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! Now, what do you want to learn about? Leave me a comment, and don’t forget to
go to youtube.com/eons and subscribe.
Credit to PBS Eons on YouTube. Thought that you would enjoy this and learn something new about the ratites.
I guess they reposted this for some reason. I watched it yesterday. PBS Eons is a great channel. I marathoned every episode a couple weeks ago. Every episode is good.
haha isn't this actual evolution and not speculative evolution though?