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The Evolution of Birds: Their Origin and Diversification

An introduction to the long evolutionary history of birds, from the Jurassic to the mass extinction at the end of the Creataceous.

DATE:
September 19, 2020
CATEGORIES:
Author
Ivan Phillipsen

Paleontology at the Dinner Table

If you’ve ever taken part in the American holiday of Thanksgiving, you’ve probably eaten some turkey, and just perhaps you have taken part in the ancient and barbaric ritual of getting the turkey’s wishbone and breaking it into two pieces... with your little brother Jimmy or your Uncle Fred or whoever. The one who gets the largest piece of the wishbone get’s their wish granted, right, because that’s how magic works.

So if you know about that weird ritual—if you’ve done that—then you’ve held in your hand a bone known as a furcula, formed by the fusion of clavicle bones in a bird. It turns out that the furcula has been one of the focal pieces of evidence that scientists have used to figure out where exactly birds came from and where they belong on the great tree of life.

What Are Birds?

What exactly are birds? Some people might answer, “Well, duh, I know what birds are. They’re those little feathered things that poop on my car. They steal my french fries when I’m sitting at an outdoor cafe. They chirp in the bushes and stuff. Etc, etc.”

Fair enough. But you and I are seeking a slightly more nuanced answer.

Birds, in their present form, can be defined by their characteristics. But a complete definition of ‘bird’ requires that we also consider their long history on Earth, looking back many millions of years.

Let’s start with a quick rundown of their characteristics, their traits:

First, birds have feathers. No other animals on the planet today have feathers.

Birds have beaks and do not have teeth in their jaws. They lay hard-shelled eggs. They’re warm blooded with high metabolic rates. They have a four-chambered heart, unlike, say, modern amphibians and reptiles, which have only three chambers.

And most birds have lightweight skeletons made of bones that are mostly hollow.

So those are some key features of modern birds, their primary defining characteristics. We can all agree on those, I hope.

Green Jay
A Green Jay, showing off some characteristics of living birds.

How Scientists Study Bird Evolution

So what have scientists been doing to figure out the evolutionary origin of birds, where their branch belongs on the tree of life? What kind of data do scientists have at their disposal for this?

Well, they have access to the anatomy and behavior of living birds. They also have fossils. They can compare the anatomy of existing birds with each other and to those of extinct, prehistoric animals. And we also have a treasure trove of genetic data. We have genome sequences for pretty much all living bird groups. Analyses of that data can provide lots of valuable insights.

Let’s trace how scientists used these various types of data to reconstruct the evolution of birds.

Archaeopteryx

We’ll start in the 1800s. Specifically 1861. That’s when Archaeopteryx was discovered. If you’ve heard of one prehistoric bird, it’s probably Archaeopteryx. It’s kind of a rock star. There’s no common name for this animal. It’s just Archaeopteryx lithografica. That’s the scientific name, the first part of which, the genus, translates as “Ancient Wing” or “Ancient Feather.”

In 1861, the first Archaeopteryx fossil was found in a limestone quarry in Germany. Today there are about 12 known fossils for this animal, and these have been dated to approximately 150 million years old.

Fossil of Archaeopteryx lithographica
One of the best fossils of Archaeopteryx lithographica.

Why is Archaeopteryx so special? It was recognized as being very important even when it was first found in the mid-1800s. This small animal, about the size of a crow, has characteristics of both birds and reptiles. Several Archaeopteryx fossils show beautifully preserved feathers. So this critter had well-developed feathers on its forelimbs and on its tail. But it also has some reptilian features. It has pointy little teeth in its skull. And it has a long bony tail, which is not present in modern birds.

Archaeopteryx was not a reptile or a bird. It was some kind of missing link between these two groups.

You may recall that On the Origin of Species by Charles Darwin was published in 1859. So just a couple years later, these Archaeopteryx fossils started turning up. That was exciting for proponents of evolution by natural selection—Darwin, and his buddy Thomas Huxley, who made a strong case for Archaeopteryx being a transition between dinosaurs and birds.

So that all happened in the 1860s. There began the long history of scientists trying to figure out what exactly birds are.

Birds are Reptiles

Fast forward to today and—spoiler alert!—we now know that birds are reptiles.

That may not jibe with what you learned in school. What I learned in school was that the vertebrate groups were fish, amphibians, reptiles, birds, and mammals. Birds were their own thing. They sure don’t look or act much like reptiles, right? You can’t blame us for thinking of these groups as separate.

But several features link the two and make it clear that birds are indeed descendants of reptiles.

So what makes birds reptiles? Well, they share skeletal features. Quite a few of them. Consider their scales. Birds do have some scales. Look closely at a chicken’s foot, check out those crazy scales. The scales of birds and those of reptiles have a unique protein in them called beta keratin. And there are similarities in the egg structures of reptiles and birds.

So there’s this suite of characteristics that unequivocally link the two. There’s no other group of animals alive today that are more closely related to birds than reptiles.

But the animals we lump together in the reptile category include several, very different, somewhat distantly related lineages: snakes and lizards, turtles, crocodiles, tuataras, dinosaurs, as well as some other extinct groups.

So where do birds fit into all this?

Birds are Dinosaurs

About 250 million years ago, before the dinosaurs had really become dinosaurs, there was a group of reptiles cruising around called the archosaurs. The archosaurs had already become distinct from things like lizards, snakes, and turtles. Millions of years after that, one lineage of archosaurs branched off and became the dinosaurs. Meanwhile, another branch of archosaurs became the crocodiles. Yet another became the pterosaurs, the famous flying reptiles of the Mesozoic.

By studying the skeletal anatomy of birds, most paleontologists could agree, early on, that birds belong in the archosaur group. Cool. But when did birds become birds? When, exactly, did they split off from the archosaur crowd to make a name for themselves?

Maybe it seems logical that the flying reptiles, the pterosaurs, evolved into birds. That would be a natural assumption, I think. Remember, though, that pterosaurs have bat-like wings. Their flapping power came not from feathers, but from membranes of skin stretched between long fingers. Pterosaurs are only very distantly related to birds. They are not the ancestors we can point to.

Okay, let’s return to the development of biologists’ understanding of what birds are. Moving forward from the time of Darwin into the early 1900s. As paleontologists were discovering and studying more and more dinosaur fossils at that time, the good ol’ furcula—the wishbone—comes back into our story.

Scientists were noticing that they weren’t finding the furcula—they weren’t seeing clavicles—in dinosaur fossils. But they were finding clavicles in older groups of reptiles, some more primitive pre-dinosaur reptiles, the archosaurs.

It’s generally understood that it’s unlikely, or even impossible, that in an evolutionary lineage, you would lose something like clavicles through natural selection, carry on for millions of years without them, and then suddenly re-evolve clavicles. That’s what scientists believed would had to have happened if dinosaurs were the direct ancestors of birds. Not likely by Jove! Birds have clavicles, but dinosaurs don’t, so dinos aren’t the ancestors of birds. Case. Closed.

“Birds are not dinosaurs.” That became the Orthodox viewpoint among most scientists. For decades. The ancestor of birds was some other kind of reptile that branched off earlier before dinosaurs really became a thing. Sure, birds are reptiles, but they’re not dinosaurs.

Decades go by. World War II, Elvis, yadda yadda...

Then, in the 1960s, the Dinosaur Renaissance began. That was when newly discovered fossils and the work of several key paleontologists began to change our images of dinosaurs. That was also when modern statistical analyses were being applied to tease apart the relationships among dinosaurs and other prehistoric beasts.

Paleontologists started to say, “Hmmm, there are so many skeletal similarities between birds and some of these dinosaurs. Maybe birds are, in fact, a type of dinosaur.”

Our friend the furcula bone did in fact turn out to be present in dinosaurs, as more fossil data was collected. The furcula and clavicles are delicate bones that don’t preserve well in fossils. So until the Dinosaur Renaissance, they had been overlooked. Suddenly, our search for the ancestors of birds no longer needed to be limited to ancient archosaur lineages. The orthodox view that birds couldn’t be dinosaurs could, at long last, be thrown out the window.

Now let’s move forward a little in time to the 1990s. Amazing dinosaur fossils started turning up in the Liaoning province of northern China. Many of these fossils—most dating to about 130 million years ago—are incredibly well-preserved. And they have feathers! This isn’t Archaeopteryx. These animals are dinosaurs of different sizes and shapes and they have freakin’ feathers. It also seemed clear that many of these beasts were NOT direct ancestors of birds. In other words, a whole mess of dinosaurs had feathers.

We now know that feathers are not unique to modern living birds. They are one important feature that links birds with dinosaurs.

But they’re also many skeletal features. And there is also some evidence that certain dinosaurs had lung structures similar to that of modern birds, with the presence or multiple air sacs. Air sacs in birds and some dinos actually penetrate the skeleton.

An Emu's foot
When you look at an Emu's foot, it's easy to see that birds are dinosaurs.

How about some more evidence linking birds to dinosaurs?!

The sleeping posture of some dinosaurs mirrors the sleeping posture of modern birds. Some dinosaur fossils show the head of the animal tucked just so underneath the arm in a sleeping posture. It’s sad to think about these little dinosaurs dying while they were sleeping. But, then again, most of us would hope to die peacefully in our sleep. So… maybe it’s not so sad. I don’t know.

There is evidence of dinosaurs taking care of their young in the way they brooded their eggs as well as the way they took care of their young, sometimes in a communal herd setting. That, too, matches up with what modern birds do.

And lastly, there’s molecular evidence. We now have heaps of genetic data to suggest the relationships among living birds. We don’t have genetic data from DNA, not really, from any extinct dinosaurs. Sorry, Jurassic Park.

But there have been multiple studies published where biologists found traces of soft tissue in dinosaur fossils. Collagen proteins, for example, have been extracted from fossils, most famously from a Tyrannosaurus femur in the early 2000s. This is amazing stuff!

The collagen protein fragments most closely match proteins in living birds. Cool, huh? Well, some of these results have been doubted by other scientists. That’s fine, that’s how science works: with a healthy dose of skepticism. And as Carl Sagan said, “Extraordinary claims require extraordinary evidence.” The claim of finding proteins from 100 million years ago was extraordinary and, at first, it seemed the evidence wasn’t extraordinary.

Nevertheless, further research has strengthened the claim. As more ancient proteins are discovered and the conclusions of these studies survive high levels of scrutiny, it seems this molecular data is indeed another compelling piece of evidence pointing to dinosaurs as the ancestors of birds.

Birds are Theropods

Now picture all the different dinosaurs you know. You’ve probably seen at least one of the Jurassic Park movies. Can you name a few dinosaur species?  T. rex is a given. How about Triceratops, Stegosaurus, and Velociraptor?

Which of these, or the many other kinds of dinosaur, are most closely related to birds?

It turns out that birds are theropod dinosaurs. Theropods are a very familiar group of dinosaurs. Most of them are carnivores that lumber about on two legs. “Theropod” translates from Greek to “Beast Foot.”

Tyrannosaurus rex was a theropod. So was Velociraptor. A suite of skeletal similarities offers the best evidence of the connection between theropods and birds. Of all the Hollywood dinosaurs that we know and love, Velociraptor is probably the closest relative of modern birds.

At long last, we have the answer of where birds fit on the tree of life: Birds are theropod dinosaurs.

Is this the end of the article? Not so fast.

The Earliest Ancestor of Birds

Let’s dig deeper.

Earlier, I mentioned that there were a bunch of different dinosaurs running around that we know had feathers. We might be tempted to call all of them ancient birds. We would be wrong in many cases.

It’s been difficult to nail down who was the actual ancestor of birds among all of those different feathered dinosaurs. There are over 30 species of dinosaurs that have feathers of one kind or another, mostly among the theropods, but simple feathers have been found in some non-theropods. It no longer makes sense to say that birds are the only animals that ever had feathers.

Interestingly, all of this evidence also suggests that feathers evolved before flight. Scientists no longer think feathers evolved as an adaptation for flight. The thinking now is that feathers originated either as some kind of insulation and or to signal member members of your own species. Think about peacock feathers. Colorful, showy feathers can make a bird look drop dead gorgeous in the eyes of the opposite sex. Well, it seems like feathers evolved for purposes like that. It was only millions of years later that feathers turned out to be advantageous for flight. So feathers first, flight later.

The head of a Secretarybird.
Feathers may have first evolved as insulation or as a display, as in this modern Secretarybird.

So there were a bunch of feathered dinosaurs skittering around in the Late Jurassic period and throughout the Cretaceous. Some species sported feathers that were more hair-like, and scientists actually refer to these as “dino fuzz.” Other species had big, beautiful feathers that resemble the wing feathers of living birds. Many of these dinosaurs clearly were not flying animals. Just because you had feathers didn’t mean you were a lean, mean flying machine.

That explosion of diversity in feathered dinosaurs didn’t last forever, obviously. All of those lineages died out. All but one… the lineage of birds. Their story began roughly 165 million years ago. Remember, Archaeopteryx dates to 150 million years ago, but that animal had pretty well developed feathers. It could glide, certainly, and maybe it was capable of some sort of powered, flapping flight. That’s uncertain. In any case, it took millions of years to evolve feathers from scales. So the best guess for the deep origin of the bird lineage is about 160 to 165 million years ago. Archaeopteryx itself is probably not a direct ancestor of living birds. But it’s a cousin of the earliest birds, at least.

Archaeornithura

Is there a single prehistoric animal we can point to and say, “There it is! That’s the earliest true ancestor of birds.”

The best answer right now is a fairly recent discovery from 2015 of a bird, known by its scientific name Archaeornithura meemannae. Say it with me: Archaeornithura meemannae. Scientific names are fun.

This critter lived about 130 million years ago in what is now northeastern China.

Archaeornithura was about a six inch tall bird, with long-ish legs. It had a beak, a fan-shaped tail, and well-developed flight feathers. In many ways, it resembles a modern bird. That’s the best candidate we can point to right now as being the oldest true bird.

The Great Extinction

66 million years ago, an asteroid smashed into our planet and the resulting devastation wiped out almost all of the dinosaurs, and so much other life on Earth. Pterosaurs that had filled the skies and had occupied many ecological niches across the world were gone. None of them made it through that great extinction. Many other forms of animal life also disappeared.

When one door shuts, another opens.

When the dust settled, opportunities were everywhere for the surviving birds—those little, flying theropod dinosaurs that had somehow made it through. Unexploited food resources were free for the taking. The same was true for nesting and foraging habitats.

In the few million years following the big extinction, birds went through an evolutionary explosion, adapting to new niches and diversifying into the many forms we know today. This process is called adaptive radiation. The same thing happened with mammals, more or less. Lucky us, right?

There are many unanswered questions in the evolution of birds. Many scientists around the world are working hard to answer them.

One question we can answer right now is: “Who are the closest living relatives of birds?”

We’ve established that birds are reptiles and, more specifically, they’re dinosaurs.

Maybe you figured this out from our brief discussion a few minutes ago about the archosaurs. The answer is: the crocodiles. Crocodiles and alligators are descendents of the archosaurs, as are birds. So that makes crocs the closest living relatives of birds. Other modern reptiles like lizards and snakes are more distantly related to birds.

It’s safe for me to assume that you like birds, which is probably why you’re listening to this podcast. But, amazingly, appallingly, there are people in the world that just aren’t that interested in birds. Maybe you know someone like that.

I suspect that many of those people who ignore birds think dinosaurs are really cool. If only they knew that birds are dinosaurs! A lightbulb would turn on over each of their heads and they would suddenly realize that, logically, inescapably, birds must be cool.

I’m not sure it would work like that, but I wish it did.

So that, my friends, was a brief look at the origin of birds. In a very small nutshell. There’s much more we could cover on this topic. But we’re trying not to get bogged down in minutiae here. We have busy lives; we got stuff to do.

I hope you have a better understanding of birds’ origin story as we currently know it.

If you weren’t already thinking of birds as dinosaurs, I hope this helps you shift your perspective. To me, the realization that birds are dinosaurs is just one of the coolest things. Dinosaurs are still with us. They’re around us every day. That big shift in perspective happened in my lifetime. I can go to the store now and look at some kids’ toys, and there are plastic dinosaurs with feathers on them. You didn’t see that 20 or 30 years ago. Kids now are looking at me and thinking, Well, yeah, of course birds are dinosaurs, you idiot. Everybody knows that. Duh.

Dinosaur toys with feathers on them
These days, dinosaur toys have feathers. This reflects our current understanding that many dinosaurs sported feathers.




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