Kendra Pierre-Louis: For Scientific American’s Science Quickly, I’m Kendra Pierre-Louis, in for Rachel Feltman.
From television shows like Land of the Lost and Terra Nova to the blockbuster Jurassic Park movie franchise, Hollywood loves to envision what it would mean for humans to live alongside dinosaurs.
But the truth is we already do. Birds, after all, are dinosaurs. But how did birds survive the extinction event that killed so many non-avian dinosaurs?
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Steve Brusatte, a professor of paleontology and evolution at the University of Edinburgh and the author of the upcoming book The Story of Birds, dug into the subject of bird survival in the May issue of Scientific American. He’s here today to speak with us about it.
Thanks so much for joining us today, Steve.
Steve Brusatte: My pleasure, Kendra. Thank you.
Pierre-Louis: So it’s funny because I think most people by now know that birds are dinosaurs, but I don’t think most of us really kind of think about how they survived while, like, dinosaur species like the T. rex didn’t. You recently wrote a feature for Scientific American digging into how birds survived. What interested you in the subject?
Brusatte: Yeah, so the article I wrote for Scientific American, it tackles that question, which really has been a mystery for a long time among paleontologists: Why is it that birds were the only dinosaurs to survive that asteroid that fell out of the sky 66 million years ago and changed the trajectory of evolution?
And this is also something I write about in The Story of Birds; it’s in one of the middle chapters. So the book tells the whole story of birds: how they evolve from dinosaurs, what it means that birds are dinosaurs, how birds survive the asteroid and then all the amazing things birds have done since then. But of that entire story, I really do think it is this mystery of “Why did birds have what it took to get through that asteroid, to stare down that asteroid, to endure that worst day in the, the history of life?”
And part of the mystery here is it just seems so unfathomable. You have these dinosaurs like T. rex and Triceratops and the long-necked dinosaurs. They had been around for over 150 million years. They lived all over the world. They were at the top of the food chain. They were the biggest meat eaters, the biggest plant eaters. They had incredible diversity. They were utterly dominant. And then all of a sudden they’re gone. But one peculiar type makes it through, and those are the birds.
It’s an incredible mystery, and it really has been a mystery for scientists until quite recently, and I do think—not that we understand it completely now. We’re dealing with fossils. We’re dealing with these clues from millions of years ago that we have to interpret like detectives. But I think we have a pretty good handle now on why birds survived.
Pierre-Louis: Okay, so before we get there, can you, like, set the scene of what it was like back when birds and dinosaurs co-existed? I mean, I know birds are dinosaurs, but, like, when birds and, like, kind of the charismatic dinosaurs that we all think of, when they overlapped, what was it like?
Brusatte: Two hundred and thirty million years ago or so, back in the Triassic period, that’s when the first dinosaurs entered the scene. And this was back during the supercontinent of Pangaea, when all of the world was globbed together into this one giant landmass. And it was on that supercontinent that the first mammals and the first dinosaurs had their origin story.
Now, the first dinosaurs were quite simple. They were small. They were the size of dogs, the size of people …
Pierre-Louis: Hold on—so you’re telling me that the early dinosaurs were the size of dogs? I could have had a pet dinosaur?
Brusatte: [Laughs.]
Pierre-Louis: [Laughs.]
Brusatte: You could’ve. You could’ve. The first dinosaurs, they would’ve been really cute. They would’ve been quite adorable. You could’ve held ’em in your arms. They looked nothing like a T. rex, nothing like a Brontosaurus, nothing like a Triceratops. Those dinosaurs would come later.
And really, the big dinosaurs, it took tens of millions of years, and it took dinosaurs surviving a great extinction at the end of the Triassic, about 200 million years ago, as the supercontinent broke apart, and you had a time of stupendous volcanism—giant volcanoes erupting all along. The earth was cracking and breaking apart, and that led to global warming, and it led to a mass extinction, and dinosaurs had to endure that.
And we don’t exactly know why and exactly how they did it, but they did. And then in the Jurassic period, the next interval of time, that’s when dinosaurs started to become the sublime creatures that we all know and love: the giant long-necked ones that were heavier than jet airplanes, the meat eaters the size of buses, the ones with horns and spikes and spines and duck bills and dome heads and all those sublime things we think of when we think of dinosaurs. That’s really the Jurassic period when that started.
And by the end of the Jurassic period, about 150 million years ago, you had another type of dinosaur enter the scene and break the bounds of Earth and fly, and these were the birds. You had small dinosaurs evolve feathers and wings and start flapping those wings, and those dinosaurs took to the skies.
And for the next many tens of millions of years, you had plenty of dinosaurs living all over the world—classic, canonical dinosaurs like T. rexes and Triceratopses—but living with them were birds, the same way that today you have all kinds of mammals living around the world, including bats, one weird type of mammal that has wings and can fly.
Now, for many tens of millions of years, the birds living with dinosaurs, they were adapting; they were changing. And a lot of the first birds living with their dinosaur cousins were quite primitive. They still had teeth. They still had big claws on their hands, like a raptor dinosaur. They still had long tails. They had small wings—they couldn’t fly particularly well.
But over time they adapted to the air. They became better flyers. They evolved bigger wings. They evolved larger muscles for flapping those wings. They turned their tail into a rudder for steering and braking. Their bones hollowed out and became full of air. They traded their teeth for beaks. And they developed this lightweight, hyperefficient, fast-growing body that was the ultimate flying machine.
And that’s where things stood 66 million years ago, on the last day of the Cretaceous period, at the moment that everything changed forever.
Pierre-Louis: That’s a really perfect picture.
In the piece, you write, “to understand why birds endured when the nonavian dinosaurs went extinct, we must first consider the overall roster of victims and survivors.” Can you talk about which species tended to survive after the Chicxulub impactor? Did I say that right?
Brusatte: You did!
Pierre-Louis: [Laughs.]
Brusatte: The Chicxulub asteroid. This is the agent of doom and destruction. So I think most of us are familiar now with this idea that an asteroid killed the dinosaurs, which is what happened, for the most part. But to unpack it a bit, 66 million years ago, at the end of the Cretaceous, the world was teeming with dinosaurs. What they had no idea about was that there was a rock, a [roughly] six-mile-wide rock, hurtling through the blackness of outer space. [Laughs.] It was a, a leftover crumb from the formation of our solar system, and it was traveling well more than 10 times faster than a speeding bullet.
And it, it could have gone anywhere; it was a piece of space junk. But it made a beeline for Earth. It crashed into what is now Mexico. It impacted with the force of over a billion nuclear bombs put together, punched a hole in the face of the Earth that’s over 100 miles wide—you can still see a lot of that crater in Mexico today, near Cancún. And that impact was so forceful that it triggered a cascade of destruction: earthquakes, tsunamis, wildfires, volcanoes going into hyperdrive. And that’s just what happened in the immediate minutes and hours and days after the impact.
The bigger problem was all the soot from the fires, the dust and the dirt and the grime from the collision of the asteroid. This stuff went into the atmosphere, circled around the Earth, formed this suffocating cloud that blocked out the sun, probably for several years, and the Earth went dark and cold. It was a long winter. And plants were starved of sunlight, and they couldn’t photosynthesize; they couldn’t make their own food. And so ecosystems collapsed like houses of cards on land because plants but in the ocean as well because of the plankton at the base of the food chain that also photosynthesize.
So you had ecosystems around the world collapse, and 75 percent of all species died—75 percent. So that means if you were alive the moment the asteroid hit, your entire species had a one-in-four chance of enduring. The odds were so low. This was the worst day in the history of Earth. I really do mean that without hyperbole. I think that was probably really the case.
What happened was a lot of animals died and all of the classic dinosaurs died: the T. rexes and Triceratopses and so on. And the reason that we think they died, in large part, was because they were big. Everything that lived on land bigger than a husky dog died, probably because they couldn’t get enough food, probably because they couldn’t hide very easily. It was hard for them to dig a burrow or find a place to hide from the fires and the earthquakes and so on. If you were big, you were in trouble.
And then beyond that, though, a lot of other animals, even that were smaller, died as well: ones that had more specialist diets, if they only ate a certain type of food; or if they were smaller but couldn’t dig burrows, couldn’t hide very easily; or if it took them a long time to grow from a baby into an adult. These were all impediments that would hold you back when the world was changing so quickly. There was no time for evolution to work through the normal processes of natural selection, shaping species and the populations over generations. No, no, you had to confront that asteroid with whatever hand of cards you already had. And if you were big and you grew slowly and you needed to eat a lot of food, game over. And that is fundamentally what explains why so many species die.
Pierre-Louis: And so birds had an advantage because maybe they couldn’t burrow, but they could at least fly to get away from predators, and overall they tended to be smaller. But not all birds survived, only one category of birds. Can you talk about that?
Brusatte: Right, and so that is exactly the case, and this is where the mystery is. So it makes sense that generally birds would have a better chance of surviving, but it’s not that every bird survived. In fact, most birds died. And the fossil record, it, it’s not perfect, of course, and we’re often dealing with fragmentary clues, but best we can tell, 90 percent of the birds that were there the day the asteroid hit, they followed T. rex and Triceratops to the grave.
And at the moment the asteroid hit, there were still birds with teeth. There were still birds with long tails. There were still birds with raptor dinosaur claws on their hands. All of those birds died out with T. rex and Triceratops. The only birds that survived were the modern-style birds, the ones that we know today.
And so these are the birds that have beaks instead of teeth, the ones that have especially big wings and big muscles on their chest for flapping those wings. Modern birds are the ones that grow fast—you barely have a chance to see the babies in the nest. They go from hatchling to an adult within a few weeks or a few months in most species. So it’s only those types of birds that survived.
And probably all of those things I just mentioned would’ve increased their odds of survival, but more than anything, we really think that the beaks were important. And it might seem really trivial. It’s, like, we don’t think about it very much: birds have beaks; they don’t have teeth. But remember, a lot of birds did have teeth, and the ones with teeth all died. The ones with beaks were the ones that survived. It’s probably not a coincidence.
And we think—we don’t know for sure, of course, because we weren’t there to witness this, thank goodness, 66 million years ago—but what we do know is that a lot of these birds were seed eaters. We know—in fact, we find seeds sometimes preserved in the stomachs as a last meal in some fossil birds. We know from modern-day birds that beaks are often very good at eating seeds.
And when the asteroid hit and blocked out the sun and the forest died and the trees died and those ecosystems collapsed, if you were an animal that ate leaves or twigs or fruits or flowers or roots or other parts of a growing tree, you were in trouble. And then if you were an animal that ate those plant eaters, you’d be in trouble, and so on. It would cascade through the food web. But seeds might have been a ticket to survival.
Why? Because seeds can stick around in the soil for a long time. They’re hearty. They’re robust. And so if you could eat seeds, you might have had access to the last remaining food source during those few years of global winter. So we think that was really important. And it’s something as subtle as that, in combination with being able to fly well and grow fast and reproduce quickly and so on, that was probably the winning hand of cards for modern birds.
And what I want people to take away if they read my articles, read my books is that when you look at the world around you today—there’s a window in front of me. There is a pigeon outside. I mean, we all know pigeons, right? They don’t seem very special or very important. They might even make us, I don’t know, feel a little bit icky: “Oh, it’s a pigeon.” But in that pigeon I’m looking at right now, that’s a dinosaur. It evolved from dinosaurs. It’s part of the dinosaur family tree. It has dinosaur blood running through its veins. And it had ancestors that survived that asteroid when the other dinosaurs couldn’t.
I mean, we know birds are awesome—when we hear a parrot mimic our speech, when we hear a songbird and its beautiful songs, when we see a crow fashioning tools. Birds are awesome. But more than anything, birds are great survivors. They’ve been through so much, and they are real-life living dinosaurs sharing the world with us, and I just think that’s really cool.
Pierre-Louis: So earlier you were talking about how the large dinosaurs were kind of the top of the food chain. They were all over the globe. They really made their mark on planet Earth. And I couldn’t help but feel parallels to humans. [Laughs.]
In doing this work and looking at extinction and looking at what survived, and then sort of looking at humanity right now and how, you know, we’re dealing with climate change, we’re dealing with kind of these big, potentially ecosystem-altering changes, do you see any parallels there?
Brusatte: I do, I do, and I, I don’t wanna get too philosophical about it, but I do think that dinosaurs and mass extinctions from Earth history, there are lessons that we can learn from them. They’re relevant to us. These aren’t just monsters from primeval times that have no bearing on us, because after all it was the disappearance of the dinosaurs, it was the asteroid knocking them off of their perch at the top of the food chain, that’s what paved the way for our mammal ancestors to take over and eventually for us to evolve. So really, the dinosaur story and the asteroid story, that is our story, too.
And I talk about this, I talk about it at the end of the book: I wanna tell a story of evolution. I want people to be thrilled and exasperated by this great evolutionary journey of birds and how they came from dinosaurs and how dinosaurs evolved feathers and wings. But in telling that story, I do wanna try to make it relevant and make that connection that the world is changing very quickly today. Climate, of course, we know is changing very quickly. Temperatures are rising, but sea levels are changing.
And dinosaurs tell us that those species that are dominant, that are at the top of the food chain, maybe they’ve been around for millions of years—and we have not, by the way; our species has been around for only 300,000 years. You know, dinosaurs like T. rex and Triceratops, their family ruled the Earth for over 150 million years. But it didn’t matter because when that asteroid came down and things changed so quickly, climates and environments changed so quickly, there wasn’t time for dinosaurs to adapt, and they were now suddenly on the back foot. They had grown accustomed to the world as it was.
And really, it tells us that the most dominant, the most sublime, the most stupendous, the most spectacular, the most successful creatures, they can be in trouble if things change quickly around them. So the lesson I hope people take is that everything’s connected. The Earth has changed before. We can learn from these past extinctions. And just because you’re dominant and you’re successful at the moment, it doesn’t mean that you’re not vulnerable to being affected in the future if things change quickly.
And that’s a lesson for humans, but it’s also a lesson, you know, for birds. The birds of today are under great peril. And it’s all because of the rapid changes in climate, in temperature, in land use, in environment that, unfortunately, we humans have brought to the world today.
Pierre-Louis: That’s all for today! Tune in on Monday for our weekly science news roundup.
Science Quickly is produced by me, Kendra Pierre-Louis, along with Fonda Mwangi, Sushmita Pathak and Jeff DelViscio. This episode was edited by Alex Sugiura. Shayna Posses and Aaron Shattuck fact-check our show. Our theme music was composed by Dominic Smith. Subscribe to Scientific American for more up-to-date and in-depth science news.
For Scientific American, this is Kendra Pierre-Louis. Have a great weekend!