Name: How 2 freak accidents shaped human evolution | Sean B. Carroll Segment
Uploaded: 2025-08-07T18:36:33.000Z
Duration: 19 min 50 s
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The odds that any given planet's going to have complex life, I mean, this is really an opinion because we don't know about life anywhere else in the universe, but most of the scientists that I know, and think about this deeply, think the likelihood of life elsewhere is very high.

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It may not be the sort of life we're used to, it's not giraffes and redwood trees, but at least things like microbes, yeah, I think there's a very high likelihood that it's out there and abundant in our universe.

You know, as thinking creatures, and we'd like to think of ourselves as among the most complex things on earth, but 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 big and, you know, macroscopic.

So, if we were to be able to go around the universe and sample life from various places, probably the odds are that most of the time it's going to be microbial.

That would be biochemically complex, but not necessarily anatomically or behaviorally complex.

So, hard to judge the frequency with which we're going to find larger, complex behaving creatures on other planets.

A huge number of things had to go right for our species to exist and for each of us individually to exist, for us to be here.

And these are at all sorts of scales, the cosmological scale, the geological scale, and the biological scale.

When you think about the long history of life on earth, and you might sort of think, well, it's been this progression.

And so, therefore, almost as though it had a direction, almost had a purpose, and, you know, that some things are predictable.

When you really unpack the geological history of the planet and the biological history of the planet, it's been a random walk through all sorts of events.

So, you know, the earth is evolving, the physical entity of the earth is evolving, and life has to evolve right along with it.

And so, we need to have this sort of, understand this relationship, this coupling between the physical planet that life exists on and what's going on with life itself.

Intersect those two, and 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, you know, 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.

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.

So, whatever went on 3.8 billion years ago, and it's murky.

It's really hard for scientists to know what's going on, the biochemical events, where this happened, can't even exclude the possibility that life arrived from somewhere else and that then seeded what happened on Earth.

So, the origin of life on Earth is a really murky.

It's incredibly fascinating, but it's really, really hard to get to.

Once you have cellular life about the grade of a typical bacterium, that story going forward, we're really comfortable with in terms of the evolution of physiology and how you harvest energy from the sun and all the chemical reactions of life on Earth and all the big geological events.

But bring it more personally to ourselves, I really marked the beginning of our story at 66 million years ago, because the asteroid impact erased essentially the previous world and opened up the world of mammals from which we evolved.

And so, I think understanding those last 66 million years is very rewarding and much more concrete than thinking about 3.8 billion years ago.

So, what's really remarkable about the time we exist and actually had a role in our emergence is the ice age.

We're living in a really unusual time with respect to Earth's climate.

For the last two and a half million years, we've been in an ice age where the ice comes and goes.

So, it advances across the northern hemisphere and then retreats, advances and retreats.

But we didn't drop into that until about two and a half million years ago, and the previous ice age was perhaps 300 million years ago.

So, from, for example, the asteroid impact 66 million years ago, the Earth was ice-free at the poles.

Imagine that, you know, forests going pole to pole and no ice at all.

So, a very warm planet, very verdant planet, but it's gone through a gradual cooling over the last 60 or so million years and dropped into this period of the ice ages just two and a half million years ago.

And so, it's interesting to wonder, well, in this unique period, what implications does that have for humanity and for the other creatures on Earth?

There's been a handful of events where about 75% or more of all species have been erased from the planet in a short period of time.

We refer to those as mass extinctions, about five in the last half billion years.

At the end of the Permian, about 250 million years ago, massive volcanism poured enormous amounts of noxious gases and climate-altering gases into the atmosphere and wiped out perhaps more than 90% of species.

It was a huge transition in life on Earth.

And then 66 million years ago, about a six-mile-wide asteroid traveling about 50,000 miles an hour slammed into the Yucatan Peninsula.

And the consequences of that impact, again, led to global change, both climatic change, wildfires, et cetera, that erased more than three-quarters of plant and animal species from the planet.

So, what we know about these mass extinctions is that they're associated with really rapid environmental change, different triggers, but really rapid environmental change.

Yet, after each of them, life has rebounded very differently.

The sorts of the winners and losers are really different after each of these events.

And the winners that took over after, for example, the asteroid impact were mammals, which were relatively small and insignificant for 100 million years prior to that asteroid impact.

There's really two big collisions that are a major reason why we're here today.

And those collisions easily might not have happened.

The first was the asteroid impact 66 million years ago.

So, imagine this, a rock six miles wide, it's probably been orbiting the solar system for, I don't know, a couple of hundred million years or maybe longer.

It happens to enter the atmosphere 66 million years ago and slam into the Yucatan Peninsula.

So, first of all, it's the largest impact we know of on the Earth or the Moon in the last half billion years.

One in a half billion year or perhaps longer event.

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 contain the right sort of chemical stew, carbonates and sulfates, that when blasted into the atmosphere, created the situation or contributed to the situation of blocking out the sun and essentially shutting down food production across the planet.

So, and asteroid had entered, you know, 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.

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, you know, 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.

Now, we'll get to the impact of the Ice Age on humanity in a minute, but the impact of this collision was to start a cooling of the planet.

And that collision is of the Indian subcontinent, the tectonic plate of the Indian subcontinent with the Asian plate.

And that happened about 40 million years ago.

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 ocean.

And the way that carbon dioxide is drawn down is by reacting with rock and then being sequestered in the ocean.

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 happen?

How did that collision sort of change the world?

Well, it turns out that little piece, that Indian subcontinent, and we call it the Indian subcontinent because that's where it is now, but that was below the equator, down near Madagascar, about 65 million years ago when the asteroid hit.

And it was zipping northward at a much faster pace than the other tectonic plates were moving around.

It was smaller, it was thinner than the other tectonic plates.

And those tectonic plates in general, which are where the continents and the oceans are riding on, those were all generated about 140 million years ago when a supercontinent broke up, sort of like the same way a kitchen plate breaks up when it hits the floor, into these various pieces.

And one of those pieces was that Indian subcontinent, which then sort of raced northward relative to the other plates and slammed in to the Asian continent.

So those two big collisions, the asteroid and the Indian subcontinent with Asia, have a lot to do with the climate that we live in today and the biosphere that we live in today.

A biosphere that doesn't have dinosaurs, but is full of mammals from which primates, our ancestors, and eventually our species evolved.

And it's pretty easy to imagine how the asteroid didn't necessarily have to happen.

And certainly if that plate had broken up in any possible way, we might still be living in a much warmer world and our species might never have evolved on the continent of Africa.

So these two collisions, one really catastrophic, one a little more subtle, they were profound events for the future in terms of humanity.

Because I can certainly say without the asteroid impact, mammals would still be a minor player on the Earth's scene.

And of course, we're mammals and we evolved from primates.

And I think the evidence for that is twofold, is that for 100 million years, mammals coexisted alongside dinosaurs, but they were ecologically much less significant.

And then they really took over once those were removed.

And the second impact, the collision that eventually triggered the Ice Age, this is really important because now you've got to think about our ancestors and actually the hominid story.

The hominid story is not really a story about warm and cold, sort of when you think about the Ice Ages, it's really about wet and dry.

And those wet and dry cycles were pretty frequent, geologically speaking, in places like East Africa.

And so that puts a lot of pressure on species to evolve.

These intervals of wet and dry, cold and warm, especially in the northern hemisphere, that's a lot of pushing and pulling going on in the plant life and on the animals that adapt to it, etc.