Factually! with Adam Conover - The Rise and Reign of the Mammals with Stephen Brusatte

Episode Date: July 20, 2022

In times of turmoil, it can be useful to take a longer view of history. Like, a LOT longer. Paleontologist and author of “The Rise and Reign of the Mammals” Stephen Brusatte joins Adam to explain how mammals took over the Earth hundreds of millions of years ago, and why we survived and achieve sentience when dinosaurs died out. Stephen goes on to discuss why taking a deep look at our history can help prepare us for the crises of the near future. You can purchase Stephen’s book at http://factuallypod.com/books Learn more about your ad choices. Visit megaphone.fm/adchoices See Privacy Policy at https://art19.com/privacy and California Privacy Notice at https://art19.com/privacy#do-not-sell-my-info.

You know, I got to confess, I have always been a sucker for Japanese treats. I love going down a little Tokyo, heading to a convenience store, and grabbing all those brightly colored, fun-packaged boxes off of the shelf. But you know what? I don't get the chance to go down there as often as I would like to. And that is why I am so thrilled that Bokksu, a Japanese snack subscription box, chose to sponsor this episode. What's gotten me so excited about Bokksu is that these aren't just your run-of-the-mill grocery store finds. Each box comes packed with 20 unique snacks that you can only find in Japan itself.

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chips made with rice flour, providing a lighter texture and satisfying crunch. Oh my gosh, this is so much fun. You got to get one of these for themselves and get this for the month of March. Bokksu has a limited edition cherry blossom box and 12 month subscribers get a free kimono style robe and get this while you're wearing your new duds, learning fascinating things about your tasty snacks. You can also rest assured that you have helped to support small family run businesses in Japan because Bokksu works with 200 plus small makers to get their snacks delivered straight to your door.

So if all of that sounds good, if you want a big box of delicious snacks like this for yourself, use the code factually for $15 off your first order at Bokksu.com. That's code factually for $15 off your first order on Bokksu.com. I don't know the way I don't know what to think I don't know what to say Yeah, but that's alright Yeah, that's okay I don't know anything Hello and welcome to Factually, I'm Adam Conover.

Thank you so much for joining me on the show once again. Just want to remind all of you that I am on tour doing stand-up across the country right now. If you live in Boston, Nashville, D.C., Arlington, Virginia, Spokane, Washington, Tacoma, Washington, or New York City, head to adamconover.net to get tickets to come see me do a brand new hour of stand-up in your area. And of course, if you want to support the show, please head to patreon.com slash adamconover where you can get bonus podcast episodes, see exclusive stand-up I don't post anywhere else, and join our live Patreon book club. That's patreon.com slash adamconover. Now, let's get to it. I know it seems like everything is on fire right now in human society, sometimes quite literally in the case of the fires in Yosemite National Park.

Oh, and then, of course, there's climate change. There's the fact that people are getting their rights taken away. There is looming economic catastrophe. Things look very bad. like this, you take some solace in things like the James Webb Space Telescope, which from its perch out there in space is doing one of the greatest scientific accomplishments in human history. It is literally peering back beyond the beyond to the very beginnings of our universe, providing an important sense of scale to our problems now. I mean, yeah, sure, we may be destroying our planet and each other right here in 2022, but with the old JWST, we can see entire galaxies being gobbled up by black holes. It's pretty fucking awesome. Now, look, I'm being a little bit hyperbolic, but there is a lot of solace in the fact that

these incredible scientific accomplishments are within the realm of human achievement, that despite all of our flaws, we can still do some incredible things. We can understand the deep origins of ourselves and of our universe. And we can do all of that despite the fact that on a geological timescale, humans have only been here for an eye blink. We've only been around a couple hundred thousand years,

whereas mammals, the class of critter that we're a part of, have been around for a couple hundred million years. And when mammals started, the Earth was dominated by giant fucking lizards walking around and stomping on things until they were destroyed by an asteroid that killed all of them, ending their reign of 165 million years. I mean, these are some crazy timescales. Now, look, you've heard all that before, right? It's one of the first facts any kid learns. Big lizards killed by a space rock. But just for a second, stop and appreciate what that means. Try to learn it again as though you're hearing it for the first time. A true fact about the spot that you stand on today is that hundreds of millions of years ago, gigantic lizards walked around on top of it until a rock from space murdered them.

And then slowly mammals gained dominance over the entire world. That is some crazy shit. So let's ask, how did it happen? What about mammals made us so incredibly successful that we were able to take over the earth? And furthermore, why were mammals the animals that became sentient in the first place? Why were we the ones that built self-driving cars and iPhone games and Supreme Courts and James Webb space telescopes? Why didn't dinosaurs get to the point of writing constitutions that gave rights to people and then taking the rights away as part of a revanchist right-wing assault on dino-democracy. I mean, seriously, this is a

serious question. How and why did this happen? Well, today on the show, we have an incredible guest to help us dive into the deep history of our species and appreciate again how wild and vast it truly is. Steve Brussati is a paleontologist and evolutionary biologist at the University of Edinburgh. He wrote a bestseller a few years ago called The Rise and Fall of the Dinosaurs, and his new book is The Rise and Reign of the Mammals, a new history from the shadow of the dinosaurs to us. He is one of the most acclaimed paleontologists and science communicators alive on the planet today, and I could not be more thrilled to have him on the show.

Please welcome Steve Grusotti. Steve, thank you so much for being on the show. Well, thanks a lot for having me. Really excited to talk mammals and books and fossils and prehistory with you today. Yeah, this is going to be a really fun one. Sometimes we do some rather serious topics on the show, but this time we're going to have a lot of fun talking about mammals. I guess it was pretty serious for the dinosaurs, though. They didn't come out so well in the story. I want to ask, the name of your new book is Rise and Rain of the Mammals. And I just want to start by asking, why do you say the reign of the mammals, considering that, you know, my understanding is there's, you know, arthropods still beat us by many, many tons

in terms of biomass, right? There's not as many mammal species as there are other animals in terms of the number and total of them on the earth. So why do you say the reign of the mammals? Well, in part, because for a book title, there's some nice alliteration there. It plays off, you know, a common, a common trope, let's say, but I think a useful trope of just thinking about evolution and thinking about what in this case is a story of 325 million years of fossils, and of evolution and of change, you know, a story of mammals and our ancestors navigating, surviving volcanoes and asteroids and global warming and ice ages ultimately culminating in us and in the 6,000 other species of mammals that share our world.

And in many ways, we do reign supreme today, at least in the sense that humans are mammals. We are a sublime species in both good and bad, and we have affected the planet unlike any animal ever. But also, there's such a range of mammals that share our world, and I think we don't always appreciate them. You just think about everything from the dogs and cats that are our pets to bats and elephants and whales. Whales are the biggest animals that have ever lived in Earth history. They're alive today and they are mammals. So although, yes, there are some groups of animals like arthropods, beetles, let's say, that have many more species than the 6,000 species of mammals, it really is a world today where mammals are incredibly important and fill so many ecological roles. They live all over the world.

And let's face it, we are a mammal. And so I think we are always interested in our own story. And that's what this story is. At least through self-aggrandizement, we'll want to say that mammals reign because we consider ourselves to reign in many ways, although it's debatable whether or not we do or whether we should. Well, OK, let's let's go to the beginning of the story. Where where did mammals come from, Steve? I think there is a bit of a misconception, let's say. And I think this comes across sometimes in textbooks or museum exhibits, this idea that the dinosaurs had their day, this big asteroid came down, killed off the dinosaurs, and then the mammals evolved to take the place of the dinosaurs. Now, in part, that's true,

and we can dive into this. There was a six-mile-wide asteroid that smashed into the Earth 66 million years ago, and T. rex was there. Triceratops was there. They did not survive. Mammals did. And mammals then filled a lot of those roles that dinosaurs once filled, and mammals got bigger, and mammals spread around the world. So mammals did assume some dominance from the dinosaurs, let's say. But mammals actually go back a lot farther. Mammals were there when the asteroid hit, and they were there long before the asteroid. And the first mammals, believe it or not, their origin story is the same as the origin story of dinosaurs. Both dinosaurs and mammals.

Yep, they go back to about 225 million years ago, the Triassic period. This was the time of the supercontinent when all of the land was gathered together as one. Pangea? Is that the supercontinent? I remember the PBS that I watched as a kid very well. I remember Pangea. It's a name that sticks in your mind, right? Like one of those names from the distant, you know, prehistory from the archaic times, Pangea. And it was real. There was a time when there was one single giant landmass stretching from North Pole to South Pole. And that was the land that both dinosaurs and mammals rose up on. And that was about 225 million years ago. Now, the story of mammals, though,

goes back even 100 million years further to about 325 million years ago. That's when the ancestors of mammals split off from the reptiles on the great family tree of life. So the story of mammals, of making a mammal, of evolution, producing a type of small, furry, smart, warm-blooded animal, that story is a story of many, many, many hundreds of millions of years. And ultimately, it is our deep history. And that's why I think it is especially interesting and especially relevant for us to understand. One of the things that I saw in the book is that mammals and dinosaurs have a common ancestor that they split off. And that, to me, really sparked my imagination, because I had never considered, wait, what would the precursor to a

mammal be? These seem like such incredibly different species, obviously. What was that precursor like and how did that split happen? If you go way back in time to about 325 million years ago, there would have been a single common ancestor. And from that ancestor, eventually, you would get everything as diverse as dinosaurs, turtles, crocodiles, pterodactyls, birds, and mammals. They would all later on evolve from that ancestor, the same way, you know, we all have grandparents and great-grandparents and so on going way back. But that common ancestor would have lived in a world that would have been scarcely recognizable to us. This was the coal age, the time of the first big jungles in earth history, when these primitive trees, these things stretched up 100 feet or more

into the canopy. They formed these dense jungles, these rainforests that clung to the coasts, all in the tropics and the subtropics. It was those trees that when they died and were buried, turned into a lot of the coal that we still mine today. And those jungles were so rich and so full of oxygen and so full of life that you had dragonflies the size of pigeons buzzing around. You had millipedes that were bigger than humans. In this weird, primitive menagerie, you had this common ancestor of all of these animals that would later evolve. And this ancestor would have been small. It would have been something you could hold in your hands. It would have been probably green or gray or brown, very drab colored. It would have been covered in scales. It would have

had arms and legs. It would have stuck out to the side. It would have sprawled when it walked. It would have moved very slowly. It would have been a very humble creature living, probably in fear of those giant dragonflies and millipedes. But from that humble ancestor, so much rich, diverse life would eventually evolve. And so we have to go back to that ancestor to start telling the story of the mammals. Can I ask, how do we know that it's a single ancestor? Because you say, hey, you know, we all come from different grandparents and great grandparents, right? Or sorry, you know, the family tree does go back, but it's not the case that all humans came from one pair, right? As is in, you know, the book of Genesis.

So how do we know that it's one ancestor species rather than, hey, there were a dozen species that all speciated in different ways? We have two lines of evidence that help us put together these family trees. And one line of evidence is what I work on and what I devote my life to. I'm a paleontologist. I'm one of those people that goes out and digs up dinosaurs and fossil mammals. And I go out looking for bones and teeth and claws and skeletons and all these bits of prehistoric animals that we can find in the rocks, the clues from prehistory. So the fossils help us put together a family tree that shows these species over time. So we have fossils of things that approximate that ancestor,

but we also have genetic evidence. We have DNA evidence from today's species, and we can see that everything as diverse from mammals to rept, to crocodiles, lizards, birds, all these things share features of the genetics that you could trace down to a single common ancestor. So although nobody was there to see this ancestral animal, we can infer that it was there back in those coal swamp days. So as that single ancestor starts to speciate, right, into different, there's a, I suppose, a reptile direction and a mammal direction. What are those super early mammals like? Yeah, so absolutely. So that ancestor would have somehow split into two populations.

We don't have the exact fossils to show it. We have to infer it a little bit, but we know there would have been some ancestor and there would have been this great divergence, this fork in the road. And one side would eventually produce lizards and snakes and dinosaurs and birds and crocodiles and turtles and all those reptiles. And the other side would lead towards mammals. And those first animals that split off from that ancestor on the mammal lineage, the mammal bloodline, they wouldn't have looked like a mammal. They would not have had hair. They would not have fed their babies milk. They would not have had big brains. Those things would come later. Those would be the things that would help define true mammals. But these ancestral mammals,

these proto mammals, let's say, the only thing that distinguished them from the other animals at the time was behind their eye socket, they had another hole in their head. And that hole was for muscles, the jaw muscles, the muscles that would open and close the jaws. So having that hole allowed them to have bigger jaw muscles and a stronger bite. And it seems like a small thing, but it was that one almost anonymous little feature of the anatomy that set the mammal line off on its own. And we still have that. We've inherited that from those early mammal ancestors. Wait, where's my hole? Is it like the—

Yeah. Go for it. You feel your cheekbone, and I see you here on camera feeling your cheekbone, and I'm doing the same. Yeah. You know, your eye is kind of above it. And you can feel underneath it. You know, there's a bit of a hole. You can feel your muscles, your cheeks.

That is the remnant of that hole. Oh. That's it. It's kind of moved down because our face is really squashed. We don't have a big, long snout or anything. Yeah. So that's the remnant of that hole. And those jaw muscles attach in that hole. And that's something that about 325 million years ago, those first mammal ancestors developed

in the coal swamps as there were those pigeon-sized dragonflies buzzing overhead, as these mammal ancestors were splitting off from the reptile ancestors, each group staking out a claim on their own. Now, that's not what I learned in elementary school as the difference between mammals and reptiles. The thing that I learned, and by the way, this is very embarrassing to me because here I am coming at you with stuff I learned when I was in fifth grade about mammals. I probably would have learned a lot of the same stuff, you know, and things are always changing in the field of paleontology because we're always

discovering new fossils all the time. So this is a rapidly changing field and that's what makes it exciting. So I think we got to embrace it. You know, the things we learned in school, they might've been cherished at the time, but we're often learning new things and future generations will too. And I want to ask you what has changed about paleontology a little bit later. But let me ask that the main thing I learned, if memory serves, is the difference between mammals and reptiles, is mammals are warm-blooded and reptiles are cold-blooded, which I believe refers to temperature regulation, like how we are able to maintain our temperature no matter where we go, as opposed to reptiles, which change it depending on the environment. How did that come about? That seems like a pretty massive change.

Absolutely. And that's and that's correct. That's still a very valid thing. Oh, thank God. Which is good. We don't you don't have to go back and track down your elementary school teachers and, you know, scold them and chastise them. So it's true. Right. So we can regulate our body temperatures. We are warm blooded. And what that is, so I grew up in the Chicago area. I live in Scotland now. I've been here about 10 years, but I'm from the Midwest. I'm from near Chicago. Those winters are cold. But we can go outside in the winter. And when we go outside, we don't just die. We can breathe. We don't just freeze. Now, a crocodile, a lizard, like they couldn't really deal with the Chicago winter. So what's the difference?

They would immediately, like we'll die after a couple of days, but they will sort of immediately slow down and not be able to handle it at all. Yeah, and the differences that mammals and also birds, you know, we have to remember birds, birds evolved from dinosaurs, which have to remember birds who birds evolved

from dinosaurs, which did evolve from reptiles. So some members of the reptile line figured out a way to do the same thing, but they figured out independently. But what mammals and birds have been able to do is regulate body temperature using internal metabolism. We are able to have a high body temperature and a body temperature that is constant. That's why if we get a fever, even if our temperature goes a little bit up, we feel really sick because we have usually a very constant temperature. So we have a high body temperature, a constant body temperature, and we regulate it internally. We have an internal furnace. it internally. We have an internal furnace, whereas a reptile, a crocodile, a lizard, a snake,

they have to go out into the sun and basically use their body as a solar panel to heat themselves up. So their body temperature fluctuates depending on if it's morning or night, if they're in the sun or in the shade. So it's really hard if you're handicapped with that sort of system to be able to live everywhere in the world. But if you're warm-blooded, you can regulate your own body temperature. Now, all of a sudden, you can live in many different places, whether it's hot or cold, whether you're on top of a mountain or down in a desert. The problem is, though, that it does come at a cost, and you need to eat a lot of food and take in a lot of oxygen in order to power that internal furnace. So our mammal ancestors, they developed this higher metabolism and ability to regulate their body temperature. They developed this around the same time they were evolving hair to keep their bodies warm.

Really, hair isn't just to keep warm, though. It's to help regulate that body temperature as part of the package deal of being warm-blooded, the same way birds that are warm-blooded have feathers to help regulate body temperature. And also around the same time, these mammals were getting bigger brains, and they were developing milk to feed their babies. So these are the things that differentiate true mammals from those ancestors. differentiate true mammals from those ancestors. So the ancestors devolved that hole in the head to have bigger jaw muscles. Then some of those early proto mammals, they develop higher metabolisms, hair, milk, and so on. They became the true mammals. And those things together, all those features I just mentioned, they are, they constitute what we call the mammal body plan or the mammal blueprint it is those things that set mammals apart from all other animals that are alive today nothing else has

hair nothing else feeds its babies milk and so on so it is it was in the triassic period then about 225 million years ago that those advanced features came together and you had proper, true mammals emerge on the scene. Can I just ask a very stupid question? That was a beautiful picture. Any time, any question. Believe me, no question is too silly when it comes to this stuff, because we're talking about fossils.

We're talking about stuff that happened hundreds of millions of years ago. Nobody was around to see it. So a lot of it's not obvious. It's not intuitive. We have to infer things. So ask away. Okay. You're making me feel so comfortable. I really appreciate it. On my own show too. You say we have an internal furnace. What is the internal furnace? Like how, hair obviously, but I'm not aware of an organ that's the particular heat organ. So how do we do it? That is actually a great question.

We do not have an extra organ. You know, we don't have a furnace organ like next to our heart or next to our lungs. The way we do it is through chemical metabolic reactions at the cellular level. So it's really there's kind of extra machinery in our individual cells. A lot of it comes down to the mitochondria, the energy powerhouses of the cell. There's a lot more mitochondria in warm-blooded animals. And there's just different chemical reactions. And that's reaching the extent of my knowledge, I have to say. I'm not a physiologist. I study the fossils. So now that's about as much as I can explain about how

warm-bloodedness works. And then I have to defer to a real expert on this subject. That's just a fascinating thing to evolve that late to me. I mean, look, I'm not an evolutionary biologist, but my understanding is divisions and how cells work seem like something that would happen a lot more in the primordial soup phase rather than when you know you've got four legs and skeletons and things like that um that's like a very fascinating adaptation but obviously it's a huge part of mammals success because then you can be present in so many more places you don't have to like restrict yourself to a particular temperature area in order to survive. You can go like, you know, for the walk through the snow banks to get to somewhere new.

Yep. And you're absolutely right. And I think it did take evolution a long time to develop that. And ultimately, I think it comes down to the cost. It's great. If you're warm blooded, you have that internal furnace. You can have a high and constant body temperature. It allows you to live in so many environments. You don't have to bask in the sun in the morning to get yourself ready to just go out and walk around and eat and whatever. But it comes at a cost because you need to eat a lot more and you need to take in a lot more oxygen. So you need to find ways to eat more and bring in more calories and breathe more efficiently in order to even get to the point where you can elevate your metabolism.

And that is just very expensive to do. But once mammals figured it out in an evolutionary sense, and they became warm-blooded and had these elevated metabolisms, then you see in the fossil record, these mammals, they spread all around the world and they start to diversify. And this is all happening, by the way, while dinosaurs are spreading around the world and dinosaurs are diversifying. And this begins about 225 million years ago. And from that point until today, the fates of dinosaurs and mammals would be forever intertwined. And at first, of course, the dinosaurs, they were destined for grandeur, and they became bigger and bigger and bigger. And some of them became as big as jet airplanes. And you had T-Rexes the size of buses, you know, the biggest pure meat eaters that have ever lived on

land. Then you had other dinosaurs evolve feathers and wings and become birds. For over 150 million years, mammals, true mammals, with hair and warm-blooded metabolisms and feeding their babies milk, true mammals, they were living with those dinosaurs as the dinosaurs were getting bigger and bigger. Those mammals were forced into the shadows. They could not become very big in a world where dinosaurs were so big and were filling those ecological roles. So the dinosaurs kept mammals small for a long time. For 150 million years, there were all kinds of mammals living with the dinosaurs, but never, as far as we know, did these mammals ever get bigger than a badger or a house cat, which is remarkable to think about. They were there for so long. But conversely, these mammals then became so good at being small,

at being anonymous, at living underground, at living in the undergrowth, at coming out at night. They became so good at living incognito, living anonymously, that they kept the dinosaurs big. And you never saw a T-Rex the size of a mouse or a Triceratops the size of a rat. You didn't see it because mammals were kings and queens in those niches. So for a long time, you had almost this evolutionary equilibrium where the dinosaurs were the big ones, the mammals were the small ones. They were both doing very well in their respective ecological roles. And then that all changed when the asteroid hit. It's pretty wild that that would be the case worldwide as well. You would, I mean, is there some, something different about the temperature or the flora or fauna of the time period that would mean that, you know, a reptile,

a dinosaur would have an easier time being large than a mammal? Because, you know, as you say, today, the largest animals in the history of the world are mammals. So, you know, it's not as though warm-bloodedness and milk are preventing you from being big. So was it just the fact that dinosaurs were occupying that ecological niche, or was there some other pressure that was, like, selecting for big reptiles, small mammals at the time? It's another great question, and it's wild to think about it, because exactly as you say, we know mammals can become big because the biggest things ever, blue whales, are mammals. And elephants are big and polar bears are big. I mean, we see these big mammals all around us. We go to zoos, we see them. We watch nature films, we see them. So mammals

can become big. They just were not able to become big until the dinosaurs were gone. And so was that because there was something that allowed dinosaurs to become big and that maybe dinosaurs were better at being big? That's possible. Dinosaurs, it looks like dinosaurs did have very efficient lungs, the same kind of lungs that birds have today that take in a lot more oxygen than mammal lungs. And we can tell that because those lungs have air sacs, these balloons that extend out from them to store extra air. And those can tell that because those lungs have air sacs, these balloons that extend out from them to store extra air. And those air sacs leave marks on the fossil bones. So we can infer that dinosaurs have those same lungs as birds. So maybe having these more efficient lungs

help dinosaurs become bigger, or maybe it was just a game of chance and contingency and timing. Maybe the dinosaurs just got bigger first, and then they seized those ecological niches and they kept them. Yeah. Or it might be a combination of both. But I think with evolution, you know, we're dealing with hundreds of millions of years. We're dealing with a lot of chance and a lot of unpredictability. And the fossils only tell us so much.

And mysteries like these will continue to be out there. It's a funny thing with evolution. There's always this desire to know why, because evolution, sort of the story of evolution often presents you with like half a why or, you know, hey, here's how something happened. But we often have this deep, well, but why did it happen this way? When in reality, kind of what evolution really gives us is the kind of, well, but why did it happen this way? When in reality, kind of what evolution really gives us is the kind of, well, here's how it happened. But a lot of it is chance. And a lot of it is like, there might not be a deeper why at the root there. Adam, you're a smart guy. I love it. I mean, you're right. You're right. And so,

you know, in addition to comedy, you should go to evolutionary biology too, because you're totally right. People ask these questions, right? Like, you should go to evolutionary biology, too, because people ask these questions, right? Like, why does the camel have a hump and why does the elephant have a trunk? And they're very difficult to answer, but we can get at the how. We can start telling the story. We can look at fossils and we can say, well, elephants actually started as these little animals the size of a poodle. And they did not have a trunk. And then they got bigger and then their nostrils moved back. And so we can use fossils and genetics

to piece together the sequence of events. We don't always know exactly why something has happened, and a lot of times the why probably is unknowable, but we can tell the how, and that's really in The Rise and Reign of the Mammals in the new book. A lot of it is storytelling, this narrative history of our ancestors leading up to the present day, the story of how our ancient mammal relatives, ancestors dealt with an ever-changing planet, changing climates, drifting continents, volcanoes, asteroids, the story of the what happened, the how it all went down. And that can make us think about the why. But I think we can at least revel in the what happened because we have so much evidence for the what happened. And the what

happened can help put our modern world into perspective, too. That is so cool. Well, I have to ask you about what happened with the asteroid, but we have to take a really quick break. We'll be right back with more Steve Brassati. Okay. We're back with Steve Brussati. So you took us through the history of mammals up until the asteroid striking the Earth. And I actually really want to dwell on this point. I could skip over it because we've all heard about the asteroid. We know the asteroid.

We all are aware of the asteroid. But I have to ask, as a kid, I remember hearing about the story of the asteroid and thinking at the time, well, this story is very convenient, isn't it? Like one asteroid comes and hits the Earth and everything changes. It sounds too apocalyptic. It sounds it's such a wild story. As you say, the asteroid six miles wide. That's very big. It's also not that big to wipe out an entire, you know, group of species from the Earth.

So let's just can we spend a moment on it? Tell me a little bit more about it. And was that really the cause of this massive change? And how do we know that it was? I'd love to talk more about the asteroid because it does seem maybe too convenient of a story, too good to be true, too apocalyptic, but it is true, it did happen.

And it probably was the single worst day in the entire history of life. Now it does sound like what I'm going to say and what I talk about in the book as well. I lay out the scenario. It does sound something like some, you know, really kitschy disaster movie where the script would be rejected by any good studio in Hollywood. It sounds like something that would be debunked. It sounds like something where you learn it in school and then paleontologists are like, well, yes, there might've been an asteroid, but it's not really that powerful. And I know the story is so good, but in fact, there were many pressures of the time. And like, that's what I would expect you to say. And yet here you are coming at me with

asteroid. So yeah, go on, please. It sounds like an urban legend. It sounds like the kind of thing the fact checkers would go nuts on. Yeah. But it actually happened. And it was only in 1980 that the geological evidence for an asteroid started to accumulate. So that's what we have to acknowledge first. We know there was an asteroid that hit the Earth 66 million years ago, right at the end of the Cretaceous period, and that no dinosaurs, other than birds, ever have been found after

that asteroid. We know that asteroid actually hit the Earth. We have evidence that was left behind in the rocks, and we can see this evidence. So geologists, in this sense, are like detectives, and they piece together this evidence. The first evidence was chemical. This chemical was found, an element called iridium, that's very rare on the surface of the Earth, but very common in outer space.

All the rocks everywhere in the world from that exact time was just saturated with this element. So this was the first sign. It was this chemical fingerprint of something extraterrestrial happening. And then other evidence accumulated, and by the early 90s, the crater was identified. And it took a while to find it because it's mostly covered by the water of the Gulf of Mexico today, this crater. But this crater, it's in Mexico, this crater is over 100

miles wide. And we can tell from the size of that crater that it would have been made by an asteroid that was about six miles wide. Wow. And so maybe that doesn't sound like a lot, you know, six miles wide. But this is a rock that was hurtling through outer space. It was traveling like ten times faster than a speeding bullet. Okay, this thing was just whizzing. I mean, just a random piece of space junk. It could have gone anywhere.

But it made a beeline for the Earth. And this was the biggest asteroid to hit the Earth in at least the last half a billion years. It is not a normal thing for a six-mile-wide rock to hit the Earth. So that is basically a rock the size of Mount Everest, or it's about three times wider than the island of Manhattan. This is big. And it was traveling so fast that when it hit the earth, it released as much energy as one billion nuclear bombs put together. And that energy instantaneously led to, you know, tsunamis, earthquakes, wildfires, the atmosphere spontaneously combusted across much of the world.

And so it was like a really kitschy disaster film, but it was real. This was all happening. And the things that were alive 66 million years ago, T-Rex was there, Triceratops was there, as were mammals, the small mammals. They're all there looking up at the sky going, oh, shit. They're going, what the hell's going on? Because they would have had no idea. You know, they would have had no clue.

They would have just one moment, there would have been this enormous explosion. And of course, it would have been much worse the closer you were to it, probably within a thousand miles of ground zero, like everything was vaporized, just boom, just turned to ghosts. But there would have been no forewarning of this. So just imagine. And so dinosaurs had been ruling the world for over 150 million years. And then bam, this horrible, unexpected, completely unpredictable, random act of space violence happens. When you describe it as moving faster than a bullet, I start to get it, that it's the amount of trauma it would cause and the amount of energy being released is really fascinating

to think about. So I'm sorry, please go on. Yeah. And so and so all of those things conspired to kill off the dinosaurs. But there was it was worse. It wasn't just the earthquakes, the wildfires, the tsunamis, all that stuff that happened in the moments and the hours afterwards. It was all of the dirt and grime and dust from the explosion and all the soot from the wildfires. All this stuff went into the

atmosphere, and it would have spread all around the world, and it would have cloaked the earth in darkness for at least a few years, maybe even up to a decade. There was a global nuclear winter and the earth went dark and cold. And so plants could not photosynthesize. They didn't have sunlight to make their own food. The forests collapsed. The plant eaters had no food to eat. They died.

The meat eaters had no food to eat. They died. Ecosystems collapsed like houses of cards. The only reason anything survived is because seeds can stick around in the soil for a long time. So when the sun eventually came out a few years later, there were still some seeds around. Those seeds could grow and reestablish the forest. But this was just an ecological disaster across the board. And the dinosaurs could not cope with that. But we had ancestors that stared down that asteroid.

And this is worth just repeating and thinking about. We, you and me, had ancestors, mammal ancestors that survived, that endured, that had what it took during that worst moment of Earth history. And if we if they didn't have the ability to survive, we would not be here talking. And what it came down to is that they had adapted so well to living in the shadows, to surviving in a world dominated by dinosaurs. They were good at being small.

They were good at hiding. They were good at eating lots of different foods. Those things all came in handy when it just went to anarchy when the asteroid hit. And it is for that reason that some mammals were able to survive. Now, a lot of mammals died too, by the way. Maybe even nine out of every ten mammal species died, depending on how you look at the fossil record and how you do your calculations. But mammals almost went the way of the dinosaurs.

But we had an ancestor that was small, that could hide, that could eat lots of different food. That ancestor made it through. And then when the dust cleared and the fires were extinguished and the sun shone again, imagine you were that ancestor. You're this small little mousy thing. And you're looking out at a mostly empty world. small little mousy thing, and you're looking out at a mostly empty world. Three out of every four species that were alive the day the asteroid hit are gone. And the world is there for you to inherit. The opportunities were abundant. That's when mammals finally, finally started to

get bigger. They took over for the dinosaurs. They moved into those niches that T. rex and triceratops once held. They got bigger bodies. They diversified. And it was from that diversification that set the stage for all the bats and know, everyone has the member of their family who does like the genealogy, who like dives into the court records and the census data to figure out who our ancestors were. You're doing genealogy for the human species in a way. Like, hey, this is, hey, guess what? You know, this is what our ancestors went through. So, you know, be grateful to great, great, great, great, great, great, great, great grandma mouse or whatever. Yep. This is like the deepest kind of deep time genealogy you can think of, which I think makes it fun. But I think also, you know, we as paleontologists, we're always talking about family trees and what's related to what. And we were talking earlier about the reptiles splitting from the mammals on the great family tree of life. We use these metaphors and we use this way of thinking because just like us, like our own human families, family trees are useful. Thinking about lineages, about genealogy, about ancestors. This is a way to understand change over time, understand where we came from.

And so we as scientists are constantly thinking about our deep genealogical past. So this diversification that you talk about, it's like this moment where we start spreading out as mammals across the world. The diversification is massive. I mean, just to end up as the largest creature in the oceans, just to dwell on that for a second, is wild. I'm assuming that mammals didn't get their start in the oceans, that we got started on land. How the hell did we end up in the middle of the Pacific Ocean, deep underwater, eating tiny shrimps? Evolution's remarkable. And I think one of the most remarkable stories is the story of whales. Because let's just start off with something that's basic, but also something that needs to be

said. And it's something, don't be embarrassed if you've ever thought this, but look, dolphins and whales, I mean, they look like fish, right? I thought they were fish until I was probably in like sixth grade or something. And, you know and saw this in a science lesson in school. Herman Melville calls them fish and Moby did. Yeah. They live in the water. They have flippers and fins. They look like fish, but they are mammals.

And they have hair. They don't have a lot of hair, but you can still see hair around the eyes and around the mouth. They use hair as sensory organs, as whiskers. see hair around the eyes and around the mouth. They use hair as sensory organs, as whiskers. And not only that, but there are placental mammals like us that give live birth to well-developed young. And so that means they have placentas. That means they had umbilical cords linking the baby to the mother. That means they have belly buttons. Whales have belly buttons. So if you ever doubt that whales are a mammal, they have hair, they have belly buttons. So they're mammals. But how did mammals get into the water? Like, you're absolutely right. Mammals started on land way back in the coal forest times. Those first proto mammals that evolved that extra hole behind

their eye to have the bigger jaw muscles, they were living on land. The first mammals that evolved hair and warm blooded metabolism and everything we've been talking about, these mammals were living on land. The ones living underfoot of the dinosaurs were living underfoot of the dinosaurs. They were burrowing, you know, into the ground. They were all on land. But then the asteroid hits, mammals survive, mammals diversify, and mammals diversify so much that they start exploring all kinds of new environments. Now, during the age of dinosaurs, there were lots of reptiles that lived in the water. They were not dinosaurs strictly.

They were cousins of dinosaurs. But there were plesiosaurs and ichthyosaurs and mosasaurs and all these things we call marine reptiles. Well, these things died with the asteroid, too. So there was new opportunity in the oceans. And so mammals seize that opportunity. And about 50-ish to 55 million years ago, we start to see fossils of a small, little, meek, little mammal. And this mammal is only about the size of a little deer, like a little baby deer. You can call it Bambi if you want. And it had

hooves. It had hooves. This was a mammal that lived on the land. But there's a few features of its skeleton. You can start to see, hmm, these things might be useful for also kind of moving around in the water a bit, but still primarily a land animal. And then we see this transitional series of fossils, one after the other after the other, showing how that type of little Bambi morphs into a Moby Dick. We see it getting bigger. We see it turning its limbs, its hands and feet into flippers. We see all kinds of changes. And so ultimately from that comes whales. And so that's what we call the major evolutionary transition. It's when evolution takes one type of animal with a body that's really well suited for behaving in a certain way and living in a certain environment, you know, a hoofed animal

with long legs for running on land. And it takes that type of animal and morphs it into something radically different with a new type of body that can now behave in new ways and live in new environments. A big animal with flippers that can swim in the water. And we have the fossils to show it. That's the amazing thing. This isn't just guesswork. We're not just assuming that whales evolved from land mammals.

We have the genetics that tell us that whales are very close relatives of things like hippos. They're the closest cousins of whales. And hippos spend a lot of time in the water, obviously. They do. So you can start to kind of see whales are part of this group of mammals with hooves that live on the land, but also spend some time in the water.

So we have the genetics and the fossils to prove it. And to me, that is really fascinating. And then once whales completely changed their limbs into flippers, there was this transitional stage where there were walking whales, where these whales could both swim and walk. They could live on land and in water. But then they totally divorced themselves from the land. They had full-on flippers. They lost their hind legs entirely. Once those whales went fully aquatic, they spread all around the world. And along with bats, which of course

evolved wings and developed the ability to fly, the whales were the first truly global group of mammals. And they still are today. Yeah. That's one of the things that I always trip out about, about whales when I'm like late at night, 2 a.m. looking up whales on Wikipedia, that like they are truly global. They live in like every ocean around the world, which is incredible. Look, we only have 15 minutes left. We have to talk about humans. Yeah, if we must. So can we take us from, you know, mammals diversifying, taking over the world to the rise of humans?

And I'm curious if there's something that, like, connects, like, what is it about mammals that cause them to, I'm going to another why here, but, you know, that cause mammals to, you know, produce humans as opposed to any of these other large groups of animals. I'll start with the caveat, with the disclaimer, and just say that the history of mammals is not just about us. You know, the 325 million years of history from the coal swamps leading up to the present, it was not some inevitable coronation march for humans. You know, we are one of more than 6,000 mammals alive today. And there are a lot of amazing mammals. Think of bats and whales and elephants and just the diversity of things mammals can do.

Those types of mammals are remarkable in their own right. As are we, of course, because we are a single but sublime species of mammal that evolved enormous brains and really high intelligence and consciousness and the ability to work in groups and the ability to shape the earth in ways that no mammals ever had. So we come from the primate group of mammals, and I save primates really for the last chapter in The Rise and Reign of the Mammals in the new book. Because I don't want people reading the book to think it's only all about humans. The only reason I'm telling this story is because it leads to us. That is a reason why we should be interested in the history of mammals, of course. We should be

interested in our cousins and our ancestors. But I want people to appreciate them in their own right. So I bring humans in and other primates in the last chapter, and I start with the asteroid. When the asteroid hit, there were lots of mammals around. Some survived. And then from those ancestors on the other side, the dinosaurs are gone. Those mammals start to proliferate, and they get bigger, and they diversify, and they develop new ways of moving, and they spread around the world, and they start eating new foods. Big diversification. Now, some of those very first mammals that we see in the fossil record, we start seeing their teeth and their jaws turn up in abundance just about 100,000 years after the asteroid are the very first primates. Now, they were small. These

were little things. You could hold them in your hands. They had gentle teeth, gentle cusps, little mounds on their teeth for eating soft vegetation and fruit. That's something a lot of primates do. That's one way we can recognize their teeth. And from these small first primates, incredible diversification would later come. And so humans are one of many, many primate species. And what we can recognize as humans, basically primates that are big and stand only on their hind legs and have hands free for doing other things and have big brains and so on. These human type of primates first appear in the fossil record about five or six million years ago. Our closest relatives are chimpanzees. The genetics show that, and the fossils show that too. And so it was about six-ish million years ago we made our split from

the chimpanzees. And the first humans, they started walking on two legs, and then they got bigger brains later. That's the evolutionary sequence. So it seems like standing upright, coming down from the trees, being able to walk on the ground, that probably freed our hands to do things, to build tools, to hunt, and that allowed us to bring in more calories and evolve these bigger brains. That's the general thinking. But we see humans start walking upright on two legs, then tools appear, then brains get big. From those first humans, things like Artipithecus, and then a little bit later, Australopithecus, Lucy, maybe, you know, many people have heard of Lucy, that famous fossil human. That belongs to a species called Australopithecus.

These were some of the first fossil humans. And from these ancestors, this big bushy family tree evolved. And what's remarkable is that from the time of the very first humans, five or six million years ago, until only about 40,000 years ago, there were always multiple species of humans, oftentimes living together in the same environments. And that seems weird to us, because we're one species. We're Homo sapiens. We're all the same. You know, no matter what differences we have in skin color, eye color, hair, or height, or whatever, we are all the same species. But in the past, there really were multiple species living together. And it kind of makes sense when you think about it. You know, there's multiple

species of cats living in today's world. There's different types of bears living in today's world. There used to be different types of humans. But it was only Homo sapiens when we moved around the world, and we hunted, and we changed environments, and we would have fought and bred with some of these other species of humans that we ended up wiping them out one way or another and having the whole world to ourselves. And now it is us alone, a single human species, having this discussion, pondering where we came from. You're saying that you had sort of a positive tone in your voice, but it sounds very sad. Like now you said we're alone now, you know? And as you're talking about Neanderthals, the Denisovans is another one of the speeches I say, right.

And so we wiped them out through what, just through not competition, but just the sort of like what hegemony that forms at when you have this many humans, all all all operating together as as we just sort of bred each other out of existence or do we just incorporate them into one species yes we we did in a sense so you and i will both have somewhere around three percent neanderthal dna in our genomes which is wild actually, all humans today that do not have an unbroken family pedigree that's been on Africa the entire time will have some Neanderthal DNA in their genome. Why is that? Well, humanity evolved in Africa from apes. The fossils show it. All these

early human species are African. And then at different times, different waves would leave Africa because, you know, these were animals. They would be chasing prey, they would be tracking changing climates, migration corridors. So you had different human species leaving Africa at different times. And the most recent one to leave Africa was our species, Homo sapiens. We see the first fossils of Homo sapiens in Africa about 300,000 years ago, and then they start to appear across Africa, and then a bit later they appear outside of Africa. Now, when our Homo sapiens ancestors would have left Africa, they would have encountered other species of humans, descendants from those earlier waves of human species that had left Africa before. And that's what Neanderthals were.

And that's what Denisovans were. And so these were other species of humans that would have been living in Europe and in Asia. So imagine you are one of these Homo sapiens ancestors of ours. You've been born in Africa and your band of humans, you're tracking prey, you go northwards, you go into Europe and you stop where the ice sheets are, because this would have been the Ice Age, it would have been mile-thick glaciers, and you would have seen these in the distance, and you would have stopped, and maybe you would have sought shelter in a cave, and you go into

this cave and you sense something is amiss. And you light a fire and you see on the cave walls, paintings in red ochre. And you see tools on the ground, stone tools. And then you hear something behind you and this shadow flickers across the wall and you turn around and you see a creature that looks like you,

a primate that's walking on two legs with a big head and eyes facing forward, but shorter, stockier, a bigger brow. So, you know, Homo sapiens meet Neanderthals, and this would have happened. And those Homo sapiens and Neanderthals, they were closely related enough that they could breed with each other the same way that, you know, lions and tigers can breed with each other. So they would have made it. with each other the same way that, you know, lions and tigers can breed with each other. So they would have made it. We have fossils of hybrids of a Neanderthal parent, a Denisovan parent, a Homo sapiens parent, you know. So we did, in a way, absorb these other species, but we also probably did fight with them over

territory and so on. We don't know the full story. It's a bit speculative. But what we definitely do know is that we would have mated with them. We would have hybridized. And the genes of Neanderthals and Denisovans, which are these other humans that were living in Asia, just discovered really over the last decade from DNA left behind in some of their bones, DNA that's so different from modern human DNA or Neanderthal DNA that it must be a new species. We still don't even have a skeleton of any of these Denisovans. The biggest prize in

paleoanthropology for all you young researchers out there is to find a skeleton of one of these Denisovans. But these Denisovans would have been in Asia when Homo sapiens left Africa. The Neanderthals would have been in Europe. We would have met them. We mated with them. the Neanderthals would have been in Europe. We would have met them. We mated with them. Our genome shows that we carry on the genetic legacy of those close human relatives that are no longer with us. And now is the reason these are different species, because you said it's successive waves of humans leaving Africa, but you know, we've also, Homo sapiens has spread around the world. And in some cases we didn't see those groups. Those groups didn't come back into contact with each other for many thousands of years. You know, you got, say, people in Japan, right?

Nobody from Europe met a Japanese person for many hundreds of years. But was it a much longer time period separating the Neanderthals from the successive wave of Homo sapiens? Is that why they ended up being different species as opposed to just, you know, different sort of differentiated members of the same species? Yeah, it's hard to know the exact sequence of events that allowed Neanderthals or Denisovans to become distinctive enough that they are classified as new species because they could mate with Homo sapiens. So in that sense, you might think, well, maybe they're all just one

big species or they're subspecies. It kind of gets down to semantics. But when it comes down to it, a Neanderthal, let's say, you know, their skeletons, their bodies, their physique was different enough from Homo sapiens that you can classify them in one group or the other. So there's distinctive features of the skeleton. And also genetically, although Homo sapiens and Neanderthals did interbreed, genetically they were quite distinctive. If you have DNA from a fossil, you can see, okay, this has the Neanderthal genome, Neanderthal signature, or this is Homo sapiens. You can build family trees based on the DNA and see the different clusters.

And so it's because they're both anatomically and genetically quite distinctive that they are called different species. it's because they're both anatomically and genetically quite distinctive that they are called different species. But we do have to remember they were similar enough and closely related enough that they could at least sometimes breed. Maybe that breeding wouldn't always be successful, but at least sometimes it was. And that's why we have these Neanderthal genes in our genome. Well, God, I could talk to you for a thousand years, but we have to let you go in a little bit. So I want to find a note to end us on here. I mean, obviously one of the reasons that you could describe mammals as reigning over the earth is that we as humans have this unparalleled ability to change the earth that we live on,

that no other species has really had to the same degree. But the account you're giving us, you know, it also gives me a sense of the immense amount of time that stretches back throughout history, the immense amount of changes that have happened throughout the history of the earth. And so, you know, it reminds me that like the current world that we live in is not the one that is going to exist forever. And that the earth is always changing. And so I'm curious from your view, when you look at this deep geologic evolutionary time, and then you look and then you refocus on the present day, what are the challenges facing mammals as a group? Right? What are what should we be worried about as a, not just species, but entire group of species? I'm not going to pretend to be a futurist. I'm not going to look too far into the future. I,

as a paleontologist, I'm comfortable looking into the past. I can tell you all about the past. And the past is relevant for context, for understanding our present world and thinking about where we may go in the future. But I don't want to get too predictive, because when you get too predictive, it becomes really uncertain. What I do know is this. Climates are changing very quickly today. Temperatures rising very fast. There have been many other times in Earth history when there have been global warming

episodes. The Earth has been through this kind of thing before, although it's happening much faster now than it ever has. It's happening over the course of a hundred years rather than tens of thousands. Exactly. And so we are kind of, and of course it's because of us too. So we are entering a state that is a bit different than anything that's come before. Yet we can look in the fossil record and see what's happened in the past when temperatures have risen. That's what I want people to do. I want people to understand the fossils, appreciate the fossils,

and see from the fossil record that when temperature's gone up X amount of degrees over a certain number of years, certain species die, certain types of species are better able to survive, and so on. We can get that context with the fossil record. I talk about this in the book, what some of the lessons are. I think when it comes down to it, ultimately, mammals are very resilient. Mammals have been through many things. Mammals have been through global warming spikes. They've been through ice ages.

They've survived the asteroid. They've survived big volcanic eruptions. They first developed on a supercontinent. They've continued to diversify as the continents have moved around. So mammals are resilient. And even during global warming episodes, they found a way to make it through. Although sometimes the dominant mammals have gone extinct and new ones have arisen in their place. Other times, a lot of mammals have actually gotten smaller when the temperatures have gone up.

That's just a physiological thing, being able better to shed body heat. So those are the lessons from the fossil record. The thing that we also see from the fossil record is that since Homo sapiens started moving around the world, something like 350 mammal species have died. And maybe that seems like a big number. Maybe it seems like a small number. But it's about 5% of all mammals. Wow. So this is not a catastrophic number in the sense of mass extinctions.

I said earlier that when the asteroid hit, 75% of stuff died. So we're not at mass extinction yet. Are we heading there? Maybe. What I will say is that mammals are probably at the most perilous position that we have been in since we had to stare down that asteroid. And it is largely because of climate change and environmental change, which is largely because of one single remarkable, but also quite destructive mammal species. That's us. But we also have this ability to understand what we are doing to the planet

and to do something about it. The dinosaurs had no way to stop that asteroid. They didn't know what it was. They couldn't understand it. We now are perhaps the asteroid to other species today, but we have the ability to know what we're doing and to hopefully mitigate that and looking into the fossil record, learning about our ancestors, learning about how real mammals, real ecosystems have responded to global warming and environmental changes in the past. That can give us important information to better prepare for our future. And that's where I want the conversation to be. I'm not going to sit here and tell you we're all doomed. It's, you know, I'm not going to sit here and tell you that X percent of mammals are all going to be dead in X number of years. I can't look into the future. What I do know is things are changing fast.

It's largely because of us. We have the ability to understand this. The fossils can help us understand this. And we have the ability to do something about it if we choose to do so. And it very much is a choice. And that is the essential message about climate change. And I really appreciate you sharing it with us. And it's so awesome that it comes from your understanding of this deep time and the changes that we've gone through. Look, I hope you'll come back sometime and we can talk more because I literally could ask you questions for another three or four hours. So it'd be fun. We talk about dinosaurs one day. We can, I love having these conversations and they're really, really fun.

So please do have me back. That's great. I absolutely will. Well, please folks check out the rise and the reign of the mammals. You can get it if you want at our special bookshop, factuallypod.com slash books. Steve Brussati, thank you so much for coming on the show. Couldn't, can't thank you enough. Adam, my pleasure. Thanks for inviting me. And what a great, fun conversation. And if, you know,

those of you that have listened, if you've had fun hearing this, I think you would enjoy the book. I'm easy to find on social media. So get in touch and let me know what you think about mammals. Great. Great. I want to thank our producer, Sam Rodman, and our engineer, Ryan Connor, and of course, everyone who supports this show at the $15 a month level on Patreon. That's Adrian, Alexey Batalov, Allison Lipparato, Alan Liska, Anne Slagle, Antonio LB, Aurelio Jimenez, Beth Brevik, Brayden, Brandon Sisko, Camu and Lego, Charles Anderson, Chris Staley, Courtney Henderson, David Condry, David Conover, Drill Bill, Dude With Games, M, Hillary Wolkin, Jim Shelton, Julia Russell, Kelly Casey, Kelly Lucas, Lacey Tyganoff, Lisa Matoulis, Mark Long, Miles Gillingsrud, Mom Named Gwen, Mrs. King Coke, Nicholas Morris, Nikki Batali, Nuyagik, Ippoluk, Paul Malk, Rachel Nieto, Richard Watkins, Robin Madison, Samantha Schultz, Sam Ogden, Shannon Grimmett, Spencer Campbell, Susan E. Fisher, Tyler Daruch, and WhiskeyNerd88. If you want to join them or support us at any level, head to patreon.com slash adamconover.

That's patreon.com slash adamconover. A big thanks to Falcon Northwest for building me the incredible custom gaming PC that I record so many of these episodes on. And thanks to Andrew WK for our theme song. You can find me online at Adam Conover, wherever you get your social media, or at adamconover.net, where you will also find my tour dates. Thank you so much for listening, and we will see you next week on Factually. That was a HeadGum Podcast.