WOMAN: About 61 million years ago on the shore of what is now the South Island of New Zealand, there lived a bird.

It stood upright, measured about a meter tall, with wings that were pretty short for its large body, and a long, narrow bill.

And it couldn't fly, but it used those short wings to propel itself into the coastal waters of the Paleocene epoch.

The scientists who describe this creature named it Waimanu manneringi.

And it's the oldest fossil that we've ever found of a penguin.

From the shape of its skeleton, paleontologists can tell that Waimanu was already a flightless water bird like modern penguins.

But they don't know what came before it, what the transition from flying bird to early penguin looked like.

The fossils that could describe that for us have yet to be discovered.

But experimental work on modern birds that can both dive and fly is giving us clues about what it took for penguins to go from the sky to the sea.

And the world those first penguins lived in, that helped shape them, too, because why penguins gave up on flying is just as interesting as how it might have happened.

Waimanu lived just a few million years after the extinction that took out the non-avian dinosaurs and all the predatory marine reptiles that had ruled the oceans of the Mesozoic era.

So in taking to the water, Waimanu managed something its non-avian dinosaur ancestors never had.

It became fully aquatic.

And Waimanu was only the first member of what would become a very diverse and sometimes strange group of diving birds.

Today, we think of penguins as smallish, waddling, tuxedo birds.

But they evolved from a flying ancestor, were actually giants for millions of years.

And some of them were even dressed a little more casually.

The thing to know about modern penguins is that they're really specialized for underwater life.

While they still have to molt and breed on land, they've evolved a ton of features that make them dynamic marine predators.

For example, they've got unique scale-like feathers that help keep them warm and dry in cold ocean waters.

And they've also got structures called rete mirabile systems in their limbs and in their head.

Incidentally, I saw this pronounced as "reetee" and "reet."

So for this episode, we're going to go with "reetee."

These networks of neighboring blood vessels transfer heat between the arteries, where the blood coming from the heart is warmer and the veins where the blood is colder.

This exchange helps keep the penguins' core temperature up by buffering the much colder blood that is returning to the heart.

Their wing joints are also stiffer than those of flying birds, which helps them produce thrust on both the downstroke, like most birds do, and on the upstroke of the wing.

And the muscles that raise the wing on the upstroke are much bigger than in other birds.

So the place they attach on the penguin's shoulder blade is also much bigger than in other birds.

This powerful double stroke allows penguins to move easily through water, which is denser than air.

Their bones are also denser than you'd find in flying birds.

After all, penguins aren't trying to stay up in the air.

They're fighting against the buoyancy of seawater.

And those dense bones are part of what gives penguins such a great fossil record.

Now we haven't yet found enough fossils to completely fill out the penguin family tree.

But we know they evolved from a flying ancestor.

The closest living relatives of the penguins are the procellariiformes, of which includes albatrosses, petrels, and storm petrels.

And they all fly.

And based on studies that use a combination of genetics, skeletal similarities, and lots of statistics, it seems that the split between the two groups probably happened in the late Cretaceous period, sometime between about 71 million and 66 million years ago.

So why would a bird trade the ability to fly for a lifestyle of full-time diving?

After all, flight is energetically expensive.

But it helps you do things like travel long distances and avoid predators.

Well, one answer is that it's hard to be really good at both in terms of anatomy and biomechanics.

And in a paper published in 2013, researchers compared energy use in two species of living birds to figure out why.

They studied thick-billed murres, which are wing-propelled divers, and pelagic cormorants, which are foot-propelled divers.

Now both of these species also fly.

So the researchers could calculate the energy costs for that too.

And they found that the more specialized wings became for diving, the worse they were for flying.

For wing-propelled diving, you need a large body size and shorter, flatter wings with dense and large bones.

Because all of these things maximize the length and efficiency of every dive.

But these features are also the exact opposite of what you need for efficient flight.

So there's a pretty straightforward trade-off in anatomy here.

And it might be that the ecological conditions in which penguins evolved probably made it easier for some birds to give up flight in favor of diving.

In the aftermath of the extinction at the end of the Cretaceous, diving predators like small mosasaurs and plesiosaurs were gone.

So their ecological niches were suddenly open.

And the larger marine reptiles and sharks that had made the sea so treacherous for diving birds were gone too.

So with a new potential source of food and fewer predators, those penguin ancestors that were better divers might have had an advantage, being able to get more food while using less energy.

And once they started down the path towards underwater flight, there was no stopping them.

They spread quickly, by geological standards, to most of the places that we find them today.

We don't know what those first penguins looked like because we haven't found any fossils that show this transition directly.

But what we can say is that they were probably around 1 kilogram because that's the biggest that living birds get that can do both wing-propelled diving and flying.

And it's also the size of the smallest living penguin.

From there, true penguins probably evolved in what's now New Zealand in the early Paleocene epoch, around 61 million years ago, with Waimanu and its relatives.

They made it to Antarctica by the late Paleocene, between 59 million and 56 million years ago.

And they arrived in South America by the middle Eocene epoch, around 42 million years ago.

By the late Eocene, between 37 million and 34 million years ago, there were at least two genera of penguins in Australia.

Oh, and did we forget to mention that many of these penguins were huge?

Well, huge for a penguin.

The largest living penguin is the emperor penguin, which stands a little over a meter tall and can weigh up to 40 kilograms or so.

It's not a small bird.

But it would have looked small next to some of its Paleocene relatives from New Zealand.

Crossvallia was just over 1 and 1/2 meters tall and weighed nearly twice as much as an emperor penguin, between 70 and 80 kilograms.

And Kumimanu was even bigger.

It stood about 1.7 meters tall, and tipped the scales at just over 100 kilograms.

Both of these species show up not long after penguins first evolved.

This tells paleontologists that not having to fly anymore meant they could go all in on becoming more efficient divers.

And several groups independently developed large body sizes as a result.

And those guys were even the biggest penguins that we've found.

Antarctica in the late Eocene was home to at least two more species of giant penguin.

Antropormis was about the size of Kumimanu, close to 2 meters tall and about 100 kilograms, very respectable for a giant penguin.

But one species in the genus Palaeeudyptes was the penguin heavyweight champ.

It stood just over 2 meters tall and weighed about 115 kilograms.

And finally, another of these late you Eocene giant penguins wasn't the dapper bird we picture today when we think of penguins.

We call it Inkayacu and it lived in what's now Peru.

It was found with fossilized feathers that contained preserved melanosomes, the parts of cells that make and store pigment.

And it's melanosomes didn't look like the ones found in living penguins.

Instead of making its feathers black and white, they looked like those of other modern birds that are gray and reddish brown.

The giant penguins were successful for millions of years.

But they'd mostly disappeared by about 23 million years ago, and were totally gone by 18 million years ago.

And the cause might have been the rise of marine mammals, especially new groups of toothed whales, that happened in the Oligocene epoch.

Toothed whales and pinnipeds like seals and sea lions might have competed with the giant penguins for food.

Or they might have seen the giant penguins as food.

And social pinnipeds might have competed with penguins for safe spaces to breed too.

But the jury is still out because testing hypotheses about competition in the fossil record is hard.

All we can say for sure is that these groups overlapped in time and space.

And the giant penguins aren't here anymore.

Today, penguins still live on all of the southern continents, from Antarctica all the way up to the Galapagos Islands, which they reached about 4 million years ago, and where cold ocean currents keep the waters cooler.

And we know a lot about their extinct relatives thanks to their dense bones and preference for living on the coast.

Because these things made them more likely to be preserved than birds living farther inland.

We're still missing the fossils that capture their transition from flying to diving.

And there's definitely more to figure out about why and how that happened.

But it looks like the story of penguins might have been tightly linked to the rise and fall of other organisms around them.

The extinction of the marine reptiles at the end of the Cretaceous left a lot of empty niches that they were able to dive into.

See what I did there?

And then they got huge, which I for one am pretty sad to have missed out on.

They spread throughout the southern continents until some of them lost out to new competitors, the marine mammals.

And while we think of them as smallish, formally dressed, flightless birds, for almost 40 million years, penguins were some of the big predators in the Cenozoic oceans, all because they could take advantage of one of our planet's major extinctions.

And the penguins that are still around today are the descendants of that ancient lineage that went from the sky to the sea.

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