The Science of Birds - The Origin of Birds

Episode Date: September 19, 2020

Episode: 1SummaryLearn about the deep evolutionary origin of birds and follow their history up to the great extinction that happened 66 million years ago.First, I discuss what we mean when we say “bird.” What characteristics separate modern birds from other animal groups?Then I discuss the evidence—both historical and recent—that scientists have used to determine the evolutionary history of birds. Finally, we arrive at the answer of where birds fit on the tree of life.Research CitationsDiscovery of collagen protein in Tyrannosaurus femur (Asara et al. 2007. Science)‍Archaeornithura, the earliest known true ancestor of birds (Wang et al. 2015. Nature Communications)Errors and ClarificationsError - 05:39 min: I say that scientists have genome sequences for 'all living birds.' What I meant was that we have genome sequences for representative species within each of the living bird families.Link to this episode on the Science of Birds websiteSupport the show

If you've ever taken part in the American holiday of Thanksgiving, you've probably eaten some turkey. And just perhaps you have participated in the ancient and barbaric ritual of getting the turkey's wishbone and breaking it into two pieces, whether your little brother Jimmy or your uncle Fred or whoever. The one who gets the largest piece of the wishbone gets 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. Hello and welcome. This is the Science of Birds podcast. I'm your host, Ivan Phillips.

and I'm really excited because this is the very first episode. I'm jazzed that you're here with me as we get this podcast off the ground. So thanks so much for being here. The goal of the Science of Birds podcast is to provide fun information about bird biology that isn't super technical, dry, stale, boring, or crusty. I do have a background in biology, so I can, if I'm not careful, go down that slippery slope into jargon world. That said, scientific terminology isn't always avoidable, and I hope we can have some good

times learning some of the specialized vocabulary of ornithology. But overall, I'll do my best to keep this digestible and accessible, something that you would enjoy listening to in the car or while you're out jogging, that kind of thing. I want the podcast to be educational, but lighthearted and fun. Perhaps you are wondering, who is this Ivan character? Alternatively, you might be asking yourself, Why do they call those little candy bars fun size? Big candy bars are way more fun, so who are they trying to fool?

We don't have time for a long philosophical discussion, so let's set aside the candy bar question for now. But as for who I am, well, briefly, I'm a guy who grew up loving nature in all of its forms. I eventually grew up, of course, and went to graduate school and got a master's degree in biology and then a Ph.D. in Zoology. I did not study birds, believe it or not. I studied other equally wonderful animals. I had a great time in academia and I got to do some cool research and got some papers published in good journals. But since then, I've transitioned to a life centered, mostly on birds.

Now I'm a professional naturalist guide. I take small groups of people to exciting places around the world to help them find and enjoy exotic birds, as well as non-avian wildlife and other interesting elements of natural history. So birds have become my focus as a naturalist and a biologist. But perhaps like you, I love all kinds of other animals. But this is a podcast about birds, dang it, and we're going to stay laser-focused on them. Okay?

Good thing is, we'll never run out of things to talk about. Today, we're talking about the origin of birds. Here's the plan. First, we should define what we mean when we say bird. We all have a sense of what they are, but let's make sure we're on the same page. Then we'll talk about what kinds of evidence scientists have, or what they have used to determine the evolutionary history of birds. And then we'll answer the question of where birds fit on the tree of life

and what are birds really, who are their closest living relatives, and so on. So let's get into it, shall we? What are birds? Some people might answer, well, 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, et cetera, et cetera. 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 and foremost, birds have feathers. No other animals on the planet today have feathers. Birds have beaks. They 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 like us, but unlike, say, modern amphibians and reptiles, which have only three chambers. And most birds have lightweight skeletons made of bones that are essentially hollow. So those are some of the key features of modern birds, their primary defining characteristics. We can all agree on those, I hope.

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 that's the primary source but also they have fossils that's also super important 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 now we have genome sequences for pretty much all living birds and analyses of that data can provide lots of valuable insights

So let's begin tracing the way that scientists used these various types of data to reconstruct the evolution of birds. 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 rock star. There's no common name for this animal. It's just Archaeopteryx lithographica. 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. So why is Archaeopteryx so special? Why do we all know that name? 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.

A number of the Archaeopteryx fossils show beautifully preserved feathers. So this critter had well-developed feathers on its fore limbs and on its tail. But it also had some reptilian features. It had pointy little teeth in its skull, and it had a long, bony tail, which is not present in modern birds. Archaeopteryx was not clearly 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 really 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. Fast forward to today and, spoiler alert, we now know that birds are reptiles. Now that may not jive 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 a number of 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. Also consider their scales. Birds do have some scales. Look closely at a chicken's foot, for example. Check out those crazy scales. The scales of birds, as well as those of reptiles, have a unique protein in them called beta-carotin. And there are also 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 that we lump together in the reptile category include several, very different, somewhat distantly related, lineages. snakes and lizards, turtles, crocodiles, Tuotaras, dinosaurs, as well as some other extinct groups. So where do birds fit into all of this? 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 arcosaurs branched off and became the dinosaurs. Meanwhile, another branch of archosaurs became the crocodiles, and yet another became the

pterosaurs, the famous flying reptiles of the Mesozoic. By studying the skeletal anatomy of birds, most paleontologists were able to 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 Arcosaur 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. Terosaurs are only very distantly related to birds. They are not the ancestors that we can point to. So 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 old furcula, the wishbone, comes back into our story.

Scientists were noticing that, hey, we're not seeing the furcula. We're not seeing clavicles in our dinosaur fossils. But we are seeing clavicles in older groups of reptiles, some of those more primitive pre-dinosaur reptiles, the archosaurs. It's important here to point out that it's generally understood that it's extremely unlikely or even impossible that in an evolutionary lineage you would lose something like the clavicles through natural selection, carry on for millions of years without them, and then suddenly re-evolve clavicles. That's what scientists believe would have happened if dinosaurs were the direct ancestors of birds. Not likely by Jove. Birds have clavicles, but dinosaurs don't, so dinosaurs 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 two elvis yada yada 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 among other beasts.

Paleontologists started to say, hmm, 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 in general

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 arcosaur lineages. The orthodox view that birds couldn't be dinosaurs could at long last be chucked out the window. Now let's move a little forward in time to the 1990s. At that time, amazing dinosaur fossils started turning up in the Liaoning province of northern China. 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 freaking feathers. It also seemed pretty 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 there are also numerous skeletal features, and there is also some evidence that certain dinosaurs had lung structures similar to that of modern birds, with the presence of multiple air sacs. Air sacks in birds, and some dynos actually penetrate into the

skeleton. We'll talk about bird lungs and breathing in a podcast episode at some point. 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. two matches up with what many 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 for 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. And 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 indeed 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. now picture all of the different dinosaurs you know you've probably seen at least one of the Jurassic Park movies can you name a few dinosaur species I'm waiting I'm waiting no T-Rex I'm sure T-Rex is a given how about T-Seratops stegosaurus velociraptor and so on

you probably have heard those names which of these or of the many other kinds of dinosaur is 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 beastfoot. 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, love, Velociraptor is probably the closest relative of modern birds. At long last, then, we have the answer of where birds fit on the tree of life. Birds are theropod dinosaurs. Is this the end of the episode? Not so fast. Let's dig deeper. Pun. 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, but we would be wrong in many cases. It's actually been a bit difficult to nail down who was the actual ancestor of birds among all of those different feathered dinosaurs. There are more than 30 species of dinosaurs that have feathers, or had 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. In other words, scientists no longer think that feathers evolved as an adaptation for flight.

We'll do another podcast episode on the origin of flight, but the idea now is that feathers originated either as some kind of insulation or as a way to signal 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 some purpose like that. It was only millions of years later that feathers turned out to be advantageous for flight. So feathers first, flight later. So there were a bunch of feathered dinosaurs

skittering around in the late Jurassic period and all through 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 and tail 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. The 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 100. 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 the very least. 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 Archiorna Thura mimony. Say it with me. Archie Ornithura, Mimini. Scientific names are fun.

This critter lived about 130 million years ago in what is now northeastern China. Archaeorna thorough was about a six-inch tall bird with longish legs. It had a beak, a fan-shaped tail, and well-developed flight feathers. So 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. 66 million years ago, an asteroid smashed into our planet, and the resulting devastation wiped out almost all of the dinosaurs, as well as so much other life on Earth. Terraosaurs 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 disappeared as well, but 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, after the extinction, 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 figure this out from our brief discussion

a few minutes ago about the archosaurs. The answer of who are the closest living relatives of birds is the crocodiles. Crocodiles and alligators are descendants 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, even though all of these things are reptiles. 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 for one reason or another

just aren't that interested in birds. Maybe you know someone like that. I do suspect that many of those people who ignore birds happen to think dinosaurs are really cool. If only they knew that birds are dinosaurs. A light bulb 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 works 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 understand it. More research will probably add more interesting elements to this story. 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 and 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 so that is it for episode one of the science of birds podcast thank you so much for being here and going on this journey with me today please do subscribe to the podcast if you love birds and you want to learn more about them you can also go over to scienceofbirds dot com and check out the website it would be awesome if you signed up for the email newsletter which you can do right there on the website. See you next time.