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When an asteroid landed on the Yucatan peninsula 66 million years ago, it turned our planet into a debris field of chemicals that, eventually, fostered human life. Sean B. Carroll, author and esteemed biologist, unpacks the consequences of this collision, and claims we, as a species, should feel fortunate that we’re on this planet at all. 

This historical cosmic event, paired with the tectonic movement of Earth’s plates and the initiation of the Ice Age, ultimately led to existence as we know it today. Without these random, chance environmental and biological encounters, the development of life would have been stunted, or even entirely nonexistent. 

Even the sequence of human conception is random and unlikely, Carroll explains, leading us to reevaluate our understanding of evolution, true survival, and the significance of each individual life.

SEAN B. CARROLL: 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?

PRODUCER: Yeah, let's do it.

CARROLL: 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.

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 condition 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.

NARRATOR: 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.

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

PRODUCER: It was fantastic.

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