Narrated by Sir David Attenborough
So, a hundred and fifty 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 three thousand million years ago.
Complex chemical molecules began to clump together to form microscopic
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
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 bodies some rigidity. So now, the seas were filled with a great variety
And then around 450 million years ago some of these armoured creatures
crawled up out of the water and ventured on to the land.
And here the tree of life branched in to 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 a stiffening rod in their bodies
had strengthened it by encasing it in bone. They increased in size. They grew
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 on to the land. They
were still not independent of water. They were amphibians with wet skins and
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 that had watertight shells. These creatures, the reptiles,
were the ancestors of today's tortoises, snakes, lizards and crocodiles. And of
course, they included the group that, back then, came to dominate the land:
So the tree of life burgeoned into a multitude of different branches.
But 65 million years ago, a great disaster overtook the Earth. Whatever its
cause, a great proportion of animal life was exterminated. All the dinosaurs
disappeared - except for one branch, whose scales had become modified into
feathers. They were the birds.
While they spread through the skies, a small and seemingly insignificant group
of survivors began to increase in numbers on the ground beneath. These
creatures differed from their competitors in that their bodies were warm and
insulated with coats of fur. They were the first mammals. With much of the land
left vacant after the great catastrophe they now had their chance. Their warm
insulated bodies enabled them to be active at all times - at night as well as
during the day - and in all places: from the Arctic to the tropics, in water as well
as on land, on grassy plains and up in the trees.