Nova (1974–…): Season 31, Episode 12 - Origins: How Life Began - full transcript

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SUBTITLED BY ANDROMEDA

In the endless reaches of the
universe Earth seems unique.

It's a planet shaped
and molded by life,

a planet that six billion
people call home today.

But when it was born, some 4.5 billion
years ago, Earth was a violent place,

so hostile it's hard to believe
life could ever begin here.

Covered in lava,

and smothered in noxious gases,

Earth was a planet under siege.

If you were a human,
going back into time,

and trying to stand on the early Earth,



it would be just like visiting
a planet that was not your own.

This was a hazardous
worid, no doubt about that.

If you were located in the
wrong place at the wrong moment,

you were simply vaporized.

It was a planet plagued by catastrophe.

If you condense all of Earth's
history to just 24 hours,

then only minutes after it formed,

the entire globe melted and reformed.

Then, to make matters worse,

another planet about the size of Mars

slammed into Earth,

a cataclysm that created our moon.

But soon after these
disastrous beginnings,

the most radical
transformation of all time



hit the planet: the origin of life.

So how did life begin?

Well, over the years,

people have come up with some pretty
creative answers to this question.

One of my favorites

comes from a 17th century scientist

who wrote down a recipe

for creating life from scratch.

Let's see,

it says here,

"Take a dirty garment,

place it in a vessel.

Next add wheat."

Then, according to the recipe,

after fermenting for 21 days,

mice will appear fully formed.

Of course, we all know that
life doesn't form this way.

But at some point in
the Earth's early years,

life did emerge out of
non-living ingredients.

And for clues to the
real recipe of life,

we have to go back
some four billion years

to a time when Earth

was nothing like the
planet we know today.

When we think of early Earth

we must recognize it
was not a Garden of Eden.

There were no clear blue oceans,

there was no clear water,
there were no plants.

There was no life at all.

The young sun was
weaker than it is today.

And its light barely penetrated
the atmosphere of carbon dioxide

spiked with the pungent
fumes of hydrogen sulfide.

Since the atmosphere was thicker

and dominated by CO2,

the Earth had a reddish tinge to it.

It didn't have the familiar blue sky.

The oceans would have had

an olive green color rather
than our familiar blue color.

For about the first 600 million years,

comets and asteroids
pounded our planet,

a time known as the
"Heavy Bombardment."

These interplanetary missiles

measured up to 300 miles across.

Their impacts vaporized Earth's oceans

and melted its crust.

With its extreme
temperatures and toxic rain,

seemingly nothing could survive here.

But we now think that in
this hellish environment,

life first took hold.

And today, hidden away in
remote corners of our planet,

conditions that in some ways
resemble the extremes of early Earth

can still be found.

Penny Boston and Diana Northrup are
microbiologists on an expedition to
investigate

how life can survive in
those harsh surroundings.

Buried in the depths of
this tropical rainforest

is a cave called Cueva de Villa Luz.

Located in southern Mexico,

it's an underground worid
laced with hydrogen sulfide,

a foul-smelling gas
that was present on Earth

some four billion years ago.

These relic, or antique
environments like Cueva de Villa Luz

offer the same kinds of environments
that we would have found on early
Earth,

and we're hoping to get clues
to work backwards from those.

As you approach the cave

you begin to get these faint
whiffs of the rotten egg smell.

And as you get closer
this becomes more intense.

Hydrogen sulfide can
be extremely poisonous,

so the scientists have to
wear gas masks inside the cave

and carefully test them for leaks.

Have you got everything in there?

I think I got everything.

At the levels at which humans can't
live very long in hydrogen sulfide

you don't smell it at all.

It will just simply cause you to
go unconscious and die very quickly.

But can any other forms of life survive

in the deep recesses of the cave

so toxic to humans?

Here, hydrogen sulfide,
an invisible gas,

escapes from the underground springs,

reacts with oxygen in the water,

and coats the cave with sulfuric acid.

The longer it sits there on the walls,

the more acid it becomes.

And so, eventually, by the
time the drop is falling on you

it's a very, very acid environment.

It's very fatiguing,

and even with the protective
masks that we have,

we pick up loads of
toxic gas through our skin

and perhaps through tiny leaks.

Look at those stalactites to your left.

Amazingly, despite
the extreme conditions,

it appears that life is
thriving inside the cave.

It comes in a strange package:

colonies of single-celled bacteria

that form slimy drips
scientists call "snottites."

The snottites are drippy,
gooey, mucusy formations

that look like stalactites.

And that's why they
were called snottites,

because they resemble strings of snot.

We believe that the snotty, gooey stuff

is to protect them
against extreme acidity

because when we measure
the drips on the snottites,

they are as extreme as battery acid.

And so, while we find that daunting,

this is where they thrive.

Bacteria are among the most primitive
and most common organisms on Earth.

Like all forms of life,

they grow, adapt to their
environment, and reproduce.

Inside each single-celled
bacterium is a molecule of DNA,

the code of life

which allows them to multiply.

There are millions of
bacteria in each snottite.

And down in the underground streams,

Penny Boston has found different
kinds of bacteria in slimy clumps

she calls "phlegm balls."

In fact, the cave is home to a
huge number of bacterial colonies.

And astonishingly,

instead of being poisoned
by the hydrogen sulfide,

these bacteria depend
on it for their survival.

They take the hydrogen sulfide and
they get chemical energy out of it.

It doesn't poison them.

It's home sweet home for them,

and this is a pretty new
finding for these organisms.

Conditions on early Earth
may have been far worse,

but these bacteria suggest

that primitive life could have thrived

in extremely hostile environments.

But where did the very
first life come from?

For more than a century,

scientists have known that life

is the result of chemistry,

the combination of just
the right ingredients

in just the right amounts.

Today, we know these ingredients aren't
things like dirty garments and wheat,

which people used to think would
spontaneously generate mice.

The ingredients of life
are actually much simpler.

All living things, from bacteria to
mice

to you and me,

are made from a small
set of chemical elements:

hydrogen, oxygen, carbon, nitrogen

?four of the most common
elements in the universe.

Combined in just the right way,

these are the fundamental
ingredients of life,

and carbon is the star of the show.

Carbon's everywhere.

It's all over the universe.

What makes carbon special is
the kind of bonds that it makes,

both with itself and
with other elements.

We know of no other atom

that has the flexibility
that carbon has to form

diverse types of compounds.

And the idea that life could have
started when carbon and other
ingredients combined

in the harsh conditions of early Earth

was first put to the test in the 1950s

by a young graduate student

named Stanley Miller.

To simulate the newborn
Earth in the lab,

Miller assembled a contraption
made out of flasks and tubes.

He filled one flask with gases thought
at the time to represent
Earth's primitive atmosphere,

and he connected that to another flask

with water to represent the oceans.

And then he did a brilliant thing.

He simply put an electric charge

through that to essentially simulate

lightning going through
an early atmosphere.

And after sitting around
for a couple of days,

all of a sudden there
was all this brown goo

all over the reaction vessel,

and when he analyzed

what was in the vessel now,

he actually had amino acids.

Amino acids are compounds that form

when molecules of carbon and
other elements link together.

They are the essential building
blocks of proteins and cells,

vital ingredients of all living things.

Stanley Miller's
experiment was headline news

and jump-started the scientific search

for the origins of life.

Life is really chemistry;

there's no question about that.

In fact, it's a chemistry that,
when you get the recipe right,

it goes, and it goes fairly quickly.

That recipe is hotly debated today,

and most scientists think the
environmental conditions on early Earth

were very different from the
ones Miller simulated in his lab.

And another debate rages

about when this recipe
first got cooked up.

On our 24-hour clock,the
barrage of asteroids and comets

lasted from about midnight
until almost 3:30 in the morning.

The assault then weakened,

but continued for more
than 100 million years.

It's hard to believe that life
could have gained a foothold

during this unstable period,

but new discoveries reveal

that life may have existed as
early as four in the morning,

or about 3.8 billion years ago.

The evidence comes from some of
the oldest rocks on the planet,

found in the remote
regions of West Greenland.

The geology of Greenland is unique.

It contains a record of

some of the earliest geological
processes that we know of on the Earth.

The rocks themselves are thought to be

between 3.7 and 3.9
billion years in age.

These rocks are so old

that any fossils they once
contained have been destroyed.

So to find out if life
existed when they formed,

Mojzsis had to look for evidence
that is far more elusive.

There may have at one
time been small fossils,

microfossils but under
the conditions of heat

and pressure that
these rocks experienced,

such fossils would have been
disaggregated and destroyed.

So what we have left behind then

are chemical fingerprints of
ancient bacteria or microbes.

To search for those fingerprints,

Mojzsis first extracts a sample
from the ancient Greenland rocks.

Then he will analyze its chemical
composition looking for carbon,

a signature of life.

But carbon comes in
several different forms.

And Mojzsis wants to know
if the carbon in this sample

is the kind left behind
by living creatures.

If so, he believes that
life may have existed

when these rocks formed
over 3.8 billion years ago,

a controversial claim.

It was a surprise for us to find
evidence of ancient life in these
rocks.

We didn't know if it would be there.

You know, just because the stage is set

doesn't mean that the
actors are present.

But these samples, here, represent
the first evidence we have,

direct evidence of a
biosphere on our planet.

If it emerged so early,

life was lucky to miss the
greatest cataclysm of all time,

an impact like no other
in our planet's history.

It happened when another rocky
sphere about the size of Mars

collided with Earth.

The outer layers of our
planet were vaporized,

and the debris from this collisi?n

coalesced to form the moon.

That impact was so powerful

that any building blocks of
life that existed on Earth

would have been destroyed.

This gives rise to speculation

that the ingredients of life

didn't form on Earth at all,

but arrived special delivery,

from outer space.

Hollywood has always been taken with
the idea that life came from outer
space.

But it's not as far-fetched
as it might sound.

Space is not very far away.

Space is only about
20 kilometers that way.

Now, that's very close
and space is vast.

And a scientist named Don Brownlee

designed an experiment
to find out if space

might actually harbor the
building blocks of life.

There are 40,000 tons of
bits of comets and asteroids

that impact the Earth every year.

This is mostly in the form of
particles that are less than a
millimeter in size.

We breathe them, they're
in the food that we eat,

but they are very difficult to find.

You can only find them
in very special places.

To see if this shower of space-dust
contains the ingredients for life,

Brownlee needed to obtain samples

uncontaminated by Earth's atmosphere.

So to get just a few micrograms of
dust,

he commissioned a former spy plane

to fly close to the edge
of Earth's atmosphere.

Sticky pads on the plane's
wings collected the space dust.

Then, Brownlee's colleagues
sliced the dust particles

into slivers less than one-tenth
the thickness of a human hair.

And they discovered

that these tiny particles are rich

in the seeds of life.

If you look on an electron microscope,

you'll see this wonderful array
of minerals and carbon
and organic materials

that are 4.55 billion years old

and we believe are the
building blocks of life.

And this extra-terrestrial dust

isn't the only possible
source of life's ingredients.

In a region of space called the
Asteroid Belt are huge amounts of
debris left over

from the formation of the solar system.

And sometimes, chunks of debris
containing metal and rock fall to Earth

bearing surprising gifts.

One such meteorite landed in the
town of Murchison, Australia, in 1969.

It's a gold mine, this
little chunk of meteorite

which fell on Australia last year.

For the past six months
they've been taking it apart

and have discovered that
it contains amino acids,

the building blocks of life.

It was the first time that
complex organic compounds

had ever been found
in material from space.

And if meteorites like it were common

perhaps they had delivered
vast quantities of

the original constituents
of life to early Earth.

Enough organics are
present here that we think

that meteorites like this

provided the early Earth

its entire budget of organics.

So all the organics in your body,
all the carbon in your body

and in your lunch you had today,

arrived on the Earth
in meteorites like this.

If they come through the
atmosphere in large enough objects,

they're like little capsules
coming in the atmosphere.

They break apart on the Earth's surface

and deposit their cargo of organics.

More than 70 kinds of amino acids
have been found in meteorites,

and many are the fundamental
ingredients of proteins

that make up living cells.

During the Heavy Bombardment,

millions of meteorites
may have seeded the Earth

with the stuff of life.

And there might have been an even
more efficient delivery system.

Comets are like giant dirty
snowballs made of ice and rock.

Some comets that hit the early
Earth were the size of mountains,

and a large portion of their mass

could have contained organic compounds.

The destructive power of comets
and meteors is astronomical.

The meteor that slammed into
Earth some 50,000 years ago,

here in Arizona,

blasted a hole in the
ground nearly a mile wide

?from here to here?

and so deep it could hold
a 60-story skyscraper.

And as if that weren't enough,

the force of the impact was so great

that it instantly vaporized
nearly the entire meteor,

three hundred thousand tons of it.

So it makes you wonder:

if the building blocks
of life were delivered

courtesy of comets and meteors,

could any of the tiny
ingredients they carried

have survived the landing?

And just what happens to
things like amino acids

when they slam into Earth
with such devastating power?

To answer those questions,

one scientist came up with
an ingenious experiment.

Using a huge gas-powered gun,

Jennifer Blank simulates the
extreme pressures and temperatures

that are unleashed when a
comet smashes into Earth.

We set out to test whether
or not materials would

survive or whether
they would break down.

And we expected that, or we were hoping
that, some fraction would survive.

We figured the parts
that didn't survive

would break down into
smaller components,

but in fact what we found
is much more exciting.

The gun fires a bullet
at 5,000 miles an hour

towards a sample that represents
the organic molecules inside a comet.

The sample consists of a solution
of five different amino acids,

two of them present
in every living cell.

The mixture is inserted
into a steel capsule.

The gun will send a
shockwave through the capsule

simulating the extreme
pressures of a comet's impact.

I think it's very hard to just imagine

what kinds of pressures we're
generating in these experiments.

If you think about going
to the bottom of the ocean,

the pressures you'll
have there are only

a hundred times atmosphere.

So these are hundreds of thousands
of times atmospheric pressures.

Will Jennifer Blank's experiment show

that the building blocks
of life can survive

a crash landing on Earth?

Clear the room...

Charging now...Okay,
bringing up the X-rays...

35. Three, two, one, fire.

Three, two, one, fire.

When they remove the
capsule it's undamaged.

But have its contents
survived the impact?

The once clear solution of amino acids

has turned a tarry brown color.

And the analysis revealed

that not only had the material
withstood the colossal pressure of the
impact,

but it had transformed
into a new compound.

Amino acids, combinations of
carbon and other basic elements,

had fused together to
form more complex molecules

called peptides.

We went from our initial small
compounds?and here's
an example of one of them,

a simple amino acid?and
we used the energy

associated with the impact

to build larger molecules.

Molecules like this?this is a peptide?

and we show that we can
use the impact energy

to grow larger molecules from the
simplest building blocks of life.

Peptides link together

to form larger building
blocks, proteins,

which make up all the
cells in our bodies.

But the leap from non-living
ingredients to a living creature,

complete with DNA which
allows cells to replicate,

is staggeringly complex.

No one knows how this process started

or what course it took.

It is hard to really
get your head around

the great leap from
non-living to living.

Well, it's hard enough
that nobody's succeeded

in doing it in the laboratory.

I think it's an
astonishing mystery, and one

that we truly don't
understand in any great detail.

While we don't yet know how
the spark of life occurred,

we can try to figure out where
it might have gotten a foothold.

And because the planet was
under such devastating assault

from comets and meteors,

the leap to life may not have taken
place up here on Earth's surface.

To take hold, life may
have needed a safe haven,

perhaps underground.

A team of scientists descends into
one of the deepest mines on Earth

to investigate whether
microbial life can survive

far below the Earth's surface.

And the mining environment
gives us this fantastic window

into the deep subsurface.

It's a unique scenario

because there is nowhere
else on planet Earth

that allows you to have access
to that sort of sample location

at two, three, three and
a half kilometers deep.

It takes 45 minutes to reach the
heart of this South African mine.

Conditions here are
extremely uncomfortable,

for humans, that is.

The temperature of the rock
is 120 degrees Fahrenheit,

and the air pressure is
twice that at Earth's surface.

Life down here survives
entirely without sunlight.

If they exist, microbes need to
find a way to live in pitch darkness,

drawing chemical energy
from water and minerals

trapped in the surrounding rocks.

Microorganisms have been shown

potentially to be able
to use these molecules

to provide themselves with energy

and support themselves completely
independent of photosynthesis.

And if we can prove that
that is the case here,

then that is very interesting
because that adds credence to the idea

that you could have life
originating in the deep subsurface.

As the miners drill into the rock,

they break into ancient
pockets of water,

havens for microorganisms.

We're not sure how organisms can
live in such extreme environments.

The major thing is there's
such low nutrient availability,

there's nothing really for these guys

to continually use
and process to survive,

and yet somehow they do.

And the question is,
"How do they do it?"

The first step is to collect
pristine samples of the water

and see if they can grow
the microbes it contains.

I'll get a very big
sense of achievement

if I can actually take something that's
been isolated for 200 million years,

put it in the laboratory and
actually find out what it is

this organism needs to survive.

In a makeshift lab near the mine,

the team attempts to recreate the
environment deep inside the rock.

And they have found that
these microbes are dining

on a variety of strange gases.

It turns out that in the deep
subsurface

there's an abundance of methane
gas and ethane and propane.

Now, for you and I that's
not a very exciting diet,

but what we think is that
these organisms may be taking

that kind of gas and actually
using that as a food to survive.

On such an exotic diet,

the bacteria draw just enough energy

to divide and reproduce only
once every thousand years,

suggesting a way that
life could have survived

deep beneath the surface
of the early Earth.

And the Earth's crust may
not have been the only place

where life could have hidden
from the Heavy Bombardment.

Another safe haven may
have been the ocean.

Volcanic activity was
intense on the early Earth.

Chemicals from deep inside the planet

spewed into the primitive seas.

Even today, marine biologists
have discovered volcanic vents

on the ocean floor.

Despite scalding temperatures,

acid eruptions and
total lack of sunlight,

they found creatures of all
types thriving down here.

And at the bottom of the
food chain are microbes

that live on the noxious
hydrogen sulfide gas

erupting from the vents.

On early Earth,

primitive life may have
survived in similar environments.

If all of the bombardment was
occurring near the surface,

survivors would be existing in just
these kinds of hydrothermal vent
communities

where there's abundant
water and nutrients

and heat and food in the
form of chemical energy.

It has been found that organisms
collected there nowadays

are genetically akin to

some of the earliest organisms

that we think existed on the Earth.

By about three and a
half billion years ago,

or five o'clock in the
morning on our 24-hour clock,

the bombardment of asteroids
and comets had ceased.

With far fewer violent impacts on
Earth,

microbial life could now survive

outside its protective hiding places.

After it reaches Earth's surface,

life would take advantage
of another source of energy:

the sun.

Up here, microbes evolved a green
pigment known as chlorophyll.

This allowed them to trap sunlight
and use it to drive a chemical reaction

that converts carbon
dioxide and water into food.

Called "photosynthesis,"
it was a clever invention

that enabled some bacteria

to grow and reproduce
almost without limit.

Once it started, photosynthesis
was a runaway success,

and today it's how all green
plants make their living.

As Earth cooled, this new generation
of cells spread across the oceans.

Immense colonies of green
slime would take over the worid,

kicking off the greatest
transformation in our planet's history.

Photosynthesis is the
great liberator of biology.

With photosynthesis,

the energy is coming from the sun,

and life could spread, literally,
over the entire planetary surface.

And this remote corner
of Western Australia

holds clues to how that happened.

These domed structures,
called stromatolites,

are built up layer by layer over
thousands of years by tiny microbes.

These microbes may be similar to life
forms

that dominated our planet
billions of years earlier.

And in the arid hills nearby, there may
be evidence of these ancient creatures.

These rocks have remained unchanged

for three and a half billion years.

Here it's possible to walk
on the surface of early Earth.

Martin Van Kranendonk spends
months at a time in this wilderness,

studying the geology and producing
maps.

In a secret location in
these hills is what could be

one of the greatest geological
discoveries of all time.

These are the oldest
fossils in the worid,

at about three and a
half billion years old,

and they're composed of stromatolites.

And at this outcrop we can see
two different types of structures

that these creatures formed.

First are these black mats

that have wrinkly textures all through
it,

and the second are these larger domes
that form these broad structures.

The most likely way these things
formed is by the growth of microbes.

Like modern stromatolites,

these ancient structures could also
have been built by colonies of
bacteria.

And not far away are
fossilized ripple marks

which suggest they might
have grown in shallow water.

And here, you can see we've
got a smaller structure

that we call the "Mickey Mouse Ears,"

which is this beautiful
doubly branching structure.

And there is nothing
else that we can think of

which would make that except something

that was growing on
the bottom of the ocean.

So perhaps the ancient stromatolites

were formed by microbes like the ones

that build these structures today.

These big stromatolites are composed
mostly of rock at the bottom,

and the only living part of the
stromatolite is a thin layer on top.

And that thin layer on top is made up
of

microscopic blue-green
bacteria called "cyanobacteria."

Named after the blue-green
color of their cells,

these cyanobacteria use photosynthesis
to collect energy from the sun.

They secrete a sticky coating

to shield them from
ultraviolet radiation.

As tiny pieces of dust and sediment
settle on top of the sticky cells,

the bacteria migrate closer to
the surface to reach the light.

The layers of sediment build up
by about half a millimeter a year.

These structures
contain living microbes,

just as they have for
thousands of years.

The amazing thing about
these stromatolites

is that the microorganisms
which build them are so tiny.

And the structures
that you see around me,

compared to their size, are enormous.

It'd be like if humans made a
skyscraper that was a hundred
and five kilometers high

by seventy kilometers across.

These are massive structures for the
size of the organisms that make them.

Many different shapes and sizes of what
appear to be fossilized stromatolites

have been found in the rock.

It seems likely that these structures
were formed by some type of microbe

living on the early Earth,

perhaps even by the ancestors
of today's cyanobacteria.

We're looking at sort of a
cross-section
through the top of these cones.

And layers that were
laid down year after year,

and the fact that they're all
different sizes on this one surface,

shows that there was a colony of
microorganisms growing on this one
bedding plane.

And that's really fascinating
because it means that life
evolved on this planet

very early and very fast.

And it's the cyanobacteria
that would bring about

the most astounding
changes in Earth's history,

a change that could have started as
early as three and a half billion
years ago.

Over time, stromatolites
spread out across the planet.

As a byproduct of photosynthesis,

the ancient bacteria
produced a waste gas: oxygen.

The oxygen was absorbed
into the oceans at first.

There, it combined with iron
erupting from undersea volcanoes

to form iron oxide particles
that fell to the ocean floor.

Over the next several hundred million
years the planet literally rusted.

There may have been
other forces at work,

but eventually, all the
iron was turned into oxide,

building up layer after layer,

one of the most valuable
mineral deposits on Earth,

iron ore.

Located in Western Australia,

this is one of the
worid's largest iron mines.

The iron here was originally deposited
on the floor of a primordial ocean.

We are at the current
position as connected.

We will fire in 10 seconds
with a five-second count down.

Every week they excavate half
a million tons of iron ore

used to make steel for everything
from cars to skyscrapers.

In a more pristine state,

thousands of ancient layers
of iron ore are preserved

in the Karijini Gorge,
just 30 miles from the mine.

The layers exist because different
amounts of iron oxide were deposited

at different times of the year.

Cyanobacteria produced
oxygen in varying amounts

as water temperatures
changed with the seasons.

All over the worid, vast amounts of
iron ore were laid down in similar
ways.

On our day-long clock, this process
continued until one in the afternoon.

Eventually, oxygen
produced by cyanobacteria

began to build up in the atmosphere.

Slowly but surely this
transformed the planet.

Over the next eight hours or so,

tiny microbes raised the level of
oxygen

from less than one percent
to today's 21 percent.

The time was about 9 p.m.

It's amazing to contemplate,

but without cyanobacteria,

there would be no oxygen

and Earth would still be
smothered in noxious gases.

Plants, animals and humans
would have never evolved.

We're sitting here today breathing

an oxygen-rich mixture of air.

We couldn't be here
without that oxygen,

but that oxygen wasn't
present on the early Earth,

and it only became present

because of the activity of
photosynthetic organisms.

Life has made this
environment what we know.

It's allowed us to live on the surface,

it allows us to breathe,

it allows large organisms like we are

to function at very
high rates of activity.

The oxygen also helped protect life

from the sun's lethal
ultraviolet radiation,

by creating a layer of ozone
in the upper atmosphere.

One of the fascinating properties of
that is that it actually screens out

?just sort of like a
sunscreen does on your skin?

screens out this harmful radiation.

With the protection of the ozone layer,

life was able to diversify
into more complex organisms.

It took only the last
three hours of the day

for all the other life forms
on our planet to evolve.

The first multicellular life emerged

at six minutes past nine in the
evening.

Then came fish, and
insects, and reptiles.

By about 10 minutes to 11 in the
evening, dinosaurs roamed the Earth.

The first primates
appeared at 20 to midnight.

And with less than 30 seconds to go,

the first humans made their appearance.

It's only been the last 10
percent of the Earth's history

where there was life on
the surface of the Earth

that you would see with your naked eye.

So, for most of Earth's history,
life has basically been
invisible on the Earth.

Over a billion or two billion years,

the amount of oxygen that
these little creatures produced

was enough to actually change the
entire atmosphere of the planet.

Multi-cellular life that
we're most familiar with

?animals, plants, their environment?

was made possible by the
slow, toilsome task of bacteria

to oxygenate the atmosphere.

Microbes ruled the planet for
more than three billion years,

two thirds of its history.

These tiny organisms had
transformed an entire planet.

Without them, complex life, humans
included, would have never evolved.