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The Accident That Helped Fish Conquer the Earth | PBS Eons | YouTubeToText
YouTube Transcript: The Accident That Helped Fish Conquer the Earth
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The evolution of tetrapods from fish onto land, initially thought to be a deliberate migration, is now understood to be a gradual adaptation for shallow water environments that incidentally prepared them for terrestrial life.
Imagine
a lungfish in a pond. It's the dry
season and the pond is shrinking. It
will soon disappear. But even though
this fish is about to be out of water,
it isn't totally out of luck. When the
water gets too shallow, the lungfish can
gulp air at the surface using its lungs.
And when the pond becomes too small, it
can use its sturdy fins to crawl across
the ground in search of a new one. Now,
these abilities seem almost tailorade
for surviving on land. You'd think this
fish was just one evolutionary step away
from becoming terrestrial. Okay, but
here's the thing. This lungfish has no
interest in land. Those terrestrial
adaptations exist to help it survive
long enough to get back to the water.
Over a century ago, one paleontologist
wondered if the lungfish strategy held
the key to understanding one of the
biggest evolutionary moments in history,
why our own ancestors left the water.
Maybe he thought the earliest tetropods
also had no interest in colonizing land.
And while parts of his hypothesis turned
out to not hold water, it looks like
lungfish have something to teach us
after all. because it's beginning to
look like our ancestors might have
originally evolved terrestrial traits in
order to stay fish.
Hundreds of millions of years ago, the
group of animals we belong to, for
limbmed vertebrates called tetropods,
conquered the land. They evolved from a
group of fish called sarcopterigians,
which include the ancestors of
tetropods, the tetropotamorphs. And for
over a century, scientists have been
trying to solve an important puzzle. How
and why did the earliest tetropods
evolve from these ancestors making the
monumental leap onto land? Through the
fossil record, we can track what
happened during this transition, more or
less. But why it happened has been much
harder to answer. During the early 20th
century, researchers contemplating what
drove our fish ancestors to leave the
water considered a bunch of ideas. Like
maybe the fish were escaping from
predators in the water or taking
advantage of food resources on land. But
in 1917, paleontologist Richard Lull
offered a different idea. He looked at
lungfish and wondered if scientists had
been thinking about this problem
backwards. LOL's idea, later championed
by paleontologist Alfred Sherwood Romer,
became known as the shrinking pond
hypothesis. LOL and Ror pointed to how
African and South American lungfish
living in aid seasonal environments use
a range of adaptations to cope with
changing amounts of water, like entering
a dormant state underground to escape
the dry season, as well as using those
skills of breathing air and sometimes
crawling around on land. and they
pointed out that while those traits
could be confused for having evolved for
life on land, their real function was
just to sustain the fish long enough to
get back to the water. LOL and ROR
viewed these fish as possible proxies
for our tetropod ancestors because
lungfish are also soprarigians closely
related to the ancestors of tetropods.
Maybe those ancestors also lived in
similar environments that changed
seasonally with bodies of water
shrinking and disappearing. And maybe
this led those tetropotammorphs to
evolve seemingly land friendly traits
for the same reason, to survive long
enough to find water again. But as new
fossils emerged over the next few
decades, the shrinking pond hypothesis
began to fall apart. For example, by the
1940s, the evidence suggested that lungs
actually evolved long before the fish
tetropod transition. In fact, lungs
likely appeared millions of years before
the arrival of tetropods, meaning they
must have developed to serve a different
purpose. Then the early 2000s brought a
ton of new discoveries of transitional
tetropottomorphs. giving us more insight
into the environments where they lived
like the famous tetropod ancestor
Tecttoik which fun fact was not named
after Tik Tok but lived during the
Deonian period in what's now northern
Canada around 375 million years ago.
Tectalic and its relatives including
panderices and alpisti are sometimes
called fishopods because they look sort
of halfway between a fish and a tetropod
and among their more tetropodike traits
were limblike fins that even had the
precursors to toes. Tectalic was also
one of the first to evolve a neck,
giving it a distinct head and torso,
unlike the classic fish body plan with
no neck. This allowed for greater
mobility of its head, and it's a key
trait that distinguishes the tetropod
body plan from that of a fish. But as
more fishopod fossils began to surface
and their habitats were studied,
scientists realized they lived in
shallow water environments like tidal
channels, braided streams, and swamps.
These weren't the seasonal droughtprone
landscapes that LOL and Ror had
envisioned. So that's that then, right?
The shrinking pot hypothesis is wrong.
Well, maybe not entirely. Lol and rumor
might have been on to something even if
they had the wrong habitat. See, amid
all the new tetropottomorph discoveries
of the early 2000s, another researcher
suggested that the tetropod body plan
might have evolved mostly or entirely in
the water, not on land. One animal she
pointed to was a canthastega from the
swamps of what's now Greenland during
the late Deonian period about 365
million years ago. While it wasn't a
true tetropod, it was getting close with
a salamander-like body and four limbs
instead of fins. This was also one of
the first tetropodomorphs known to
evolve a rearpowered mode of locomotion.
That is, its movement was powered
primarily by its back limbs instead of
its front limbs. This form of locomotion
would one day become a feature common to
tetropods. But while a Kenthga looked
like a tetropod, its arms and weakly
built rib cage seem unlikely to have
been able to support its weight on land,
suggesting it was entirely aquatic.
Those seemingly land-friendly features
like its hands were instead probably
being used for navigating the waters of
its swampy habitat and holding on to
plants in the water. And when tectonic
was described around this time, its
discoverers suggested that its
tetropodlike traits might have evolved
in order to walk underwater as well. Or
actually be more like this cuz it has
four limbs. While they thought it might
have come ashore occasionally, most of
its time was probably spent navigating
stream beds and shallow channels
fluttered with plants and debris. It was
looking like some of the key pieces of
the tetropod body plan represented an
adaptation, a trait that initially
evolved as an adaptation for one thing,
but then later became used for something
else. In this case, our fish ancestors
evolved tetropod traits while adapting
for life in the shallows. And only later
was that body plan co-opted for life on
land. It was the old lungfish paradox
all over again. Traits that looked like
adaptations for land were actually just
better ways to be fish. And more
evidence supporting this aquatic picture
continued to emerge. For example, a 2012
study reexamined the limbs of
Ichthyostga, another tetropottomorph
that shared Aanthastega's environment.
Scientists originally thought its limbs
could have supported its body on land,
but these researchers found that the
range of motion would have limited
Echostga's terrestrial movement to
dragging its body like a seal. Not
exactly the most effective evolutionary
strategy. Ichthyastega probably did
spend some time on land, but not a lot.
Then a 2013 study found that jaws of
earlier fishopods like tict wouldn't
have allowed them to swallow prey on
land. And that's a big deal because it
means that even if they did occasionally
venture onto land, the fishopods would
have been mostly incapable of taking
advantage of prey there. This suggests
that at this point in their evolution,
tetraotamorphs were still very much
shallow water dwellers and the abundant
resources on land probably played a
minimal role in their evolution. And
most recently, there was a study from
2021 that analyzed bone micro structure
of tetropods from Nova Scotia that lived
in the early carbonifpherous period,
several million years after athosta and
ichthyostga. It found that the stress
patterns in their bones were consistent
with aquatic habitats and inconsistent
with movement on land. So, it looks like
not only were most tetropot fishes
mostly or fully aquatic, but that even
the later first true tetropods still
weren't living on land. Okay, but now we
should be careful about oversimplifying
the water to land transition overall.
Not all tetropottomorphs form a
perfectly neat progression in their
traits or through time, and their
habitats weren't all identical. There
was probably tons of variation, but many
scientists are beginning to think that
most transitional tetropottomorphs were
primarily shaped by shallow water
habitats choked with plants and other
debris. So, L and Ror were right about
one crucial thing. The colonization of
land was probably an evolutionary
accident. So, these animals crawl onto
land by accident. And now I have to pay
taxes. Those limbs, necks, and other
adaptations that later help
tetropottomorphs conquer continents
initially evolved to navigate the
shallows, not escape them. And
possessing these traits beforehand
probably made the transition to life on
land much easier for our ancestors when
they eventually did come ashore and
stayed there. So, in the end, or rather
in the beginning, our ancestors didn't
set out to revolutionize life on Earth.
Our success on land and that of all
tetropods from frogs to dogs to
dinosaurs was just a lucky side effect
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[Music]
Okay, what do horseshoe crabs, ginko
trees, and lungfish have in common?
They've all been referred to as living
fossils. And while these organisms
appear to have persisted for millions of
years unchanged, they also show us that
evolution is always at work. Learn more
in our episode, living fossils aren't
really a thing. And thanks to this
month's eontologists who give us a big
step up. Addie, Annie and Eric Higgins,
Carl Wolfful, Jackie Scott Rston, Jake
Hart, John Davidson, Ing, Juan M.
Melanie Lamb, Carnival, Nico Robin,
Rafael Hassa, Tony Dye, and Steve.
Become an eonite at patreon.com/eons.
And you can get fun perks like access to
a monthly digital puzzle of paleo art
commissioned just for eons. And as
always, thanks for joining me in the
Annie and Eric Higgins studio. Subscribe
at youtube.com/eons
for more fabulous fossils. [Music]
[Music]
I just want everyone to know I'm doing
resort casual for this shoot. This is
vacation dad. [Music]
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