What are the chances of YOU existing? A biologist explains

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 does something 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 4 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 going to 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 that was probably the temperature of a baking oven. It set off wildfires and the devastation of 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 evolve 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 1 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

And, 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 this is, 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 in to 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, you know, 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 brain, 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 subcontinents 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 8 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 one 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

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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 million miles of Earth. We can see the political bias breakdown of the 120 news sources that reported on this event

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That was a long, meandering riff, but hopefully there's bits and pieces you can find in there. So

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