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Narrator: There is a theory that scientists once dismissed as absurd:

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that long ago the earth became so cold, every inch of it was entombed in ice.

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The whole planet became one vast snowball,

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and it lasted for 10 million years.

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[Theme Music]

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Earth has no more terrible force than an ice age.

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Only 5 times in its 4 billion year existance

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has our planet suffered this kind of catastrophe.

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No one is certain what started them, or why they happened,

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but there is one thing about ice ages on which most scientists agree:

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the conventional view is that no matter how ferocious the advance of the cold in any ice age,

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there will always be parts of the world that are safe and warm.

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The freeze spreads out slowly from the poles,

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but never reaches the hottest part of the planet, the tropics.

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It has long been assumed that this is a basic law of nature.

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The tropics can never freeze.

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But then, this basic law of nature can't explain what happened 600 million years ago in Namibia.

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[Jazz Music]

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Paul Hoffman is one of those defiant geologists who disputes the conventional view.

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He has spent years trying to convince his colleagues about the snowball earth.

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As a scientist, I was completely convinced it was right.

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But I knew I was going to have a struggle on my hands.

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Because any idea as radical as this is going to be difficult to get across,

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and there are going to be skeptics for sure.

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Narrator: The place Hoffman and his colleague, Dan Schrag,

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come back to study again and again is Namibia.

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It is not an obvious place in which to seek evidence of a catastrophic ice age,

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it is a land of relentless heat.

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One of those locations that has been in the tropics for hundreds of millions of years.

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The conventional laws of nature say an ice age could never have penetrated here.

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But lurking among these sun-baked rocks is something that the conventional laws of nature

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cannot explain away.

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600 million years ago, these hills were at the bottom of the ocean,

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and lying on the old sea bed are mysterious boulders.

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[Eerie Music]

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Geologists call them drop stones,

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and drop stones could only have been brought here by one thing: a glacier.

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To recreate drop stones in the lab, first you need ice.

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This is going to take a while.

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Narrator: The ice needs to float on shallow water above a sandy sea floor.

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This is to simulate a glacier as it flows off the land, onto the surface of the sea.

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Because it is carved through the landscape, a glacier always collects a huge amount of rock debris.

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The ice would then get filled up with all sorts of boulders and cobbles,

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and in this case, we're just using some gravel,

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which we can pour on top of this layer of ice in our tank here.

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Narrator: Having recreated a boulder-filled glacier, you need to melt the ice.

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[Crashing sound]

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As the ice melts, the boulders are released.

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While most of the ice has melted at this point,

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you can see that all the rocks that the glaciers carried out to the ocean

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are now dropped to the sea floor.

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In addition to all of this--in the real world--boulders,

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there would also be a lot of sand, and a lot of much finer gained sediment.

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And so these rocks would get filled in with very fine-grained sediment as well.

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Narrator: In a few thousand years, the fine-grained sediment will cement itself together to form rock.

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With the boulders trapped inside.

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And that's how drop stones are made.

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All you need is a glacier.

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The message from the drop stones suggests just one thing:

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Namibia, in the tropics, a place that everyone says has been hot for hundreds of millions of years,

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must once have been covered in ice.

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The most compelling evidence here is the juxtaposition of a large boulder like this

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enclosed within very fine and delicately layered sediment.

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And so it's difficult, if not impossible to imagine any way to account for this

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other than a very extensive glaciation.

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But for 50 years, since the idea that an ice age must have smothered the tropics was first proposed,

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it has been deemed so radical that most scientists have dismissed it as absurd.

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The theory has its roots in the 1940s, when a young geologist called Brian Harland,

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made a series of intrepid field trips deep into the arctic.

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Conditions were very different then.

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Man hauling in the old Scotch tradition.

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It was cold, but, um, you get used to it.

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You can get used to anything, really.

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Narrator: In the arctic, Harland had ample opportunity to learn all there was to learn

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about that tell-tale sign of glaciation, drop stones.

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Glacier deposits were a very dramatic occurrence, very distinctive, easy to identify.

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And when I saw them, I knew from my own experience that this is glacial.

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I didn't have to think very hard about it.

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Narrator: But it was when he returned from the arctic, armed with his new-found knowledge,

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that Harland made his scientific breakthrough.

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With his now expert eye, he analyzed rocks from all over the world.

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What he discovered amazed him.

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Almost everywhere he looked, in rocks formed about 600 million years ago, he found drop stones.

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Classic evidence that there had once been thick ice, even on the hottest continents.

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The drop stones were so wide-spread that Harland began to contemplate the unthinkable.

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The fact is that we've got evidence of a global glaciation.

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Narrator: He knew it made no sense.

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He knew that at the peak of the last ice age, which ended just 10,000 years ago,

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ice spread down to where New York is today, but no further.

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But the evidence was staring Harland in the face.

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Evidence which seemed to flout the laws of nature.

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600 million years ago, the drop stones said, ice had spread over every continent.

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Right across the tropics, to the equator itself.

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It meant there must have been an ice age of unimaginable ferocity.

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In fact, too unimaginable.

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When Harland presented his theory of global glaciation, colleagues dismissed it as laughable.

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The skeptics had a whole host of reasons why Harland had to be wrong.

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Reasons that were utterly convincing,

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as they were based on the most basic rules about how the world works.

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For a start, it's a given that the tropics, that wide belt around the earth,

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has always received the full glare of the sun.

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As they have always basked in sunlight, so they must always have remained warm.

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Even during the last ice age, geological evidence suggests the tropics were pleasantly hot.

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And they had another reason why the snowball theory must be wrong.

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There was a far simpler explanation for drop stones in the tropics,

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one that was absolutely consistent with scientific knowledge,

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and which didn't involve a ludicrous idea like the tropics freezing over.

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Continents drift.

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They scrape across the surface of the earth at about the same pace that your fingernails grow.

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This means that in about 400 million years, continents can shift from one end of the globe to the other.

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Obviously, the doubters said, 600 million years ago all the continents had drifted into icy polar regions.

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Once there, glaciers would have formed, and drop stones would be common in all rocks of this area.

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And after those drop stones had been formed at the poles,

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the land and the drop stones would have been carried back down to the tropics by continental drift.

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For years, this remained the accepted explanation for why drop stones appeared everywhere.

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The only way to make the theory of a global glaciation credible

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was for someone to make the impossible possible.

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They would need to find a way for the tropics to freeze over.

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And this is just what happened next.

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[Counting in German]

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[Explosion]

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It was the 1960s, the height of the cold war.

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The world was obsessed with calculating the odds of surviving a nuclear holocaust.

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It was known that a series of massive nuclear explosions

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would create clouds of dust, smoke and soot.

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Sunlight would be blocked out.

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Hypothetically, the earth would enter a nuclear winter.

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A man-made ice age.

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In the Soviet Union, finding out how severe this man-made ice age could be

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became the task for one climatologist.

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Mikhail Budyko was that man.

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Long ago, probably 25 or 30 years,

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I compiled a number of studies which could be used to describe origin of ice ages.

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Narrator: What Budyko was to uncover would fly in the face of conventional wisdom.

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He would show how those predictions, that the tropics couldn't freeze over,

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were complacent and unfounded.

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Budyko knew that because the land and oceans are dark,

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they absorb most of the heat coming from our sun's rays,

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and that is how our planet is warmed up.

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But sheets of ice are white.

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They reflect sunlight like a mirror.

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So an ice-covered earth absorbs far less solar heat.

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During an ice age, as the freeze spreads, the earth grows whiter.

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More heat is reflected away, so less and less heat is absorbed.

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And so the earth grows ever colder.

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It means that potentially, the Earth could be

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caught in an vicious circle of unstoppable freezing.

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Budyko converted this hypothetical scenario into a mathematical formula,

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and that formula produced a terrifying prediction.

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The Earth's climate has a theoretical breaking point.

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[Dramatic Music]

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Narrator: As long as the ice sheets remain close to the poles,

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the Earth is safe but if the freeze continues,

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such as might happen in an nuclear winter,

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they could advance down to about where Texas is today.

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Once the freeze had reached that point,

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so much of the Earth would be covered in white ice

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but over half the solar heat that normally warms the planet

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would be reflected back into space.

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At that point, there wouldn't be enough heat left to warm up the Earth.

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Once this happens, there could in theory, be a runaway freeze.

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A freeze that nothing could stop.

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Temperatures plummet, ice sheets spread across all the continents,

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the oceans and eventually, even the tropics.

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If this was ever to happen, the entire planet would be trapped in ice.

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There would be a snowball Earth.

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What was most disturbing about Budyko's calculations

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was that an Earth encased in ice would reflect so much solar heat,

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it could never warm up enough to thaw, ever.

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It was one billion and twenty years ago that the system

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will be stable for a very long time and possibly,

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all life will disappear.

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Narrator: This was Budyko's paradox.

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He had shown that a runaway freeze could ice over the tropics in theory,

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but he had also shown that it could never have happened in practice

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because if it had, the world would still be a snowball today

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and that would mean we would never have existed.

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For 15 years, nobody could solve the paradox of the runaway freeze.

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The snowball Earth theory remained a logical impossibility.

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To resurrect the theory, believers would have to do two things.

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First, they had to prove that ice really had reached

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the hottest parts of the planet, the tropics and that the

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dropstones hadn't arrived there merely through continental drift.

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Second, they had to come up with a theory that offered

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an escape from the runaway freeze.

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[Music]

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That was when a new scientist had entered the picture.

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In the late 70s, Joe Kirschvink was one of geology's rising stars

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and back then, he thought the idea of a snowball Earth was mad.

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The willingness to accept a nutty idea like freezing the whole planet

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really needs to be supported by incredibly strong evidence

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and overall, if you had asked me to place a wager in the late 70s,

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I would've been on the conservative side on that one.

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Narrator: Kirschvink's conservatism came about because he believed

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the simple conventionalist explanation of why there were dropstones at the equator today.

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600 million years ago, he thought, all the continents must've been

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up near the icy poles when the dropstones were formed.

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There was however only one technique to test this theory,

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and it just so happened that Kirschvink had dedicated

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his entire professional career to that technique.

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It involved magnets and Kirschvink is a man in love with magnets.

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I have wiped out my credit cards so many times by playing with strong magnets,

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I don't like to remember it.

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Narrator: Few people realize it but every rock is a weak magnet.

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Each one contain magnetic minerals and the direction

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these minerals pull in is fixed forever the moment the rock is first formed and

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what gives these minerals their magnetic direction is the magnetic field around the Earth's core.

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This means that rocks formed at the poles will always have a

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magnetic direction that points up and down.

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Rocks formed at the equator will always have a

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magnetic direction that points from side to side.

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The rocks have the ability to preserve magnetic direction

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because of magnetic minerals in them.

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They act like little compass needles and will line themselves up.

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Narrator: By finding a rock's magnetic direction, Kirschvink is able to

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identify where in the world it was originally formed,

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no matter where continental drift might've taken it in the millions of years that followed.

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In the 1980s, Kirschvink heard of a team of Australians who had run

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tests which they claimed showed evidence of glaciers in the tropics.

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Ever the skeptic, Kirschvink decided to duplicate their tests on his own, more sophisticated equipment.

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It meant examining dropstones from some of the hottest parts of Earth.

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The question was were they already at the equator when they were

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formed by the ice or had they been created at the poles

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and then carried to the tropics by continental drift?

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To answer the question, Kirschvink turned to his pride and joy:

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a custom built, hyper-sensitive magnetometer.

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It's an incredible instrument.

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You lower the sample into the actual magnetometer chamber.

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The weak magnetic field of the sample generates an electric current

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and so it allows us to measure the magnetic moment of an extraordinarily weak magnet.

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One billionth say the level of a ordinary hand magnet type thing.

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Narrator: Attached to the magnetometer was a computer which plotted

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each rock's origin on an electronic outline of Earth as seen from above.

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The center of the circle represented the poles, the circumference, the equator.

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If the dropstones had been formed at the poles, as Kirschvink and everyone else thought,

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they would show up at the center of the circle on the computer.

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If they had been formed at the equator, they would show up at the circle's edge.

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The machine gave its answer.

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The dropstones had been formed in the tropics.

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600 million years ago, there must have been a snowball Earth.

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Here's a fundamental theorem in science.

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If a theory is in conflict with data, modify the theory.

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There's so many wonderful, beautiful theories that have been slaughtered by a nasty little fact.

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Narrator: But Kirschvink was racked by the insurmountable paradox of the runaway freeze.

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That if a snowball Earth had ever happened,

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then science said that we should still be entombed in ice today.

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How do you get out of it?

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Obviously, the climate modelers had assumed that that was

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a irreversible step and that you would never get out of it

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and yet, we're out of it now and if we'd been in it before,

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at some point, we must've gotten out of it.

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Narrator: To get out of the deep freeze, what Kirschvink needed was

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a power that would stay hot, even when the whole planet had frozen over.

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Something that Budyko hadn't thought of, something that could burn forever, something like hell.

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Looking at an active volcano, realize that magma tens or hundreds of kilometers

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below the surface couldn't care less whether there was a thin layer of ice over the oceans.

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It would still emerge.

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Narrator: Volcanos survive ice ages because they have a direct channel

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to the molten rock deep within the Earth,

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rock that reaches temperatures of over a thousand degrees but that would

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only melt ice in their immediate area.

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Kirschvink has spotted something else about volcanos.

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They also produce gas 10 billion times a year.

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One gas volcanos emit in huge quantities is carbon dioxide,

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a gas that causes the greenhouse effect and global warming.

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Today, carbon dioxide is being pumped into the atmosphere by both volcanos and industrial activities,

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but what stops the Earth from overheating is that we have a natural way

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of removing the excess carbon dioxide from the atmosphere.

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Rain is the Earth's natural cleaning agent.

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As it falls through the atmosphere, each droplet of rain

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absorbs carbon dioxide and cleans the air but Kirschvink realized

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that on a snowball Earth, there could've been no rain.

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The snowball was so cold, all the water on the planet's surface was frozen solid.

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Without liquid water, nothing could've evaporated into the air

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so there would've been no clouds and without clouds,

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there can be no rain.

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There would've been nothing to stop the carbon dioxide from the volcanos

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from building up over millions of years.

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It would've caused global warming on an inconceivable scale.

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Kirschvink came across calculations showing that after 10 million years without rain,

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the atmosphere would've been 10% carbon dioxide.

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Today, it's far less than 1%.

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This extra carbon dioxide would've created a greenhouse effect

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that raised the temperature to an average of 50 degrees Centigrade,

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hotter than the Earth has ever been.

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Hot enough to melt the ice.

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Kirschvink: And that seemed to be a natural and plausible escape.

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Certainly enough to break the snowball or the ice condition.

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Narrator: Joe Kirschvink had cracked it.

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He had found the way out of the runaway freeze,

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a way that made perfect scientific sense.

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A way that was consistent with the laws of nature.

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Kirschvink: The realization that we may have found

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the way out of the snowball, it was wonderful.

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Narrator: By 1990, Kirschvink had evidence that the tropics

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had frozen over for 10 million years and he had

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come up with a theoretical escape route from the runaway freeze,

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but the problem was it was just a theory.

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He had no physical evidence to prove the ice had melted

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because of an extreme greenhouse effect and without that evidence,

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the snowball Earth theory continued to be ignored.

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[Music]

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In 1992, Paul Hoffman entered the story.

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One of geology's most respected but open-minded figures.

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He was to turn into one of the snowball Earth

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theory's most fervent disciples.

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I loved the snowball Earth theory.

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I mean it's almost a religious ferocity with which

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I want to get across this saga of Earth history with

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so many twists and turns and events that are

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so far outside our own experience.

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Narrator: Hoffman's mission was to find that

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hard evidence that had eluded Kirschvink.

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The final piece of the puzzle that would

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prove the snowball had ended because of the greenhouse effect

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and he knew just where to start looking.

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Namibia.

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Hoffman was drawn to Namibia first by the dropstones

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but even more by what sat immediately on top of them.

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Towering directly above were huge formations called cap carbonates.

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Calcium carbonate crystalized into rock.

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These bizarre structures seemed to appear right out of the blue.

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Above this line right here, ??, no more stones whatsoever

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so the glaciation must've come to an abrupt end

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and sitting directly on top is a thick pile of carbonate

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and it's puzzled geologists for generations.

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[Dramatic Music]

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Narrator: What really puzzled geologists was that caps made

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from calcium carbonate are usually formed in warm water,

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but in Namibia, they had suddenly appeared on top

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of an ice cold glacier.

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What this indicates is that you go from glacial conditions

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to an ocean that is warm in a flash.

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Narrator: To most geologists, this instant change from

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cold to hot defied their understanding

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but Hoffman suspected it might be a huge clue.

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That the sudden appearance of mountains of

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calcium carbonate must somehow be connected to

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the death of the snowball in a colossal greenhouse effect.

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But how?

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Hoffman needed someone to explain why

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calcium carbonate would be formed not just because

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of warm conditions in general but specifically in the

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conditions created by a melting snowball Earth.

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So he turned to a geochemist, Dan Schrag.

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Painstakingly, they analyze the snowball's theoretical

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final moments, stage by stage,

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working out what chemical and climatic processes were at work.

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[Music]

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First, they realized that as the ice melted,

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it would produce liquid water.

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The water would then evaporate, and create clouds

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and those clouds would cause a change in the weather,

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the likes of which has never been seen by human eyes.

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Daniel Schrag: When the ice is retreating,

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this is probably the most spectacular climate change

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in Earth's history.

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In some ways, we're going from the coldest climate

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the Earth's ever experienced to the warmest climate

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the Earth's ever experienced.

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We immediately say that this was the mother of all climate changes.

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Narrator: The most elemental powers of nature

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would be unleashed upon the Earth.

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One would predict hurricanes of such intensity that are unimaginable

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so it's quite possible you could've had waves of

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100 meters in amplitude coming in and crashing

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in on the shore.

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It would be unbelievably violent.

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Narrator: And above all, there would be rain.

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The first rainstorms for millions of years.

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Rainstorms that would last at least a century

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and it wouldn't be just any rain.

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The rainwater would react with the vast quantites

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of carbon dioxide in the atmosphere and form carbonic acid.

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This acid rain would then deluge the Earth.

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The acid rain would pound the exposed rocks,

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producing a violent chemical reaction.

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It would break the rocks down into their constituent parts,

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one of which was calcium.

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This would then fuse with the carbon in the acid rain.

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The result, mountains of calcium carbonate,

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exactly as they could see right above the dropstones in Namibia.

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Suddenly, it became clear that the natural expectation

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of a prolonged global glaciation ending in extremely

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high levels of carbon dioxide was that you would expect

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these very unusual thick carbonate rocks should

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immediately follow the glacial deposits.

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[Music]

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Narrator: Hoffman and Schrag had found the final evidence.

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Schrag: The beauty of the snowball Earth theory is exactly that

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it links the chemistry, the geology, the planetary science

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and bam, bam, bam, all of the facts

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are consistent with the predictions of the theory.

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[Jazz Music]

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Narrator: In 1998, Hoffman and Schrag made an

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triumphal tour of the world's universities,

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promoting the snowball theory,

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and then someone threw a spanner in the works.

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An entire scientific discipline, the biologists took one look

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at the 10 million year long snowball Earth and said

401
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it was quite simply impossible.

402
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One scientist who had problems with the snowball was Guy Narbonne.

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He like many other biologists could see there was a

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fundamental problem reconciling the snowball Earth theory

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with what he knew.

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The geological evidence speaks very strongly

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of ice sheets marching on the equator.

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The biological evidence speaks very strongly of open water

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being available at this whole time.

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Narrator: Narbonne's problem was another set of nature's law.

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Nothing less than the laws that governed life itself.

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600 million years ago, when the snowball was set to have struck,

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there was nothing living on the land.

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It all existed in the sea and it was not life as we know it.

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The most common organism on the planet was this.

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Cyanobacteria.

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This mass of cyanobacteria is so ancient,

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it predates even the snowball.

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The oldest fossils that we have in the world

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are stromatolites and these are colonies like you see here.

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They're made out of literally billions of cyanobacteria.

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Narrator: What made cyanobacteria and more sophistocated

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ancient organisms like green algae essential to the snowball

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debate was that they photosynthesize.

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Just like plants today under the water, these lifeforms

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absorb sunlight and convert it into energy.

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So these species' chances of surviving the snowball

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would've depended on whether there was enough

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sunlight for photosynthesis in the snowball seas

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at a time when every inch of the planet was

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allegedly covered in thick ice.

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Light penetration through ice is not great if the ice

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is more than a couple tens of meters thick.

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Narrator: And that was the problem biologists pointed

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to in Hoffman and Schrag's work.

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Their calculations had reckoned on the snowball

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producing ice sheets up to a kilometer thick,

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and even in the warmest places around the equator,

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they would've been tens of meters thick.

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More than enough, biologists believe, to block out

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any sunlight from the oceans and that meant death

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to most things that depended on photosynthesis for life.

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Narbonne: You cut off photosynthesis and within a few years,

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let alone a few million years, mass extinction.

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Narrator: It meant that many of the photosynthesizing

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species should've become extinct 600 million years ago

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and the trouble was they hadn't.

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Cyanobacteria grow in rock-like colonies all over the tropics today

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as do other photosynthetic plants, dating from before

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the snowball like green algae.

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For many biologists, there seemed to be only one possible conclusion:

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there may have been a severe ice age but not a fully

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fledged snowball that covered all the oceans

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because lifeforms that should've died out are still with us.

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Unless someone could find a way for marine life

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to survive under a thick sheet of ice,

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the whole theory would be back in the bin.

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[Music]

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Then along came the man from NASA, Chris McKay.

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Chris McKay: I'm planetary scientist with an interest

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in life in extreme environments.

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In particular, environments that are cold and dry.

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Several years ago, I saw the papers about the snowball Earth,

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indicating the geological evidence that the Earth was ice covered

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and had been ice covered for millions of years.

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Plus the evidence that there are organisms (bacteria and green algae)

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that survived this ice covered state and that's the puzzle.

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How could these photosynthetic organisms survive?

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Narrator: McKay was intrigued because it just so happened

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he had visited a place on Earth today that was so cold,

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it could actually be compared with the snowball of 600 million years ago.

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Antarctica, the coldest place on Earth.

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Here, temperatures can fall to -30.

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Almost as severe as the snowball at its warmest point, the equator

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and deep in Antarctica are the dry valleys where there are lakes

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smothered by an ice sheet many meters thick.

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McKay: We think these little lakes represent a good

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model for looking at the snowball Earth.

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Could there be life underneath the ice cover on a snowball Earth?

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Narrator: Scientists were eager to discover if there was

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enough light for life to survive in the water below the thick ice.

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They were about to be astonished.

483
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[Music]

484
99:59:59,999 --> 99:59:59,999
There was light, lots of it

485
99:59:59,999 --> 99:59:59,999
even though over head,

486
99:59:59,999 --> 99:59:59,999
the ice was five meters thick.

487
99:59:59,999 --> 99:59:59,999
There was so much light,

488
99:59:59,999 --> 99:59:59,999
the water was filled with plant life and bacteria,

489
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including two organisms dating from the time

490
99:59:59,999 --> 99:59:59,999
of the snowball that would've needed sunlight to live.

491
99:59:59,999 --> 99:59:59,999
McKay: When we're diving in the lakes

492
99:59:59,999 --> 99:59:59,999
and swimming around, there's life in the water.

493
99:59:59,999 --> 99:59:59,999
There's bacteria, cyanobacteria

494
99:59:59,999 --> 99:59:59,999
but there's also green algae.

495
99:59:59,999 --> 99:59:59,999
Narrator: And McKay had a hunch as to why this

496
99:59:59,999 --> 99:59:59,999
photosynthetic life could survive.

497
99:59:59,999 --> 99:59:59,999
It was something about the nature

498
99:59:59,999 --> 99:59:59,999
of the ice itself.

499
99:59:59,999 --> 99:59:59,999
McKay: The transparency of this ice is very high

500
99:59:59,999 --> 99:59:59,999
and it's due to the fact that it's freezing very slowly

501
99:59:59,999 --> 99:59:59,999
underneath this thick ice cover.

502
99:59:59,999 --> 99:59:59,999
Now let's imagine this snowball Earth where

503
99:59:59,999 --> 99:59:59,999
there's also a thick ice cover,

504
99:59:59,999 --> 99:59:59,999
the ice on the bottom is freezing very slowly as well,

505
99:59:59,999 --> 99:59:59,999
it should have the same optimal properties

506
99:59:59,999 --> 99:59:59,999
as this clear, clean ice.

507
99:59:59,999 --> 99:59:59,999
Narrator: By freezing slowly, water rejects impurities

508
99:59:59,999 --> 99:59:59,999
such as salt or dirt which make it cloudy.

509
99:59:59,999 --> 99:59:59,999
So the ice becomes clean and transparent.

510
99:59:59,999 --> 99:59:59,999
Because of this transparency,

511
99:59:59,999 --> 99:59:59,999
life giving sunlight would've been reaching down

512
99:59:59,999 --> 99:59:59,999
into the snowball seas.

513
99:59:59,999 --> 99:59:59,999
McKay realized that even at its height,

514
99:59:59,999 --> 99:59:59,999
there would've been havens around the snowball's

515
99:59:59,999 --> 99:59:59,999
equator where the ice was thin enough

516
99:59:59,999 --> 99:59:59,999
for photosynthetic life to cling on below but there was more.

517
99:59:59,999 --> 99:59:59,999
Although disciples of the theory accept

518
99:59:59,999 --> 99:59:59,999
the snowball would've been death to most things,

519
99:59:59,999 --> 99:59:59,999
they argue whatever life did survive

520
99:59:59,999 --> 99:59:59,999
would've emerged into a new world almost

521
99:59:59,999 --> 99:59:59,999
devoid of competition.

522
99:59:59,999 --> 99:59:59,999
The perfect conditions for an explosion of evolutionary change.

523
99:59:59,999 --> 99:59:59,999
[Music]

524
99:59:59,999 --> 99:59:59,999
These fossils show the new species which appeared

525
99:59:59,999 --> 99:59:59,999
soon after the time of the snowball.

526
99:59:59,999 --> 99:59:59,999
All display a massive increase in size and complexity

527
99:59:59,999 --> 99:59:59,999
from the simple organisms that lived before.

528
99:59:59,999 --> 99:59:59,999
These are the first multi-cellular lifeforms.

529
99:59:59,999 --> 99:59:59,999
Even those with some questions about the snowball theory

530
99:59:59,999 --> 99:59:59,999
recognize a new climate after a mass extinction

531
99:59:59,999 --> 99:59:59,999
could be a spur for evolution.

532
99:59:59,999 --> 99:59:59,999
[Music]

533
99:59:59,999 --> 99:59:59,999
But if the snowball can change the course of life on the planet,

534
99:59:59,999 --> 99:59:59,999
there remains one final question.

535
99:59:59,999 --> 99:59:59,999
No one knows why the snowball happened

536
99:59:59,999 --> 99:59:59,999
or if it could happen again.

537
99:59:59,999 --> 99:59:59,999
The sun is hotter than it was 600 million years ago

538
99:59:59,999 --> 99:59:59,999
and is getting hotter all the time.

539
99:59:59,999 --> 99:59:59,999
Climate trends would have to change dramatically.

540
99:59:59,999 --> 99:59:59,999
If it ever did happen again,

541
99:59:59,999 --> 99:59:59,999
it almost certainly wouldn't be for millions of years

542
99:59:59,999 --> 99:59:59,999
but if it ever did, that would mean death to almost everything

543
99:59:59,999 --> 99:59:59,999
including us.

544
99:59:59,999 --> 99:59:59,999
[Music]