Exactly how did birds evolve from dinosaurs? It’s a mystery that has been with us for more than 150 years, and palaeontologists are still hunting for pieces of the puzzle today.
Among them is the University of Edinburgh’s Professor Steve Brusatte, whose latest book, The Story of Birds, tells the whole fascinating story. We caught up with him recently to find out more.
Of all the great dinosaur subjects, why this story?
I’ve always been fascinated by birds. They are all around us and there’s such a stunning diversity and variety. As a palaeontologist I specialised early in the theropod (two-legged) dinosaurs. This is the group that includes T.rex and Velociraptor – and gave rise to birds.
The more I studied theropods, the more I became more curious about the modern-day animals that descended from them. Back in the early 2010s my PhD was about the origin of birds. Its core involved building a big new family tree of theropod dinosaurs to understand where birds slot in, how they evolved from dinosaurs, and how their body features came together.
I wrote about the dinosaur bird connection in my first book, The Rise and Fall of the Dinosaurs (2018), but that was just one chapter. It made me think it would be really fun to do an entire book on the subject. That was how my new book, The Story of Birds, came together.
Is there still any debate about birds evolving from dinosaurs?
I think people have generally heard that birds descended from dinosaurs. In the newer Jurassic World films you even see feathers on some of them. And yet it hasn’t really broken through to the public consciousness that today’s birds really are dinosaurs. They are part of the dinosaur family tree. They just happen to be a peculiar group of dinosaurs that got small and evolved wings, took to the skies and have survived until today.
It was Charles Darwin’s great disciple, Thomas Henry Huxley, in the 1860s who first noted similarities between the skeletons of some dinosaurs starting to be found in Europe and those of modern birds. This was back before anybody knew what DNA was, for instance.
Huxley’s idea did enter the public consciousness, at least in Victorian Britain. Darwin added it to the later editions of On the Origin of Species. But then it went out of favour. This was the great era of exploration, especially in the US and Canada. The frontier was being pushed westwards, and all these new dinosaurs were being found – Stegosaurus, Brontosaurus and later Brachiosaurus and T.rex.
Read more: The story of birds: a new history from their dinosaur origins – extract of Steve Brusatte’s new book
None look anything like birds. I think dinosaurs obtained this stereotype as giant reptilian monsters, and this still largely dominates the public consciousness today.
Yet there were also a lot of smaller dinosaurs. Many had feathers and wings, and many were very bird-like. It’s really only in the past few decades that the idea that birds evolved from dinosaurs has become scientific consensus. The discovery of feathers on dinosaurs in the 1990s really sealed the deal on that.
What mysteries remain?
There are of course still things we don’t know, like how dinosaurs started to fly. How did they start to move their wings in a way that generated enough lift and thrust to get them airborne? Did they run on the ground and use their wings to defy gravity? Did they do it from the trees down, using these wings as a way to manipulate gravity? That’s one of the biggest mysteries.
Another area of uncertainty is which dinosaurs were the closest relatives of birds. The more fossils we find, especially feathered dinosaurs in China and other places, the more it’s clear there was a whole bunch of small dinosaurs with feathers. A lot had wings, some had wings only on arms, some on arms and legs. Some had wings of feathers. Some had wings of skin like a bat.
There was a huge diversity of them right around that point in the family tree where proper modern-style birds evolved with big arm wings that they flap to keep airborne. Each new fossil gives us more information but also another layer of complexity. It makes it just a little trickier to untangle the knot of exactly which dinosaurs were the closest rivals of birds. You still see new discoveries being made every year.
You say in the book that wings evolved not to fly?
The fossils tell us clearly that feathers evolved long before any of these animals were flying. Many dinosaurs had simple feathers; they looked like little strands of hair. In fact most dinosaurs probably had them – they just don’t normally preserve because they decay away so quickly. It’s in spectacular fossil sites where lots of dinosaurs were buried quickly, usually by volcanic eruptions, where you see a lot of these feathers (Liaoning province in north-eastern China is a good example).
But these feathers were not used for flying. There’s clear evidence from the fossil record that feathers evolved in a simpler form for other reasons. Our best hypothesis is they evolved for insulation, to help them stay warm – just like hair in mammals.
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Later on, these feathers evolved on some dinosaurs into quills that made up wings. But the fossil record shows that the first wings that show up in dinosaurs between the sizes of sheep and horses. Those wings were only about the size of laptop screens, and by the laws of physics, those could not keep an animal of that size in the air.
That hints that wings probably also evolved for another reason and were only later co-opted for flying. We can tell a lot of these feathers had flamboyant colours and patterns, so one leading idea is that wings first evolved for display, to attract mates; to intimidate rivals. This is still true today, of course.
You can imagine if those wings got bigger over time, more flamboyant, more ornate, at some point the laws of physics would take over and they would generate some of those aerodynamic forces. It’s not like we have fossils of the exact dinosaurs that were the first to flap their wings, but that is at least what the fossil record is telling us.
Did dinosaurs have to get smaller for flying birds to evolve?
This is a big part of the story. Some dinosaurs, such as T.rexes, got bigger over time, but the dinosaurs that evolved into birds had been getting smaller for tens of millions of years. We don’t know why exactly, but there’s all kinds ecological niches where it pays to be small: it’s easier to hide, you can grow more quickly, and so on.
So it seems you had this group, that their bodies were getting smaller, and their wings were getting bigger. At some point you had a wing that was big enough to keep a body that was small enough in the air. At that point, natural selection could take over and start refining these dinosaurs into ever better flyers.
Is it an accident of evolution that flying creatures the size of elephants don’t exist?
Animals that need to flap wings to fly can’t be that big. The biggest flapping flyers today are wandering albatrosses, and their maximum wingspan is about 3.5 metres. We have fossils of birds that were bigger: the Pelagornithids were giant soaring birds that went extinct right before the ice age. They had wingspans that were something like 7 metres long. But beyond that, I think it would be very hard to flap wings to fly.
It makes total sense to me that it was probably a crow-sized to lapdog-sized raptor dinosaur that first started to flap as opposed to some dinosaur the size of an albatross. It’s just that the stereotype of dinosaurs being huge makes it harder to envision some small dinosaurs flapping and flying.
How did birds survive the asteroid?
That was a big mystery for a long time. There were proper birds at least 150 million years ago, which means they lived alongside their dinosaur cousins for some 80 million years. Then the asteroid comes down around 66 million years ago and all the dinosaurs die except the birds – why is that?
The reality is that lots of birds went extinct at the same time as the other dinosaurs. Many birds were still quite primitive and would have looked a lot like their dinosaur cousins. The only ones to survive were very modern-style birds. They had beaks instead of teeth, big wings and large chest muscles, and could grow really quickly like birds today.
A lot of recent research has clarified why they survived. What it comes down to is: the asteroid was a shot out of the darkness of outer space, a six-mile wide rock that smashed into the Earth one day. It changed everything instantaneously. There were earthquakes and tsunamis and wildfires. There was dust blocking out the sun, giving rise to a nuclear-style winter that lasted several years. Natural selection can’t work on that timeframe, so when the asteroid hit, all the animals had to confront the situation with the features they already had.
Most of the dinosaurs were big, and nothing bigger than a husky dog survived on land. With all these fires and acid rain and storms, simply being outside and exposed to the elements would have been bad. If you were smaller you could hide away more easily.
Also, modern-style birds had a bunch of features that turned out to be beneficial. They grew to adult within year, so it didn’t take too long for them to nurture the next generation. They could fly away from danger. But crucially they also had beaks, which could have allowed them to eat seeds.
When the Earth went cold for many years, ecosystems collapsed. Plants did not have sunlight to photosynthesise. So plant-eaters died, which meant meat-eaters died. Seeds were probably the last foods that survived. If you could eat them, it could probably have got you through those lean years.
We have gut content of birds from the Cretaceous period (145 to 66 million years ago) and we can tell a lot of them did eat seeds. So the modern-style birds had a good hand of cards just as the world became this fickle casino and survival was a matter of the odds.
Which bird species appeared after the asteroid?
Bird fossils from the Cretaceous (meaning before the asteroid) are limited because it’s hard to fossilise birds. They’re small and their bones are really delicate. But we do know there’s birds like Vegavis and Asteriornis that lived in that period and were respectively members of the modern groups of ducks and chickens.
It doesn’t mean other modern species like owls or falcons weren’t there, but certainly they were not a major component of the ecosystems at the time. Then the asteroid hit and we start to see in the Paleocene (66 to 55 million years ago) fossils of things like penguins, mouse birds and multiple other modern groups.
Yet the really strong evidence about what happened is from the DNA of modern birds. Researchers are using whole genomes now. They can compare the similarities and back-calculate to predict when two groups would have diverged. When you do this, it predicts there was a big bang of bird evolution right around that time – including species like owls, parakeets, falcons and hawks.
It makes sense that if you have a mass extinction that kills 75% of species, there would have been abundant opportunity for whatever survived. But we’re still waiting for fossils to confirm this directly. It’s a real target for people doing fieldwork to confirm this story by finding the fossils of birds up to 5 to 6 million years after the asteroid.
You write that great birds have come and gone – talk us through some of those
There are more than 10,000 species of birds today, basically double the number of mammal species, so in that sense we’re still in a dinosaur world. But there are even more incredible extinct birds, some of which went extinct quite recently because of us, as we’ve spread around the world and changed the environment very quickly.
A lot of these fantastic birds got their start in the ecological vacuum after the asteroid. There were birds that became basically born-again T.rex and Triceratops – filling the top predator/top plant-eater role in a lot of ecosystems.
In South America were the “terror birds” (Phorusrhacidae). They stood taller than a person, had a head the size of a horse head and a massive hooked gnarly beak. They were the top predators there for tens of millions of years. South America was an island for lot of that time; only later did jaguars and big dogs arrive.
In many places, birds were the biggest plant-eaters. Australia had birds called demon ducks (Dromornithidae) that lived for tens of millions of years. Think of the modern duck and super-size it by 100. Some were heavier than cows.
Elsewhere there was New Zealand’s moa and Madagascar’s elephant bird. Elephant birds were maybe the heaviest birds of all time. They laid eggs the size of watermelons. Many of these birds couldn’t fly. They gave up that ability as a trade-off to allow them to become really big.
The Pelagornithids also really fascinate me – the birds that were double the wingspan of an albatross. They lived for tens of millions of years, sailing the world’s thermals like giant kites. They would have been utterly spectacular animals.
We only know about most of these birds because of fossils – except for some like the moas and elephant birds and demon ducks, which did meet humans but didn’t last long, unfortunately.
Is it surprising birds never became as intelligent as humans?
When I was growing up in the late 1980s and through the 1990s, it was an insult to say “you’re a bird brain”. It’s such an unfair biological slur, because birds are very smart.
It’s just that they have small brains – I don’t know how many hummingbirds could fit into the head of an elephant. But when it comes to the size of the brain relative to the size of the body, which is largely what matters for cognition, problem-solving and so on, birds are right up there with mammals.
Song birds learn intricate songs. Similar to a human language, they learn them from tutors, they babble when they’re young and make mistakes, then master their avian language later on.
Parrots can mimic human speech. And whereas plenty of animals use tools in a rudimentary way, some crows can make their own tools. It’s really only crows and humans and maybe some close primate relatives that do that. Crows take sticks and branches and twist and turn them. They make hooks out of them and use them to probe for food.
Since the asteroid, there were probably long stretches where it was actually birds that were the cognitive superstars. It was maybe only a few million years ago when some primates eclipsed birds in having the biggest brain relative to body size.
When did birds start singing?
Sound doesn’t fossilise, of course. But we can look at the family tree of modern birds. We can look at the songbird group and use DNA to predict when they would have originated. We can then look at the fossil record of the skeletons of birds, and see if they more or less match up with what the DNA suggests.
This tells us that song birds go back in Australia as long as 50 million years ago. Songbird evolution then probably went into overdrive about 27 million years ago. This was probably triggered by tectonic events such as little microplates, and islands moving around and forming new corridors and environments in South East Asia.
It’s only in the past 20 million years or so where you’ve had songbirds moving around the world. Nowadays, more than half of birds are song birds.
Anything else that is a priority?
The very first birds in the fossil record – proper flapping flight birds like Archaeopteryx – are from about 150 million years ago. Archaeopteryx had big feathered wings that could flap, but also teeth in its jaws, as well as big claws and a long tail. It’s the quintessential evolutionary link in transitional species, and has been known since the 1860s, when Huxley and Darwin wrote about them. Archaeopteryx was integral to their idea that birds evolved from dinosaurs.
We still haven’t discovered anything much older. We have some new fossils from China that are about the same age. Yet these birds must have had ancestors that were a bit more primitive, that could only fly in more of a rudimentary way. That’s one thing we’re waiting for, maybe from the Late Jurassic (162 to 143 million years ago) or even Middle Jurassic (174 to 162 million years). Those fossils would give us proper insight into how flapping flight really originated.
The Story of Birds US edition publishes on April 28, while the UK edition publishes on June 11 and is available for pre-order.
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To read an extract from the book, click here.
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Stephen Brusatte publishes books with HarperCollins and Picador. He receives funding from the Swedish Research Council, European Research Council, National Geographic, and Leverhulme Trust.
This article was originally published on The Conversation. Read the original article.
