In 1861, a man working in a Bavarian quarry discovered a fossil that would change the way we think about birds.

The fossil was of a single, beautiful feather, exquisitely preserved in limestone from the Jurassic.

But the feather turned out to belong to an animal that was unlike any other bird humans had ever seen.

Nearby, the remains were found of Archaeopteryx-- a creature that by all accounts was bird-like.

But it had some very weird features, like a long bony tail and claws on its fingers.

It also didn't have the large breastbone of modern birds.

And it had ... teeth.

Experts are still arguing over whether Archaeopteryx was a true bird, or a paravian dinosaur, or some other kind of dino.

But regardless of what side you're on, how did this fascinating, bird-like animal relate to today's birds, like ducks, ravens, emus, that sort of thing?

Well, the path from Archaeopteryx to modern birds wasn't a straight line.

Those teeth were a clue that this story goes all the way back to what we now call the non-avian dinosaurs.

Scientists started to realize that birds actually are dinosaurs not long after Archaeopteryx was discovered in the 1860s and 70s.

That's because, around the same time as that find, another discovery was made in those same Bavarian quarries.

It was the skeleton of a tiny meat-eating dinosaur called Compsognathus And it was the first more-or-less complete skeleton of any dinosaur ever found.

Compsognathus had a lot of traits that seemed bird-like -- it had a curved neck, it stood on two legs, and it had three main toes.

And we now think it might've even had hair-like feathers.

But it was indisputably not a bird.

It had very short arms instead of wings.

And it had a pelvic bone, called the pubis, that pointed forward instead of backward, as it does in birds.

So Compsognathus was a non-avian dinosaur that looked kind of like a bird.

Then, in the 1870s, American paleontologist O. C. Marsh described two more species that illuminated the transition between dinosaurs and birds, found in late Cretaceous rocks from Kansas.

One was Ichthyornis, which looked pretty much like a seagull.

And the other was Hesperornis, which resembled a huge, flightless loon.

Ichthyornis was clearly avian -- it had wings without claws, a short tail, and a large breastbone, like most modern birds do.

But both it and Hesperornis also had teeth, like most non-avian theropods did.

So, Marsh and other experts began to hypothesize that birds evolved from dinosaurs.

In these specimens, Marsh saw how birds gradually acquired their curved necks, their two-legged stance, and their three toes.

He saw modified arms transition to feathered wings, and the loss of claws and the long tail.

Hesperornis and Ichthyornis were soon classified as ornithurines, a group of Mesozoic birds whose names means "bird tail."

These ancient birds had short tails, instead of the longer bony tail of Archaeopteryx.

Most of them also had teeth.

And they tended to live on the ground.

We know this because they were usually found in environments near shorelines, instead of inland forests, and they didn't have the adaptations that are associated with perching, like curved foot claws, or claws on their wings.

So by the late 1800s, many scientists believed that it was this group, the ornithurines, that gave rise to today's birds.

They didn't know what happened to their teeth, but they knew the connection was there.

But still, not everyone was convinced that birds derived from dinosaurs at all.

In the early 20th century, some researchers - - led by Danish paleo-artist Gerhard Heilmann - - argued that the similarities between Compsognathus and birds were convergent, and that birds evolved from some other, mystery reptile.

It wasn't until 1970 that, quite by accident, paleontologist John Ostrom came across the key that would unlock the origin of birds.

He was studying fossils of pterosaurs from the same German limestones that produced Archaeopteryx and Compsognathus.

But one of these fossils was not from a pterosaur, as many had thought.

It turned out to be the hand of an animal that was very similar to Archaeopteryx.

Actually, for a long time, Ostrom and others thought it was an Archaeopteryx.

But in 2017, it was redescribed and assigned to a new genus, named after Ostrom himself.

Now, the hand of this animal looked just like the hand of a non-avian dinosaur that Ostrom had uncovered just a few years earlier in Montana, called Deinonychus.

Both animals had short thumbs, long second fingers, and third fingers of medium length.

And in both specimens, most bones in the wrist were fused together into a half-moon shaped block that allowed for movement from side to side, but not up and down.

Living birds have this same half-moon feature in their wrists, which they use to fold their wings.

So, Ostrom's discovery revived the theory that birds evolved from small theropod dinosaurs.

And yet, we had no idea how incomplete our understanding was, until the 1980s and 90s, when dozens of new fossil discoveries showed us how truly diverse birds were in the Mesozoic.

Most of the fossils found around this time belonged to a whole new lineage of extinct birds that we didn't know existed.

They came to be called the enantiornithines or "opposite birds."

Which is a terrible name, because they totally were birds.

They were only called opposite birds because one of their shoulder joints was the reverse of what's seen in living birds.

But, unlike the ornithurines, which mostly lived on the ground, these opposite birds mostly lived in trees.

Their fossils were commonly found in rocks that formed in ancient forests, and most of them had curved claws like perching birds do today.

And these turned out to be the most common and diverse group of birds in the whole Mesozoic era, filling many of the same roles as modern birds.

For example, Bohaiornis may have hunted small vertebrates 120 million years ago in ancient China.

And it lived alongside Longipteryx, a perching bird with a long snout that may have dove for fish from tree branches, like kingfishers do today.

Most of these opposite birds had teeth and clawed fingers.

And instead of having a fan of feathers in their tails, they had blunt rear-ends that sometimes sported just two long, ribbon-like feathers.

So it was clear to scientists that these "opposite birds" belonged to a different lineage than the likes of Hesperornis, Ichthyornis, and the other ornithurines.

Meanwhile, experts were also making some important discoveries about the ornithurines, too.

Especially, what happened to their teeth.

For example, in the early 2000s, Chinese researchers reported finding the oldest fossil evidence of a toothless bird.

They named it Confuciusornis, and it lived 130 million years ago -- way back in the early Cretaceous.

And the strangest thing was: Confuciusornis was neither an ornithurine nor an enantiornithine.

It turned out that toothlessness was a convergent trait that showed up in lots of different groups at different times.

Some opposite birds didn't have teeth, and some early ornithurines didn't either.

So, where did their teeth go?

Well, for a long time, scientists thought that birds lost their teeth to make their bodies lighter.

But now we know that some toothed birds were able to fly, so it wasn't like having teeth was keeping them on the ground.

As more fossils of Mesozoic birds were found, more clues started to present themselves.

For instance, experts began to notice that, even though most ancient birds had teeth, they tended to have fewer teeth than their dinosaur ancestors.

And fossils also revealed that birds started to have fewer teeth after they developed gizzards.

Gizzards are a muscular part of the stomach where, with the help of swallowed pebbles, birds can grind up their food.

So with the advent of gizzards at least 120 million years ago, the need for teeth started to fade.

And as birds' teeth changed, so did their beaks.

Most opposite birds retained teeth at the very front of their mouths.

But in ornithurines this same area became covered in keratinous beak tissue, while they kept the teeth that were farther back in their jaws.

Now, some experts think that replacing teeth with beak tissue may have had something to do with making it easier to eat certain foods, like seeds.

But another theory, proposed in 2018, suggests that beaks may have been more advantageous for baby birds.

Teeth take a long time to grow in developing embryos, but if you shorten that step or cut it out altogether, then eggs can hatch earlier.

And the shorter a bird's incubation time is, the less vulnerable its young will be to predators.

So birds may have lost their teeth for many reasons.

But one thing we know for sure is that, in the Cretaceous, teeth disappeared in the group of ornithurines that would give rise to today's birds.

These are the neornithines.

Their fossils are still very rare, but in the past couple of decades, paleontologists have found bones of the very earliest members of this group.

One of the best known is a 68 million year old duck relative from Antarctica called Vegavis.

And its remains show that, even by the Late Cretaceous, it was already well adapted to swimming and diving.

So today, we finally have enough fossils of different kinds of birds to know that, about 66 million years ago, there were both toothed birds and toothless birds from many different groups.

And generally, they were all doing just fine.

Until, the Cretaceous Period ended in extinction.

There were many causes behind the extinction event that killed off the non-avian dinosaurs.

But when it came to birds - and most everything else - , the worst was probably the asteroid impact in the Gulf of Mexico.

The impact set off huge fires that destroyed entire forests and vaulted dust into the atmosphere, probably blocking much of the sunlight for years.

For birds, their odds of survival might have come down to a number of factors, including how big they were, where they lived, and what they ate.

For example, if you lived in trees, like most opposite birds did, you were in the worst possible place in the aftermath of the impact.

Not only was your habitat suddenly gone, but if you ate plants, it would be weeks or months before any new growth would be available.

And if you ate meat, there was almost nothing around to hunt.

But if you were smaller and lived on the forest floor, there's a chance you could have found shelter.

And it might've been a lot easier for you to find food, like seeds on the ground.

Likewise, if you were aquatic, your habitat and your food supply, might have been less disturbed by the fires.

So, out of what may have been thousands of species of dinosaurs living at the end of the Cretaceous, only three groups of neornithes had that perfect survivor's combination of being small, ground-dwelling or aquatic, and seed-eating.

One of these lineages was the paleognathes, which today include ratites like ostriches and emus, as well as weird South American birds called tinamous.

The galloanserans also survived, like Vegavis and its kin, possibly thanks to their aquatic lifestyle.

Today they include chickens, ducks and other kinds of fowl.

And the third lineage is the neoavians, which includes essentially every other living bird.

Free of competition from the tree-dwelling opposite birds, a lot of neoavians quickly took to life in trees at the dawn of the Cenozoic.

These birds all have diverse diets and habitats today.

But by analyzing the earliest known members of each group, scientists have found that the last common ancestor of all living birds was indeed a ground-dwelling seed-eater.

Each of these three lineages might have had only a handful of species at the time of the extinction, but they are the ancestors of the roughly 10,000 species of birds that are alive today.

There are many things we have yet to learn about the story of birds.

But our understanding has grown tremendously since that lovely fossil of a feather was first discovered in Germany, more than 150 years ago.

And, just within my own lifetime, a wealth of bird fossils has been discovered that are only beginning to reveal the wonderfully weird and diverse world of the Mesozoic, when birds had teeth.