How Mammals Rose From the Doom of the Dinosaurs


An illustration of an extinct giant ground sloth.

Megalonyx, an extinct ground sloth.
Illustration: Steve Brusatte / The Rise and Reign of Mammals

Deep time is a headache. So many changes have happened in the cosmos, on our planet, and through evolution to create the world we know today. One of the most recent major changes, relatively speaking, was the K-Pg extinction, the mass death of the dinosaurs and many other creatures in the immediate aftermath of an asteroid impact 66 million years.

The post-impact devastation turned out to be a boon for mammals, who suddenly had the ability to grow much larger and evolve in ways they couldn’t when giant reptiles roamed the planet. Fast-forward a bit, and those mammals (namely Homo sapiens) have made more sweeping alterations to the planet than any creatures before.

How did we get here? I spoke with Steve Brusatte, a paleontologist and evolutionary biologist at the University of Edinburgh. He’s the author of The Rise and Reign of the Mammals, a new book detailing the evolutionary and environmental changes that allowed mammalian life to diversify and evolve into the biological tableau that exists today. The work follows Brusatte’s previous book The Rise and Fall of the Dinosaurs. Below is our conversation, lightly edited for clarity.


Isaac Schultz, Gizmodo: First, a simple question—or perhaps the most complicated one. Why write this book?

Steve Brusatte: The easy answer is, “well, I did one on dinosaurs, what am I going to do next? I’ll do it on mammals.” I started out as a dinosaur specialist; I studied the origin of dinosaurs and the evolution of birds from dinosaurs and the extinction of the dinosaurs. As a researcher, the next logical step was to think about what came after the dinosaurs. How did the world change? That [answer], of course, is mammals.

The more I’ve studied mammals, the more I’ve realized how fascinating they are and how much the story of mammals is our story. As awesome as dinosaurs are, in many ways they’re alien creatures—there’s nothing really like them. We have birds, of course, but mammals are us. The evolution of mammals is very much our story, our deepest ancestry, which has ultimately produced us and many other mammals today.

Gizmodo: Your book basically covers from the K-Pg extinction 66 million years ago to today. Was there anything in particular about the way the public thinks about those intervening millions of years that you were seeking to address?

Brusatte: I think there’s this misconception that dinosaurs had their time, they thrived, and they died out, and mammals evolved to take their place. And certainly mammals did quickly replace dinosaurs as the big preeminent animals on land. But I think a lot of people don’t realize that mammals and dinosaurs go back to the same time, same place. Both of those groups originated about 225 million years ago, back in the Triassic period on the supercontinent Pangea, and they went their separate ways.

The dinosaurs were destined for grandeur. Some of them became enormous. Mammals were relegated to the shadows. They couldn’t get big. The dinosaurs held the resources and the ecosystem. So mammals and dinosaurs actually lived together for over 150 million years. And mammals never got bigger than a badger during that time, but they were very diverse, there were lots of species of mammals. They were scurriers and climbers and diggers and swimmers. Some glided on wings of skin. There were mammals that ate dinosaurs for breakfast! There is a mammal with dinosaur bones in its stomach, preserved as a last meal.

Pleistocene animals roam on the book's cover.

Brusatte’s book walks readers through the evolution of mammals.
Illustration: Harper Collins

But these mammals were small, and they mostly were nocturnal, and you would probably not even notice them if you were around then. They were diversifying in the shadows. They needed to survive, they needed to adapt. And that’s where things like hair and milk and warm-blooded metabolism and fast growth and huge brains, acute senses of smell and hearing— all these classic mammalian things—they were evolving as mammals and their ancestors were trying to endure in the dinosaur-dominated world. And so I want to get that across.

And then, of course, the asteroid. The dinosaurs die. Some mammals survive by virtue of their smallness and their adaptability. And then we have this new world to take over.

Gizmodo: Name one adaptation someone reading this has but they don’t think about much—one that’s crucial for the way that they (and all of us) experience life.

Brusatte: Our ancestors had lots of bones in their lower jaw. The dinosaur has lots of bones in its lower jaw. Mammals only have one, we have one single bone—the jawbone, all of our teeth are in that bone, and all the muscles attach to that bone. As our mammal ancestors evolved, basically they took the jaw of their ancestors and they simplified it down.

That meant there were all these the extra bones. But what to do with them? Some of those bones just disappear (and sometimes things disappear in evolution), but some of those bones stuck around, and they took on a new purpose. They became tiny, smaller than a grain of rice in us. They moved into the ear and they amplify and transmit sound from the eardrum to the cochlea, which is on the part that transmits to the brain. So these things that used to be jawbones in our ancestors became earbones in mammals.

This is what allows mammals to hear so well. Mammals hear really, really, really well. We have a great sense of hearing compared to say, birds or lizards or snakes, and these extra little bone that came from our jawbone that allow us to do it. We start to develop in the womb—those bones actually start out attached to our jaw—and over the course of our gestation, they shrink and they move up into the ear. It’s a really cool evolutionary story.

Gizmodo: How much did recent academic scholarship shape the story that you tell in your book, versus more canonical understandings of mammalian evolution?

An ancient monkey fossil.

A 47-million-year-old monkey fossil.
Photo: Mario Tama (Getty Images)

Brusatte: There’s a lot of recent science, new science in the book. From the same ecosystems where the feathered dinosaurs were preserved as fossils, entire ecosystems were buried by volcanoes, almost Pompeii style. It locked in so many fossils, and it’s really only [in] these mammal [fossils] that you’ve finally got this detailed preservation, where you’re able to preserve these little mouse and rat-sized things as fossils. And that’s totally changed our understanding.

People used to think that the early mammals that lived with the dinosaurs were all dull and dreary. Yeah, they were all tiny, they were all kind of generalized. They couldn’t do very much; they’re eking out this meager existence in the undergrowth. But now we know that they seized on those small niches. So that’s one example.

There’s also just a lot of new evidence from DNA that I have in the book. We have ancient DNA now from some fossil mammal groups that for centuries have been puzzling. There’s some mammals in South America in the fossil record. They don’t live anymore—the last ones died out during the Ice Age—but they’re bizarre. They’re very big. Indigenous people in South America encountered their bones and tried to to understand what they were. When the Beagle dropped anchor in South America, [Darwin] went in and collected so many fossils he didn’t know what to do with them. He sent them to London. They seemed like they were a weird kind of Frankenstein monster mash-ups, you know, a little bit of rodent here, a little bit of horse there, a little bit of elephant there. Just completely odd, because South America used to be an island unto itself, for many tens of millions of years.

So what are these things? We didn’t know until a few years ago, until somebody found some DNA and some protein in some of the fossil bone and was able to pull out that molecular information, subject it to the paternity test in our DNA. And lo and behold, these things are members of the horse group. They’re are odd-toed hoofed mammal. The DNA revolution has really helped us understand mammals. There’s a lot new in this book, and I think it would’ve been hard to write it a decade or two ago.

A herd of modern horses.

A herd of wild horses in Germany.
Photo: KEVIN KUREK/DPA/AFP (Getty Images)

Gizmodo: Do you have a favorite epoch of mammalian life or a favorite timeframe in which mammals quickly evolved, quickly proliferated?

Brusatte: The 10 million years after the asteroid is the time called the Paleocene Period, and we don’t know a lot about the mammals that lived at that time. We have a lot of fossils, but they’ve been very puzzling mammals. We can tell that they’re placental mammals like us that would have given live birth to well-developed young. But it’s very hard to figure out what they’re related to, where they fit on the family tree. These fossils have been known for centuries, and they are the key to understanding how mammals survive the extinction and how they proliferated afterwards and how they set the foundation for today.

I do a lot of fieldwork in New Mexico with my students, and my colleagues are looking for the fossils of these Paleocene mammals. Each new fossil is a clue that might tell us something about what these animals were like, who their relatives were, or whether any of them are actually early cousins of ours, early primates or early dogs, or early horses, and what they were like. They were the ones that stared down the asteroid and then soon thereafter took on the world as their own as the dinosaurs were gone. Now mammals could get bigger. Within a hundred or 200,000 years, there were mammals the size of pigs—remember, they never got bigger than a badger during the previous 150 million years.

An illustration of the elephant-like Deinotherium.

Deinotherium, an ancient proboscidean.
Illustration: Steve Brusatte / The Rise and Reign of Mammals

Gizmodo: What were some of the challenges that mammals faced in those millions of years after the K-Pg extinction? And how do those challenges compare to our current period of mass extinction?

Brusatte: In the 66 million years since the asteroid hit, there have been so many changes to the Earth, especially with temperatures and with the ecosystem. After the asteroid, temperatures were really hot for a good 10 million years or even more. This was the hottest Earth had been in quite a long time. And there were global warming spikes and mammals had to adapt, but then the Earth started to cool down, and there’s this long-term cooling trend the mammals had to deal with. Now, that also changed the environment. What once was a lot of jungle turned into a lot of open land. That’s where grasses came in. This was only when grasses began to spread globally, got proper grassland, savannas, prairies, starting about 20 million years ago. And then temperatures got even colder in the ice age… mammals have been along for the ride. They had to adapt to this roller coaster of climate. And all along, mammals diversified.

Gizmodo: What is the weirdest species that’s no longer around? I mean, what should we be really frustrated we don’t share the planet with?

Brusatte: There’s a bevy of extinct mammals that are superb, sublime, and spectacular things we would never know ever existed if we didn’t find them as fossils. I think these things called chalicotheres are the weirdest things. They look like they’re a hybrid between a gorilla and a horse. They’re hoofed mammals that are completely extinct, but they walked on their knuckles. They only died out fairly recently. In fact, some of our ancestors would have encountered them. There’s nothing alive like them today. If they still persisted, they would undoubtedly be the most popular exhibit at zoos.

And the other thing I’ll say is that there are still some sublime mammals that are with us today. It really bears emphasizing that the biggest animal that has ever lived, the biggest organism that ever lived in the entire four-and-a-half-billion-year history of the Earth is the blue whale. And it is still alive. This thing is longer than a basketball court; it’s like submarine size. It weighs over 100 tons. Its babies are the size of steamboats when they’re born, and it can dive to depths of thousands of feet. I don’t think we appreciated it enough, but I think we can imagine an alternative reality where blue whales are extinct and all we have are some petrified bones. I think in that sort of world, whales would be as iconic as any dinosaur. So, let’s make sure we conserve them.

The skull of a glyptodont found in Venezuela.

A fossilized armored glyptodont from Venezuela.
Photo: JUAN BARRETO/AFP (Getty Images)

Gizmodo: Is there anything about your book you’d like to emphasize for someone about to open it for the first time?

Brusatte: I’d like people to see this book as a story of us—but of our deepest evolutionary history. Humans only appear towards the end. It is not a book about humans but a book about all mammals, and it puts us humans in perspective. After all, we are one of many types of superlative mammals that have evolved over time, from bats and whales to elephants and even those dinosaur-eating Cretaceous mammals. It’s not all about us! With that said, we are a particularly sublime species, with our intelligence and consciousness and ability to work in groups and build and create. These things allow us to have such impact on our planet, both good and bad, like no mammal before. They are things that make us the most dangerous mammals that have ever lived, and the mammals that most affect and harm other mammals. Yet, these talents may be our salvation. We can choose to change.

Gizmodo: You also worked on the latest Jurassic Park movie. How do you see your role working on the film, and are there any mammals we’ll be lucky enough to see in it?

Brusatte: I watched the first Jurassic Park film in the cinema in 1993, with my dad and brothers. I was 9 years old. The dinosaurs were so realistic, like actual animals, so different from the images in the books in the library and at school. To work on the sixth film, nearly 30 years later, is surreal. My role was to consult and advise. I was on call to answer any questions that the director, the character designers, the artists ever had. Mostly these were facts about dinosaurs—how big they were, what they looked like, how they behaved. I saw it as my role to make sure the real science, the real fossils were always in the ear of the people making the Hollywood magic. And to make sure that knew the real science, so that could take that into account when designing their movie monsters.

To my great pleasure, there are proper feathered dinosaurs for the first time in the Jurassic series, and there are two mammal ancestors! You will see Dimetrodon and Lystrosaurus: two early synapsids, members of the ancestral stock that gave rise to mammals. I am psyched for these mammal antecedents to finally get their share of the limelight!



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