Transcript for: Randomness and Chance in Earth's History
When you think about the
long history of life on Earth, you might sort of think, "Well,
it's been this progression almost as though it had a
direction, almost had a purpose." Not at all. It's an incredible series of accidents that's given us the world we know today- and this is really a deep
philosophical rub for humanity. You know, for millennia, philosophers and theologians have sort
of asked the question, "Does everything happen for a reason or do some things happen by chance?" And I would really say it's only about the last 60 years or so that
scientists would be saying "Oh my goodness, it's a remarkable series of events that were
required for us to be here and that so many things
could have happened in a different way that we
wouldn't be here at all, both individually for sure
and certainly as a species." I'm Sean B. Carroll. I'm a biologist, an author
and a film producer, and my most recent book is "A
Series of Fortunate Events: Chance and the Making of
the Planet, Life and You." When you think about the history of Earth, if you visited this planet for the first four billion years of its existence, you wouldn't necessarily be
that impressed with its life; it was largely unicellular
for that time period. Only in the last half billion years has life gotten macroscopic. In the book I wrote, "A
Series of Fortunate Events," I almost decided it's
like a new origin story. I'm gonna forget the first almost four and a half
billion years of life on Earth and start with the asteroid
impact 66 million years ago because that was such a
reset for life on the planet; that is really the catalyst to the making of the world we know now, and it was the eraser for
the world that preceded it. If you give me a chance
here to riff for a second? - Yeah, let's do it. - So there's really two big collisions that are a major reason
why we're here today. The first was the asteroid
impact 66 million years ago. So imagine this, a rock
six-miles wide happens to enter the atmosphere and
slam into the Yucatan Peninsula- sort of like a BB hitting
the side of a barn but it turns out one of
the important explanations is that that BB was going
50,000 miles an hour. So it drilled an enormous hole 120-miles wide in the
Yucatan, which blasted massive amounts of material
into the atmosphere and beyond. And when that material rained back down, it was like raining trillions of red hot meteors back upon
the surface of the Earth, and it was probably the
temperature of a baking oven. It set off wildfires and the devastation to the plant community
and then all the soot and all of the impact debris
that was in the atmosphere blocked out the Sun for probably a period of at least say one to three decades. It was hell on Earth, only
a few things squeaked by. Things that burrowed, things
that were semi-aquatic and smaller things because
smaller things are dependent upon less food and they also
have a rapid reproductive rate. Nothing really over about 25 kilograms in size survived on land. What implications does
that have for humanity and for the other creatures on Earth? Well once you sort of
took out the big reptiles, that created a lot of
opportunity for mammals and of course we're mammals
and we evolved from primates. But it's pretty easy to imagine how the asteroid didn't
necessarily have to happen. So first of all, it's the
largest impact we know of on the Earth or the Moon in
the last half billion years- so it's rare. Second of all, it could have
easily missed the planet or third of all, it could have hit somewhere
else on the planet. It turns out that geologists think that where it hit really matters, that those rocks on the Yucatan Peninsula contained the right sort of chemical stew, carbonates and sulfates, that when blasted into the atmosphere, contributed to the situation
of blocking out the Sun and essentially shutting down food production across the planet; and perhaps only one to
13% of the Earth's surface contains that mixture of rocks. So if this asteroid had entered half an hour earlier or half an hour later and hit, for example,
in the Atlantic Ocean or the Pacific Ocean
and missed the Yucatan, well, dinosaurs might still be here, everything else alive at that time or at least most of those
groups might still be here, mammals would still be a minor group, and we wouldn't be
having this conversation. So that's one thing that had to go right, and it's a random collision of a big rock with an enormous rock that
happened 66 million years ago. The second big collision, and I think most people have
heard the story of the asteroid but the second big collision
people don't really know about and that is the collision that
set in motion the Ice Age. About 65 million years ago, the tectonic plate of
the Indian subcontinent was below the equator down near Madagascar and it was zipping northward
relative to the other plates and slammed into the Asian continent, and that started building,
for example, the Himalaya. And that rock building, what that does when rock is exposed on
the surface of the planet, that rock actually draws carbon dioxide out of the atmosphere. And that process has, over
then millions and millions of years, eventually drawn
enough CO2 out of the atmosphere to get CO2 levels low enough
to tip us into this Ice Age. You might say "Well, wow, how did that collision
sort of change the world?" It's during the early and middle part of that Ice Age that our brains
expanded threefold in size. Now this is remarkable. The Ice Age was sort of the test. It was this very challenging conditions out of which a line of apes emerged, big-brained, tool-making, habitat-making, complex-behaving apes and
those are our ancestors. So we really have the Ice Age
to thank for our existence, and we have the Indian
subcontinent's collision with Asia to thank for the Ice Age. I think we should all feel
very lucky to be here, first as a species for the
reasons of understanding that things on the planet could
have gone very differently. The asteroid, the Ice Ages, etc. But as individuals, as we've learned more about human biology, we understand really how lucky we are. Picture a spherical object surrounded by lots and lots of meteors and there's an impact of just one and then there's big chemical changes. Well it turns out, I'm not
describing the asteroid impact. I'm describing fertilization
of an egg by a sperm. In human fertilization, there's about 100 million sperm on average and those 100 million sperm are carrying all sorts of genetic
combinations from the father and that one individual
egg is carrying one out of about eight million different genetic combinations from the mother. So when that one lucky sperm makes it and combines with that
one egg at that moment, that's about a 1 in 70 trillion event, genetically speaking. Furthermore, there are
individual mutations in every sperm and in every egg
randomly distributed in DNA, mutations that didn't exist
in mom, didn't exist in dad and wouldn't exist by chance
really in any sibling. So the genetic recipe is getting mixed up in every generation on
an astronomical scale. So yeah, you are truly genetically unique and there will never, ever, ever be anyone like you ever again on
the planet or ever before and so, yeah, I think
we should feel fortunate that we are individually here. - Hey, Big Thinkers. Have you ever found
yourself in this scenario: A news story breaks and
you want to stay updated and understand what just happened and why? But every source you can
find includes some level of spin or framing that
makes it difficult to assess whether you're actually reading
good quality information. Wouldn't it be great if you could assess the informational quality
of the news you are reading and if you could
understand the general spin or slant a news organization
is prone to taking? If you're like me, the answer to that hypothetical
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monitoring organizations. Let me show you how it works: The video we just watched discussed how one big asteroid impact millions of years ago affected life on Earth. Thankfully, large asteroid impacts are exceptionally rare,
with catastrophic impacts occurring once every
300,000 to 500,000 years. But news organizations report
on large asteroids regularly. Let's check out this
relatively recent story about a skyscraper-sized asteroid that passed within 1.7
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link, ground.news/bigthink, you'll receive 40% off unlimited access to the platform with the Vantage Plan. - That was a long,
meandering riff but hopefully there's bits and pieces
you can find in there, so. - It was fantastic. - Want to dive deeper? Become a Big Think member
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