigidity.

So now the seas were filled

with a great variety of animals.

And then, around 450 million years ago,

some of these armoured creatures

crawled up out of the water

and ventured onto land.

And here, the tree of life branched into

a multitude of different species

that exploited this new environment

in all kinds of ways.

One group of them developed

elongated flaps on their backs

which, over many generations,

eventually developed into wings.

The insects had arrived.

Life moved into the air

and diversified into myriad forms.

Meanwhile, back in the seas,

those creatures with the stiffening rod

in their bodies

had strengthened it

by encasing it in bone.

A skull developed, with a hinged jaw

that could grab and hold onto prey.

They grew bigger and developed fins

equipped with muscles

that enabled them to swim

with speed and power.

So fish now dominated

the waters of the world.

One group of them developed the ability

to gulp air from the water surface.

Their fleshy fins became

weight-supporting legs,

and 375 million years ago,

a few of these backboned creatures

followed the insects onto the land.

They were amphibians, with wet skins,

and they had to return to water

to lay their eggs.

But some of their descendants

evolved dry, scaly skins,

and broke their link with water

by laying eggs with watertight shells.

These creatures, the reptiles,

were the ancestors of today's

tortoises, snakes,

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