曾被科学家们认为是荒谬的一种观点-- There is a theory that scientists once dismissed as absurd -
很久以前地球曾一度极度严寒 that long ago the Earth became so cold
冰盖遮覆了每一寸土地 every inch of it was entombed in ice.
整个地球被冻成一个巨型雪球 The whole planet became one vast snowball
这种状态持续了一千万年 and it lasted for ten million years.
地平线 雪球地球
没有比冰河时代更可怕的自然力量了 Earth has no more terrible force than an ice age.
在地球四十亿年的历史中 Only five times in its four billion year existence
遭受了五次这种灾难 has our planet suffered this kind of catastrophe.
没人能确定冰河时代何时开始和出现原因 No one is certain what started them or why they happened,
但大多数科学家都认同冰河时代的 but there is one thing about ice ages on which most scientists agree.
传统观点： The conventional view
任何一次冰河时代中，无论寒流推进得多么凶猛 is that no matter how ferocious the advance of the cold in any ice age
世界上总有一些地区很稳当地温暖 there will always be parts of the world that are safe and warm.
冰冻从两极缓慢蔓延 The freeze spreads out slowly from the poles,
但从未蔓延至地球最热的地区--热带地区 but never reaches the hottest part of the planet, the tropics.
长久以来都认为一项基本的自然法则是 It has long been assumed that this is a basic law of nature.
热带地区永远不会被冻结 The tropics can never freeze.
但这项基本的自然法则 But then this basic law of nature
无法解释六亿年前发生在纳米比亚的事情 can't explain what happened 600 million years ago in Namibia.
保罗·霍夫曼是那些挑战传统观点的地质学家中的一员 Paul Hoffman is one of those defiant geologists who disputes the conventional view.
他花了数年时间想让同事们相信地球也曾是个雪球 He has spent years trying to convince his colleagues about the snowball Earth.
作为一名科学家，我完全相信它是对的 As a scientist I was completely convinced it was right,
但我也知道 but I knew
要靠双手实现奋斗目标 I was going to have a struggle on my hands,
保罗·霍夫曼教授 哈佛大学
因为观点全新 because any idea as radical as, as this
又很难被认同，肯定会有人提出质疑 is going to be difficult to get across and there are going to be sceptics for sure.
霍夫曼和同事丹·施拉格多次到纳米比亚展开研究 The place Hoffman and his colleague, Dan Schrag, come back to study again and again is Namibia.
灾难性冰河时代的证据并不在很明显的地方 It is not an obvious place in which to seek evidence of a catastrophic ice age.
这片土地高温从不间断 It is a land of relentless heat,
几亿年来都是热带地区 one of those locations that has been in the tropics for hundreds of millions of years.
传统的自然观点说冰河时代永远不会蔓延到这里 The conventional laws of nature say an ice age could never have penetrated here,
但沉积地层中的巨砾 but lurking among the sun-baked rocks
用传统自然观点无法解释 is something that the conventional laws of nature cannot explain away.
六亿年前，这些山丘位于海底 600 million years ago these hills were at the bottom of the ocean
这些神秘的巨砾截然分布在古老的海床上 and lying on the old seabed are mysterious boulders.
地质学家把这些巨砾称为坠石 Geologists call them drop stones
坠石只能是冰川所带来 and drop stones could only have been brought here by one thing: a glacier.
要在实验室还原坠石产生过程，需要先准备冰层 To recreate drop stones in the lab first you need ice.
要等一阵才结冰 This is going to take a while.
冰层需要浮在沙质海底上的浅水中 The ice needs to float on shallow water above a sandy sea floor.
这是为了模拟 This is to simulate a glacier
冰川从陆地滑动到海面时的过程 as it flows off the land onto the surface of the sea.
冰川滑动时修整了地貌 Because it has carved through the landscape
大量的岩石碎片脱落被冰川裹挟而动 a glacier always collects a huge amount of rock debris.
冰层中被各种巨石和鹅卵石填充 The ice would then get filled up with all sorts of boulders and cobbles
丹·施拉格教授 哈佛大学
我们就照样子把一些砾石 and in this case we're just using some gravel
平铺在水箱冰层之上 which we can form on top of this layer of ice in our tank here.
重现出一块填满砾石的冰川，接着融化冰层 Having recreated a boulder-filled glacier, you need to melt the ice.
随着冰层融化，砾石从冰层中脱落下来 As the ice melts the boulders are released.
当大部分冰融化的时候 When most of the ice has melted is a point
可以看到进入海洋的冰川中所有砾石 you can see that all the rocks the glacier's carried out to the ocean
都沉到海底 are now dropped to the sea floor.
沉积层除了冰川带来的砾石 In addition to all of this,
同时沉降的 in the real world of boulders,
有大量的沙子和很多很细的颗粒沉积物 there'd also be a lot of sand and a lot of much finer grain sediment
从而砾石被很细的颗粒沉积物所包裹 and so these rocks would get filled in with very fine grain sediment as well.
经过几千年的地质变化 In a few thousand years
细颗粒沉积物粘合在一起包裹着巨砾 the fine grain sediment will cement itself together to form rocks,
形成岩层 with the boulders trapped inside
这就是坠石产生的过程 and that's how drop stones are made.
产生这一沉积层就是冰川作用 All you need is a glacier.
沉积层中存在大量坠石的唯一原因是 The message from the drop stones suggests just one thing:
热带地区的纳米比亚 Namibia in the tropics,
一个在学界人人都认为已经热了几亿年的地方 a place that everyone says has been hot for hundreds of millions of years,
一定曾经被冰覆盖过 must once have been covered in ice.
这里最有说服力的证据是 The most compelling evidence here
在极细微的层状沉积物中 is the juxtaposition of a large boulder like this
包裹着一个像这样的大砾石 enclosed within very fine and delicately layered sediment
and so it's difficult, if not impossible, to imagine any way to account for this
other than a very extensive glaciation.
但自从五十年前 But for 50 years,
首次提出热带地区曾被冰层覆盖的观点以来 since the idea that an ice age must have smothered the tropics was first proposed,
学术界认为这个观点太过激进 it has been deemed so radical
大多数科学家都认为它很荒谬 that most scientists have dismissed it as absurd.
The theory has its roots in the 1940s
when a young geologist called Brian Harland
made a series of intrepid field trips deep into the Arctic.
Conditions were very different then -
manhauling in the old Scots tradition.
It's cold but you get used to it,
you can get used to anything really.
In the Arctic Harland had ample opportunity
to learn all there was to learn about that telltale sign of glaciation - drop stones.
Glacial deposits were very dramatic,
currents very distinctive, easy to identify
and when I saw them I knew from my own experience
that this is glacial.
I didn't have to think very hard about it.
But it was when he returned from the Arctic,
armed with this new-found knowledge,
that Harland made his scientific breakthrough.
With his now expert eye,
he analysed rocks from all over the world.
What he discovered amazed him.
Almost everywhere he looked,
in rocks formed about 600 million years ago,
he found drop stones:
classic evidence that there had once been thick ice even on the hottest continents.
The drop stones were so widespread
that Harland began to contemplate the unthinkable.
The fact is we've got evidence of a global glaciation.
He knew it made no sense.
He knew that at the peak of the last Ice Age,
which ended just 10,000 years ago,
ice spread down to where New York is today,
but no further,
but the evidence was staring Harland in the face,
evidence which seemed to flout the laws of nature.
600 million years ago the drop stones said
ice had spread over every continent,
right across the tropics to the Equator itself.
It meant there must have been an ice age of unimaginable ferocity,
in fact too unimaginable.
When Harland presented his theory of global glaciation
colleagues dismissed it as laughable.
The sceptics had a whole host of reasons why Harland had to be wrong,
reasons that were utterly convincing,
as they were based on the most basic rules about how the world works.
For a start, it's given that the tropics,
that wide belt around the Earth,
has always received the full glare of the sun.
As they have always basked in sunlight,
so they must always have remained warm.
Even during the last Ice Age
geological evidence suggests the tropics were pleasantly hot
and they had another reason why the snowball theory must be wrong.
There was a far simpler explanation for drop stones in the tropics,
one that was absolutely consistent with scientific knowledge
and which didn't involve a ludicrous idea like the tropics freezing over.
Continents drift.
They scrape across the surface of the Earth
at about the same pace that your fingernails grow.
This means that in about 400 million years
continents can shift from one end of the globe to the other.
Obviously, the doubters said,
600 million years ago all the continents had drifted into icy, polar regions.
Once there, glaciers would have formed
and drop stones would be common in all rocks of this era,
and after those drop stones had been formed at the poles
the land, and the drop stones,
would have been carried back down to the tropics by continental drift.
For years this remained the accepted explanation
for why drop stones appeared everywhere.
The only way to make the theory of a global glaciation credible
was for someone to make the impossible possible.
They would need to find a way for the tropics to freeze over,
and this is just what happened next.
(RUSSIAN COUNTDOWN) OK, 5, 4, 3, 2, 1, 0!
It was the 1960s, the height of the Cold War.
The world was obsessed with calculating the odds of surviving a nuclear Holocaust.
It was known that a series of massive nuclear explosions
would create clouds of dust, smoke and soot.
Sunlight would be blocked out.
Hypothetically the Earth would enter a nuclear winter,
a man-made ice age.
In the Soviet Union finding out how severe this man-made ice age could be
became the task for one climatologist.
Mikhail Budyko was that man.
Long ago, probably 25 or 30 years,
I compiled a number of studies
which could be used to describe origin of ice ages.
What Budyko was to uncover
would fly in the face of conventional wisdom.
He would show how those predictions
that the tropics couldn't freeze over were complacent and unfounded.
Budyko knew that because the land and oceans are dark
they absorb most of the heat coming from the Sun's rays
and that is how our planet is warmed up;
but sheets of ice are white.
They reflect sunlight like a mirror,
so an ice-covered Earth absorbs far less solar heat.
During an ice age as the freeze spreads
the Earth grows whiter,
more heat is reflected away so less and less heat is absorbed
and so the Earth grows ever colder.
It means that potentially
the Earth could be caught in a vicious circle of unstoppable freezing.
Budyko converted this hypothetical scenario
into a mathematical formula
and that formula produced a terrifying prediction:
the Earth's climate has a theoretical breaking point.
As long as the ice sheets remain close to the poles the Earth is safe,
but if the freeze continues,
such as might happen in a nuclear winter,
they could advance down to about where Texas is today.
Once the freeze had reached that point
so much of the Earth would be covered in white ice
that over half the solar heat that normally warms the planet
would be reflected back into space.
At that point there wouldn't be enough heat left to warm up the Earth.
Once this happens there could, in theory, be a runaway freeze,
a freeze that nothing can stop.
Temperatures plummet,
ice sheets spread across all the continents, the oceans,
and eventually even the tropics.
If this was ever to happen the entire planet would be trapped in ice.
There would be a snowball Earth.
What was most disturbing about Budyko's calculations
was that an Earth encased in ice would reflect so much solar heat
it could never warm up enough to thaw, ever.
It was my opinion twenty years ago
that such a system will be stable for very long time and possibly all life will disappear.
This was Budyko's paradox.
He had shown that a runaway freeze could ice over the tropics in theory,
but he had also shown that it could never have happened in practice
because, if it had, the world would still be a snowball today
and that would mean we would never have existed.
For 15 years nobody could solve the paradox of the runaway freeze.
The snowball Earth theory remained a logical impossibility.
To resurrect the theory believers would have to do two things.
First, they had to prove that ice really had reached the hottest parts of the planet, the tropics,
and that the drop stones hadn't arrived there merely through continental drift.
Second, they had to come up with a theory that offered an escape from the runaway freeze.
That was when a new scientist entered the picture.
In the late 70s Joe Kirschvink was one of geology's rising stars
and back then he thought the idea of a snowball Earth was mad.
The willingness to accept
a nutty idea like freezing the whole planet
really needs to be supported by incredibly strong evidence
and overall if you had asked me to place a wager back in the late 70s
I would have been on the conservative side on that one.
Kirschvink's conservatism came about
because he believed the simple, conventional explanation
for why there were drop stones at the Equator today.
600 million years ago, he thought,
all the continents must have been up near the icy poles
when the drop stones were formed.
There was, however, only one technique for testing their theory
and it just so happened
that Kirschvink had dedicated his entire professional career to that technique.
It involved magnets and Kirschvink is a man in love with magnets.
I have wiped out my credit cards so many times by playing with strong magnets
I, I don't like to remember it.
Few people realise it, but every rock is a weak magnet.
Each one contains magnetic minerals
and the direction these minerals pull in
is fixed for ever the moment the rock is first formed
and what gives these minerals their magnetic direction
is the magnetic field round the Earth's core.
This means that rocks formed at the poles
will always have a magnetic direction that points up and down.
Rocks formed at the Equator will always have a magnetic direction
that points from side to side.
The rocks have the ability to preserve magnetic direction
because of magnetic minerals in them.
They act like little compass needles and will line themselves up.
By finding out a rock's magnetic direction
Kirschvink is able to identify where in the world it was originally formed,
no matter where continental drift might have taken it
in the millions of years that followed.
In the 1980s Kirschvink heard of a team of Australians
who had run tests which, they claimed, showed evidence of glaciers in the tropics.
Ever the sceptic, Kirschvink decided to duplicate their tests
on his own, more sophisticated equipment.
It meant examining drop stones from some of the hottest parts of the Earth.
The question was: were they already at the Equator when they were formed by the ice,
or had they been created at the poles and then carried to the tropics by continental drift?
To answer the question Kirschvink turned to his pride and joy:
a custom built, hypersensitive magnetometer.
It's, it's an incredible instrument.
You lower the sample into the actual magnetometer chamber.
The weak magnetic field of the sample generates an electric current
and so it allows us to measure the magnetic moment of an extraordinarily weak magnet,
one billionth, say, the level of an ordinary hand magnet I think.
Attached to the magnetometer was a computer
which plotted each rock's origin on an electronic outline of Earth as seen from above.
The centre of the circle represented the poles,
the circumference the Equator.
If the drop stones had been formed at the poles,
as Kirschvink and everyone else thought,
they would show up at the centre of the circle on the computer.
If they had been formed at the Equator
they would show up at the circle's edge.
The machine gave its answer:
the drop stones had been formed in the tropics.
600 million years ago there must have been a snowball Earth.
There's a fundamental theorem in science:
if the theory is in conflict with data
modify the theory.
There's so many wonderful, beautiful theories
that have been slaughtered by a nasty little fact.
But Kirschvink was racked by the insurmountable paradox of the runaway freeze,
that if a snowball Earth had ever happened
then science said we should still be entombed in ice today.
How do you get out of it?
Obviously the climate modellers had assumed
that that was an irreversible step
and that you would never get out of it
and yet we're out of it now
and if we had been in it before some point we must have gotten out of it.
To get out of the deep freeze
what Kirschvink needed was a power that would stay hot,
even when the whole planet had frozen over,
something that Budyko hadn't thought of,
something that could burn for ever,
something like hell.
Looking at an active volcano
you realise that magma tens or hundreds of kilometres below the surface
couldn't care less whether there was a thin layer of ice over the oceans.
It will still emerge.
Volcanoes survive ice ages
because they have a direct channel to the molten rock deep within the Earth,
rock that reaches temperatures of over 1,000 degrees,
but that would only melt ice in their immediate area.
Kirschvink had spotted something else about volcanoes:
they also produce gas,
ten billion tons a year.
One gas volcanoes emit in huge quantities is carbon dioxide,
a gas that causes the greenhouse effect and global warming.
Today carbon dioxide is being pumped into the atmosphere
by both volcanoes and industrial activity,
but what stops the Earth from overheating
is that we have a natural way of removing the excess carbon dioxide from the atmosphere.
Rain is the Earth's natural cleaning agent.
As it falls through the atmosphere
each droplet of rain absorbs carbon dioxide and cleans the air,
but Kirschvink realised that on a snowball Earth there could have been no rain.
The snowball was so cold
all the water on the planet's surface was frozen solid.
Without liquid water nothing could have evaporated into the air,
so there would have been no clouds
and without clouds there can be no rain,
there would have been nothing to stop the carbon dioxide from the volcanoes
from building up over millions of years.
It would have caused global warming on an inconceivable scale.
Kirschvink came across calculations showing that after ten million years without rain
the atmosphere would have been 10% carbon dioxide.
Today it is far less than 1%.
This extra carbon dioxide would have created a greenhouse effect
that raised the temperature to an average of 50 degrees Centigrade,
hotter than the Earth has ever been,
hot enough to melt the ice.
And that seemed to be a natural and possible escape,
certainly enough to break the snowball, the ice condition.
Joe Kirschvink had cracked it.
He had found the way out of the runaway freeze,
a way that made perfect scientific sense,
a way that was consistent with the laws of nature.
The realisation that we may have found the way out of the snowball was wonderful.
By 1990 Kirschvink had evidence that the tropics had frozen over for ten million years
and he'd come up with a theoretical escape route from the runaway freeze,
but the problem was it was just a theory.
He had no physical evidence to prove the ice had melted
because of an extreme greenhouse effect
and without that evidence the snowball Earth theory continued to be ignored.
In 1992 Paul Hoffman entered the story.
One of geology's most respected, but open-minded figures,
he was to turn into one of the snowball Earth theory's most fervent disciples.
I love the snowball Earth theory.
I mean it's almost a religious ferocity
with which I want to get across this, this saga of Earth history
with so many twists and turns
and, and events that, that are, are, are so far outside the, our own experience.
Hoffman's mission was to find that hard evidence that had alluded Kirschvink,
the final piece of the puzzle
that would prove the snowball had ended because of the greenhouse effect
and he knew just where to start looking: Namibia.
Hoffman was drawn to Namibia first by the drop stones,
but even more by what sat immediately on top of them.
Towering directly above were huge formations called cap carbonates,
calcium carbonate crystallised into rock.
These bizarre structures seemed to appear right out of the blue.
Above this line right here...
creates... on top no more stones whatsoever,
so the glaciation must have come to an abrupt end
and sitting directly on top is a thick pile of carbonate
and it's puzzled geologists for generations.
What really puzzled geologists was that caps made from calcium carbonate
are usually formed in warm water,
but in Namibia they had suddenly appeared on top of an ice-cold glacier.
What this indicates is that you go from glacial conditions
to an ocean that is warm in a flash.
To most geologists this instant change from cold to hot rock defied their understanding,
but Hoffman suspected it might be a huge clue,
that the sudden appearance of mountains of calcium carbonate
must somehow be linked to the death of the snowball in a colossal greenhouse effect,
but how?
Hoffman needed someone who could explain why calcium carbonate would be formed
not just because of warm conditions in general,
but specifically in the conditions created by a melting snowball Earth,
so he turned to a geochemist, Dan Schrag.
Painstakingly they analysed the snowball's theoretical final moments stage by stage,
working out what chemical and climatic processes were at work.
First they realised that as the ice melted it would produce liquid water.
The water would then evaporate and create clouds
and those clouds would cause a change in the weather
the likes of which has never been seen by human eyes.
When the ice is retreating this is probably the most spectacular climate change in our history.
In some ways we're going from the coldest climate the Earth's ever experienced
to the warmest climate the Earth's ever experienced.
We immediately said that this was the mother of our climate changes.
The most elemental powers of nature would be unleashed upon the Earth.
One would predict hurricanes of such intensity that our unimaginable,
so it's quite possibly you could have had waves of,
you know, 100 metres in, in, in amplitude coming in and crashing in on the shoreline,
so it would just be unbelievably violent.
And above all there would be rain,
the first rain storms for millions of years,
rainstorms that would last at least a century
and it wouldn't be just any rain.
The rainwater would react with the vast quantities
of carbon dioxide in the atmosphere and form carbonic acid.
This acid rain would then deluge the Earth.
The acid rain would pound the exposed rocks,
producing a violent chemical reaction.
It would break the rocks down into their constituent parts,
one of which was calcium.
This would then fuse with the carbon in the acid rain.
The result: mountains of calcium carbonate
exactly as they could see right above the drop stones in Namibia.
Suddenly it became clear that the natural expectation of a prolonged global glaciation
ending in extremely high levels of carbon dioxide
was that you would expect these very unusual thick carbonate rocks
should immediately follow the glacial deposits.
Hoffman and Schrag had found the final evidence.
The beauty of the snowball Earth theory is exactly
that it links the chemistry, the geology, the planetary science and bam-bam-bam,
all of the facts are, are consistent with the predictions of the theory.
In 1998 Hoffman and Schrag made a triumphal tour of the world's universities
promoting the snowball theory,
and then someone threw a spanner in the works.
An entire scientific discipline, the biologists,
took one look at the ten million year long snowball Earth
and said it was quite simply impossible.
One scientist who had problems with the snowball was Guy Narbonne.
He, like many other biologists,
could see there was a fundamental problem
reconciling the snowball Earth theory with what he knew.
The geological evidence speaks very strongly
of ice sheets marching on the Equator.
The biological evidence speaks very strongly
of open water being available at this whole time.
Narbonne's problem was another set of nature's laws,
nothing less than the laws that govern life itself.
600 million years ago,
when the snowball was said to have struck,
there was nothing living on the land.
It all existed in the sea,
and it was not life as we know it.
The most common organism on the planet was this:
cyano-bacteria.
This mass of fossilised cyano-bacteria is so ancient it predates even the snowball.
The oldest fossils that we have in the world are stromatalytes
and these are colonies like you see here
and they're made out of literally billions of cyano-bacteria.
What made cyano-bacteria and more sophisticated ancient organisms like green algae
central to the snowball debate was that they photosynthesised.
Just like plants today under the water
these life forms absorbed sunlight and converted it into energy,
so these species' chances of surviving the snowball
would have depended on whether there was enough sunlight
for photosynthesis in the snowball seas
at a time when every inch of the planet was allegedly covered in thick ice.
Light penetration through ice is not great
if the ice is more than a couple of tens of metres thick.
And that was the problem biologists pointed to in Hoffman and Schrag's work.
Their calculations had reckoned on the snowball producing ice sheets up to a kilometre thick
and even in the warmest places around the Equator
they would have been tens of metres thick,
more than enough, biologists believe,
to block out any sunlight from the oceans
and that meant death to most things that depended on photosynthesis for life.
You cut off photosynthesis
and within a few years, let alone a few million years, mass extinction.
It meant that many of the photosynthesising species
should have become extinct 600 million years ago
and the trouble was they hadn't.
Cyano-bacteria grow in rock-like colonies all over the tropics today,
as do other photosynthetic plants
dating from before the snowball like green algae.
For many biologists there seemed to be only one possible conclusion:
there may have been a severe ice age,
but not a fully fledged snowball that covered all the oceans
because life forms that should have died out are still with us.
Unless someone could find a way for marine life to survive
under a thick sheet of ice
the whole theory would be back in the bin.
Then along came the man from NASA, Chris McKay.
I'm a planetary scientist with an interest in life in extreme environments,
in particular environments that are cold and dry.
Several years ago I saw the papers about the snowball Earth
indicating the geological evidence that the Earth was ice-covered
and had been ice-covered for millions of years,
plus the evidence that there were organisms,
bacteria and green algae, that survived this ice-covered state
and that's the puzzle: how could these photosynthetic organisms survive?
McKay was intrigued because it just so happened
he had visited a place on Earth today that was so cold
it could actually be compared with the snowball of 600 million years ago:
Antarctica,
the coldest place on Earth.
Here temperatures can fall to -30,
almost as severe as the snowball at its warmest point (the Equator)
and deep in Antarctica are the dry valleys
where there are lakes smothered by an ice sheet many metres thick.
We think these little lakes represent a good model
for looking at the snowball Earth -
could there by life underneath the ice cover on a snowball Earth?
Scientists were eager to discover
if there was enough light for life to survive in the water below the thick ice.
They were about to be astonished.
There was light, lots of it,
even though overhead the ice was five metres thick.
There was so much light the water was filled with plant life and bacteria
including two organisms dating from the time of the snowball
that would have needed sunlight to live.
When we're diving in the lakes and swimming around there's life in the water,
there's bacteria, cyano-bacteria, but there's also green algae.
And McKay had a hunch as to why this photosynthetic life could survive.
It was something about the nature of the ice itself.
Transparency of this ice is very high
and it's due to the fact that it's freezing very slowly underneath this thick ice cover.
Now let's imagine the snowball Earth where there's also a thick ice cover.
The ice on the bottom is freezing very slowly as well,
yet you have the same optical properties as this clear, clean ice.
By freezing slowly, water rejects impurities such as salt or dirt which make it cloudy,
so the ice becomes clean and transparent.
Because of this transparency
life-giving sunlight would have been reaching down into the snowball seas.
McKay realised that even at its height
there would have been havens around the snowball's Equator
where the ice was thin enough for photosynthetic life to cling on below.
But there was more.
Although disciples of the theory accept the snowball would have been death to most things,
they argue whatever life did survive
would have emerged into a new world almost devoid of competition,
the perfect conditions for an explosion of evolutionary change.
These fossils show the new species
which appeared soon after the time of the snowball.
All display a massive increase in size and complexity
from the simple organisms that lived before.
These are the first multi-cellular life forms.
Even those with some questions about the snowball theory
recognise a new climate after a mass extinction could be a spur for evolution.
But if the snowball can change the course of life on the planet
there remains one final question.
No one knows why the snowball happened
or if it could happen again.
The sun is hotter than it was 600 million years ago
and is getting hotter all the time.
Climate trends would have to change dramatically.
If it ever did happen again
it almost certainly wouldn't be for millions of years,
but if it ever did
that would mean death to almost everything,
including us.
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