Three thousand years ago, in the leafy green forests of New Zealand, the big beady eyes
of a giant locked onto what would be its final meal.
The giant in this case was a North Island Giant Moa, a bird that stood 3 meters tall.
And its quarry was a tasty fern.
But just as it leaned over to take a bite, another bird swooped down from above.
The enormous Haast Eagle weighed less than a tenth of the Moa, but speed was on its side.
Its massive, muscled wings propelled it down fast enough to knock the Moa off its feet.
And once on the ground, the slashing of the Eagle’s talons did the rest.
This scenario could have been avoided, had the Moa been able to fly.
But Moas were Ratites, a group of birds that today includes ostriches, emus, cassowaries,
and kiwis – and almost all ratites are flightless.
Yet despite this, ratites have spread all over the world, to Africa, Australia, New
Zealand, and South America.
And there are fossils of Ratites in Europe, Asia, and North America too.
Now, that’s a lot of ground to cover for birds that can’t fly – especially since
these continents have been completely separated for about 80 million years.
So… how did Ratites end up all over the world?
And how and why did they stop flying and still manage to survive?
To explain how different ratites got to where we find them today, for a long time scientists
followed a hypothesis that came to be known in the 1990s as … wait for it ... “Moa’s
Ark.”
Under the Moa’s Ark hypothesis, there’s one simple explanation for how flightless
birds colonized the world: They all shared an ancestor that was flightless, and then
members of this already-flightless group were separated as the continents drifted apart.
So the ark, in this case, was the supercontinent of Gondwana, which finished splitting apart
80 million years ago.
This hypothesis is the simplest, because it requires that ratites lost flight only once,
and have been flightless ever since.
It’s also called the Vicariance hypothesis, because it refers to the type of speciation
that happens when members of a once-related group become separated by geographic barriers.
So, this model supposes that the moving continents split an ancestral group into isolated populations,
which then took different evolutionary paths and became separate species.
And this idea held together pretty well for a very long time.
After all, the bones of modern and extinct ratites all look remarkably similar and seem
to support the idea of a common, flightless ancestor.
But in the late 2000s, new research into ancient DNA blew this theory into pieces.
If the Moa’s Ark model were correct, then ratites that lived closest together would
be more closely related to each other than to those on different continents.
So, kiwis should have been closely related to Moas, for example, because they’re both
native to New Zealand.
And Ostriches of Africa should have been close relatives of the extinct Elephant Birds in
Madagascar.
Likewise, the Rhea should be a close relative of the Tinamou, another South American ratite,
though the only living one that can fly.
But a study in 2014 analyzed the DNA of modern ratites, as well as that of the extinct Elephant
bird and the Moa.
And the results showed that the relationships among these birds was very different from
what we thought.
It turned out that most ratite groups that lived on the same continent weren’t closely
related, like at all!
So, Kiwis weren’t closely related to Moas, even though they both lived in New Zealand.
Instead, kiwi DNA showed that they were closer to the Elephant Bird, which lived all the
way over in Madagascar!
And Moas turned out to be more closely related to the tinamou in South America
than they were to any other ratite.
And Ostriches… well, they turned out to not be closely related to any of the others,
belonging instead to their own lineage that branched off from the rest much earlier.
Now, this genetic data not only revealed which birds were more closely related.
It also showed when their evolutionary paths diverged from each other.
Using that method we’ve described before, the molecular clock, the DNA data showed that
many of these groups split during the present Cenozoic Era, long after Gondwana had broken
up.
For instance, according to this genetic research, Elephant birds and Kiwis diverged in the Eocene
Epoch, about 50 million years ago.
And Moas and Tinamous parted ways about 60 million years ago, back in the Paleocene.
But those old ostriches split apart from other ratites way back in the Mesozoic Era, some
76 million years ago, long after Africa split from the rest of Gondwana.
So all of this meant that the Moa’s Ark hypothesis couldn’t possibly be right.
Suddenly, the story of the ratites was infinitely more complicated.
Because now there could’ve been only one way that all of these related birds could
have gotten from one continent to another: They had to have flown.
But that would require that ratites flew to different continents and then became flightless,
multiple times instead of just once.
In fact, our new understanding of their family tree shows that all of the closest shared
ancestors of ratites could fly.
And that, in turn, means that ratites had to have independently lost the ability to
fly at least 6 different times.
Plus, because many of these birds evolved to get really big, but their flighted ancestors
were likely small, this suggests that ratites also evolved into their enormous sizes at
least 5 times.
And all of this new genetic data also allowed scientists to more accurately place fossil
birds that were too old to yield DNA, in the family tree.
With this data in hand, scientists in 2017 showed that the oldest of the ratites weren’t
actually from Africa – they were the Lithornithids, a fossil group from North America.
So ratites originated in North America, then flew to different continents over and over
again until about 27 million years ago.
And in each new place, they eventually had descendants who lost their ability to fly,
and also often became gigantic.
These adaptations were so successful that almost all of the living ratites are large
and flightless – and the flighted ones are mostly extinct.
But, flight is one of the best superpowers an animal can have.
I mean I wish I had it!
So why did the ratites lose it?
Well, flight is energetically really expensive.
And if you can get all your food on the ground, and not be too worried about predators, then
sure, flight isn’t really necessary.
Even if you are a bird.
But living on the ground may have only become an option in the first place, because of one
key event: the extinction of the non-avian dinosaurs!
That’s right!
As the Cenozoic Era dawned after the extinction of the dinos, many ecological niches were
left empty, especially on the ground.
For example, the Terror Birds were quick to adopt the niche of apex predator in South
America, which the non-avian dinosaurs had left empty.
Likewise, another bird group, fondly known as the Demon Ducks, lost their flight at about
the same time as the ratites, and filled an herbivore’s niche in Australia.
In the absence of the hadrosaurs, triceratops, sauropods, and many other big herbivorous
non-avian dinosaurs, there was suddenly plenty of room for ratites to browse without competition.
Which is why most of them are herbivores, though some, like the ostrich and the kiwi,
have become omnivorous.
And many ratites may also have become really big to help them live on the ground.
After all, large body size can help protect from predators.
But it’s not clear which evolved first in some species: Were they flightless, and then
became big?
Or did they become big, and simply outgrow their wings and lose the ability to fly?
And!
If being a massive, ground-dwelling bird has proven to be so great for ratites, then why
are the biggest ones, the Moa and the Elephant bird, extinct?
Well that one’s easy.
It’s because of us.
Humans arrived in Madagascar 2500 years ago, and about fifteen hundred years after that, elephant
birds disappeared from the fossil record, although there were some rumored sightings
of them up until the 1500’s when Europeans arrived.
And in New Zealand, moas went extinct about 500 to 600 years ago,
just a century or two after the arrival of humans.
In both places, archeological sites show that humans hunted the big birds -- but not just
the birds themselves.
What we were really after was their massive eggs.
And for ratites, that was a REAL problem.
That’s because ratites don’t lay as many eggs as other birds do.
Instead, they invest a lot of energy into fewer, larger ones.
Producing and protecting such big eggs puts a burden on the mother, but it also means
that the young are much more developed and require less care after they hatch.
So, after just a few centuries of overhunting by humans, Moas and elephant birds could no
longer replace their populations and vanished into extinction.
So it wasn’t huge eagles that brought about the end for these birds, nor was it their
inability to fly.
But if the evolution of ratites has taught us anything, it’s not to underestimate their
stubborn evolutionary trajectory.
Most of them stopped flying, and it proved to be an adaptation that served them well.
And the same could be said for many of the other flightless birds that aren’t part
of the ratite family.
So who knows.
Maybe someday, thousands or millions of years from now, we’ll see massive, herbivorous
birds walk the Earth once again.
Thanks for joining me for today.
And BIG thanks to our Eontologists: Jake Hart, Jon Ivy and mah bestie STEVE!
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