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The Hardest Problem Evolution Ever Solved | Hank Green | YouTubeToText
YouTube Transcript: The Hardest Problem Evolution Ever Solved
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Summary
Core Theme
The transition of life from water to land was an incredibly challenging evolutionary process, requiring numerous complex adaptations that fundamentally reshaped organisms, demonstrating that terrestrial life is deeply rooted in its aquatic origins.
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It is just so hard to be on land. Like,
we're used to it, but it is kind of
amazing that we are doing it. Out here,
it's hot. It's bright. Temperatures
swing wildly. We're constantly being
bombarded by the sun's radiation. Fish
don't have to fight gravity to stay
upright. They have buoyancy. They don't
need reinforced skeletons or tight skin
or lungs that won't dry out. Down where
it's wetter, there are no freezing
winds, no dry heat, no need to worry
about desiccating into a crispy fish
chip. Also, eggs. Eggs are a huge deal
for a lot of animals. And in the water,
you don't have to worry about them
drying out. But land, land strips all of
this away. We are here. We're used to
it. We like it this way. But for life,
life on land, in the air, under the sun.
At first, it was intolerably harsh.
Moving from water to the land is like
moving to an entirely different planet.
It's a new world out here. A whole new
world. That's a different movie. It is a
harder, much harsher world. But that
didn't stop some fish from getting
ambitious. I have recently become
obsessed with this. I think I could
write a book about this. Unfortunately,
I am already writing a book and I do not
have time. So, you get a YouTube video
instead. The things we needed to do to
get onto land are so complex that
sometimes I am amazed vertebrates ever
did it. And look, it is important to
note that for a very long time, we
didn't do it. There were vertebrates for
tens of millions of years before any of
them left the water. And then for
millions of years after that, they
remained reliant on being near the
ocean. The earliest land vertebrates,
like this guy, ate digits and lungs, but
also gills, a weak rib cage and limbs
too flimsy to support its own weight on
dry land. They were still basically
aquatic, just with legs and lungs. And
lungs, we're going to get to this, but
you would think lungs are a very big
deal. Uh, it turns out not really. I
mean, obviously you can't leave the
water without lungs, but as we will see,
that is not what held us back because
these environments are so different.
This was not a single leap from the sea
to the soil. This was an awkward climb.
Some lineages tried and failed. Most
didn't bother. And the transition only
stuck. Like we only really conquered
land when we solved a whole bunch of
problems in parallel. And we're not
going to talk about all of them, but we
are going to talk about some of them.
Each one was its own survival puzzle and
each one came with its own trade-offs.
So the question isn't how did fish make
it onto land. It's how did they
immediately not die trying? But this is
the thing about evolution. They did die
trying. They died so much. Evolution
happens when organisms die a lot. That
makes it possible for little advantages
to win out. And after doing a bunch of
research on this, you are going to be
surprised by which thing I think was
hardest because it was definitely not
the lungs and it also wasn't the legs,
though they were pretty hard. And that
is where we're going to start. So fish
fingers, not those actual fingers and
actual fish. Fish that are alive right
now in the world in the water swimming
around have fingers kind of. They're
called lobe finned fish and they're
built differently from the ones you've
probably kept in an aquarium or eaten
for dinner. Instead of thin, flexible
rays like trout or tuna, their fins are
thick, fleshy stocks, little muscular
limbs with robust bones inside. And
these bones are arranged in a
suspiciously familiar pattern. That's
right. You can actually identify the
radius and ulna of these fish. You could
see metatarscils. You could see some
digits. And this isn't just like, well,
that looks familiar. We actually have
direct fossil evidence of these bones
moving from lobefinned fishes through
time into terrestrial fish-like things
like tictalic that were pushing
themselves around on the ground. These
are animals that were definitely
spending some amount of their lives
outside of the water. Now,
interestingly, ray fish also have made
some headway into getting onto land, but
not particularly successfully.
Mudskippers are perhaps my third
favorite fish. They are not loed, but
they can get around on land. They are
rayfinned fish, which along with the
loed fininned fish is the other kind of
bony fish. Mudskippers do actually prop
themselves up on their little ray fins
and they push themselves around with
their little tails. But lacking robust
bones in their fins, they never really
ventured much farther from the water.
They did however evolve some very
familiar tricks for staying out of the
water for a long time, but you'll have
to wait until the eyes section for that.
Right now, we are talking about limbs.
These fleshy fins with big bones were
vital for getting on land, but they
appeared to have been a drawback since
there are now only three lineages of
lobefinned fish. The celicants, the
lungfish, and a fairly successful
lineage of all land vertebrates, the
tetropods, or as I like to call them,
landfish. You, me, eagles, dinosaurs,
landfish. This is where the fish don't
exist science fact came from. If you try
to create one taxonomic group that
includes all of the fish, you
necessarily have to include all of the
terrestrial vertebrates. Celicants now
live quite deep. They go nowhere near
the land, but they evolved from fish
that spent some of their time on land,
which is why they evolved those bony
fins both in the front and the back.
Limbs were very important. I guess you
know why, but because of gravity, which
is a terror. And fish in the water don't
have to worry about gravity because of
buoyancy. They can just get as big as
they want, which is very nice for them.
So, a secret. We're talking about fish
moving onto the land in this video, but
a lot of what we're actually talking
about is neofunctionalization.
This is where some old structure or a
protein or gene in an organism is
adapted for a new function. I have just
said the word neofunctionalization too
many times and now I feel like I need to
figure out what the root of that word is
cuz I would think it would be function.
But shun makes me think that the root is
funk. So now I need to look that up from
the nominative Latin funio performance
on execution noun of action form fund.
Funct past participle stem of fungi not
related by the way to the fungus from
the protoindo-uropean bung to be of use
or be used and curiosity satisfied. Back
to the video. Neofunctionalizing the
loed fins into legs was very hard. The
bones had to get thick. Lots of muscle
attachments needed to evolve. It had to
happen on both the front fins and the
back. But all of this kind of makes
sense. You can see the bones and what
they turned into. Now, let me hit you
with a harder one. Eyes. So, eyes very
useful. And it is amazing that we have a
literal evolved lens in our eyes. And
look, since we're talking about
neofunctionalization, the genes that
coded for the proteins that
neofunctionalized into the vertebrate
eye lens are descended from genes that
coded for proteins called alpha
crystallins. Did alpha crystallins
initially evolve to become these
transparent eyeens proteins? No, the
eyeens proteins neofunctionalized from
proteins that help other proteins not
come unraveled when they get put under
stress from heat or UV light. So they're
basically like heat stress protection
proteins, which has nothing to do with
eye lenses. We still have these in our
bodies now, but if you stack them, it
turns out they're transparent and so
they got neofunctionalized into lens
proteins. But more important for this
story, they did that in such a way that
bent light basically the same amount as
water. So when light went from water
into the eye, that light kept going
basically straight. But air, air is a
whole new optical game. It bends light
less than water. And fish eyes evolved
to see in water. Fish eyes can't see out
of the water, just like we can't see in
the water. The fact that your vision
gets blurry in water is not because it's
like putting pressure on your eyes or
something. It is actually because the
light bends when it moves from the water
into your eye. And it bends in a way
that your eyes are not set up for. So to
fix the fact that fish couldn't see well
on land, early land vertebrates had to
rework the physics of their eyeballs.
And it turns out that easier than
changing the refractive index of the
lens was curving the cornea. So our
lenses actually are still evolved to see
in water, but the shape of the eye has
changed to compensate for that. We also
developed muscles that squeeze and shape
the lens. Fish eyes actually focus by
moving the entire lens forward and
backward like a camera lens. While
landfish, like us, squeeze and stretch
the lens with tiny muscles. We basically
had to reinvent autofocus. Also, fun
bonus fact. Air dries things out, but
your eyes need to be wet. So now you
need eyelids and tear glands and
drainage systems for your eyeballs and
pigmentation to protect your retinas
from solar radiation. You know, just a
whole new category of problems. So yeah,
fins into legs was hard, but turning
underwater cameras into air adapted
self-lubricating solar shielded
shape-shifting binoculars in my opinion
harder. Also, you know who else
independently evolved eyelid like
things? Mudsk skippers. Which is, I
think, why I like them so much. They
just look a little more like interesting
and alive when they can close their
eyes. But, of course, they did this in a
totally different way that
neofunctionalized different parts of
their body. Mudskipper eyes blink by
pulling the eye down into the body and
having a flap of skin that automatically
covers them when that happens. So,
there's way more here and I would really
love to talk about it, but this is a
YouTube video and not a book. And as
much as I would love to write this book,
I don't think it would be particularly
easy to get people to buy it. So, I'm
not going to do hearing or eggs or sex
or heat or protection from UV radiation.
All of which are definitely interesting.
And the last one's going to be lungs.
We're going to get to lungs and it's
going to be interesting and you're going
to love it. But the next one, the next
one is actually, I think, both the most
interesting and the most difficult of
all of the evolutions that had to happen
for us to take on the land. And I don't
know, perhaps people won't want a book,
but maybe they will want a shirt. So, I
didn't actually talk about this when I
was recording the video, which is the
kind of mistake I would make. But, I
commissioned an artist to make a shirt
featuring a bunch of fish going about
their daily business being like doctors
and vacationers and crossing guards and
business people. And I didn't record
that during the video. So, here I am
recording this right before I upload
from my brother's guest room. But only
for the next two weeks. After years of
owning a merch company, I know how to do
this. You can order the shirt for two
weeks. At the end of those two weeks, we
close orders and then we print that
exact amount. That way, we don't print
too few and we don't print too many and
there's no waste. However, this does
mean that it will not be available after
the pre-order window is over. Okay, and
now you know that it's time to go back
and explain these words and also the
part of this story that I am most
excited about. Harder than seeing or
breathing or defying gravity. Land is
dry. It is so dry. And life, as everyone
knows, is wet. The fundamental solvent
of all living chemistry starts
evaporating the moment you step out of
the water. And life needs it. Every cell
in your body is basically a squishy
little aquarium full of the ocean we
originally evolved in. But if there's
water on one side and air on the other,
the natural equilibrium is going to push
toward the same amount of water inside
and outside. And that means instant
death on land. You don't just need to
drink water. You have to hold on to it
constantly. As landfish moved away from
the water, their entire existence became
a game of minimizing leaks and
evaporation. This is a massive problem
because remember, evolution doesn't
build new things from scratch. It
remodels. Your ancestors weren't blank
slates crawling out onto the beach. They
were fish covered in scale and skin
designed to let water flow through it,
gasping in an atmosphere that evaporated
their bodies one molecule at a time.
Life couldn't happen without water. So,
our ancestors had to somehow carry the
ocean with them out of the water and
onto the land. As the shirt says, we
never left the water. So, how do you do
that? Well, if you need to carry the
ocean out of the water with you, you
have to make your skin waterproof. You
have to build yourself into a water
balloon. And nothing in the biochemical
arsenal of fish was up to this task. But
there is a protein-based structure that
all land vertebrates have. All of us,
frogs, birds, snakes, people. And no
fish have it. This is by my account
actually the most important evolutionary
adaptation that allowed fish to take
over the land. It is the adaptation that
made it possible for us to carry the
ocean out of the ocean. Caratinized
skin. This is keratin, a tough fibrous
protein stuffed into the outermost layer
of skin where the cells are already dead
and sealed shut. It along with collagen
and some other structures forms a
flexible armor against evaporation. It's
waterproofish, microbresistant,
flexible, and surprisingly expendable.
And when I found out about this, I
assumed that keratin would have evolved
from some other structural protein like
collagen. But collagen could not do this
on its own. It's too squishy. It's too
porous. It's not strong enough.
Apparently, there was no path to
neofunctionalize collagen into something
as strong as keratin. But fish also have
bone, but that is too stiff. And if you
just make bone plates, the spaces
between the plates would lose too much
water. We needed something that was
flexible and stretchy and that was tough
and waterproof. We had to invent an
entirely new structural system seemingly
from scratch. And this was a huge
mystery for a while. Collagen and
keratin are just too different to be
coded for by related genes. So, we knew
that it wasn't that. What we found blew
my mind. Fish actually do kind of have
keratin, but it has a very specific
purpose that has nothing to do with the
skin and hair and nails that we use it
for. Fish scales are not made out of
keratin, but there is keratin in their
bodies somewhere. Their cells have it in
long strands that build the internal
structure of their cells. It's part of
the stuff that gives cells their shape
and their mechanical strength. Our cells
have this cell skeleton keratin inside
of them too. Doing this, these keratens
are a vital part of cellular structure.
But when fish needed to take over the
land, a weird new function evolved. Fish
whose skin cells had more keratin in the
cell could carry more water around,
evaporating less. Eventually, that
evolved into an even more structural
function where the cells would fill
almost entirely with keratin and then
release all of their water into the
body, sticking together along with some
collagen binding proteins to form this
tough, thick, but still flexible skin.
Keratinized skin is, in my opinion, the
most important adaptation that allowed
fish to conquer land. Evolution took the
initial cellular scaffolding and started
plastering it onto the surface of the
organism. That's neofunctionalization
again. Your inner support beams
repurposed into outer armor. I love this
so much. And once keratin started being
used in this way, evolution just went
nuts with it. Scales, claws, beaks,
feathers, horns, hair, foot pads, little
toe beans. If it's solid and it came out
of the skin, it's probably keratin. And
the best part, even frogs have it. Just
enough to keep them from leaking, but
not enough to stop some air exchange
through the skin. Amphibians usually
don't have lungs or skin good enough to
handle land on its own. They make it
work with a thin leaky compromise that
they periodically shed and consume.
That's not really related to anything in
this episode, but it's a good fact cuz
you know, life sometimes means recycling
your own raincoat by putting it inside
of you and turning it into fuel. Now, I
need you to know that this is not an
exhaustive list. There are a bunch of
things that I know and I'm not telling
you. And also, I'm sure stuff that I
don't know about this transition and the
physical, genetic, and biochemical leaps
required. But this video is already
getting quite long. But I feel like it
would be a real problem if I did not
bring up lungs. But the thing I didn't
realize about lungs until I started
researching this is that they didn't
initially evolve to help with the
transition to land. Lungs evolve as
early as 420 million years ago in the
ancestor of all bony fish. This blew my
mind. But this makes a ton of sense. It
takes time for oxygen to diffuse into
water. There's way more oxygen in the
air than there is in the water. And
sometimes, especially in muddy, shallow
areas, all the oxygen in the water can
get used up. If you're a fish, living in
that muck, having a little air sack that
lets you gulp atmospheric oxygen is a
massive survival advantage. But you can
still ask, what structure was
neofunctionalized to form the lungs, and
it's just the gut. It's the digestive
system. Crazy thing, this actually works
a little bit for people right now. In
certain very strange circumstances when
ventilating the lungs isn't possible,
scientists have investigated whether
filling the rectum with oxygen could
help keep a patient alive. Just
oxygenating some wet internal tissue
that has lots of blood vessels can
result in some gas exchange that could
help keep a person alive. But our
ancestors didn't do this through the
rear end. They did this through the
mouth and into the gut. Which is why to
this day we breathe and eat out of the
same hole. Even though this does
sometimes cause problems, even with
basically zero change, gulping some air
into the gut would result in some small
amount of extra oxygen in a fish's
blood. This is why I say this wasn't
actually that big of a deal. It's
evolutionarily very obvious and a small
increase in function leads to a large
survival advantage. So, you would expect
to see it, which is why it happens so
early and so many times, which we will
get to in a moment. That air sack off
the digestive system is the proto lung.
just pouches branching off the gut with
enough blood vessels to absorb oxygen
from the air. But the weirdest thing
possibly in this whole video is that
obviously lots of fishes don't need
lungs. But it turns out to be super
useful to have a pouch of air inside of
them for controlling their buoyancy,
which is why they have swim bladders.
And if you had asked me a couple of
months ago, I would have definitely
said, "Well, lungs must be a
neofunctionalized swim bladder." Swim
bladders are things that water fish
have, and lungs are things that landfish
have. and water fish came first. So,
swim bladders must have come first. But
no, y'all, swim bladders are
neofunctionalized lungs. I don't know if
this is as cool to you as it is to me,
but I literally ran to my wife when I
found out about this. But then fish that
had lost their lungs, sometimes turning
them into swim bladders, occasionally
would need lungs again, which is why air
breathing in fish has evolved like
dozens of times. There are many
different ways that they do it. Catfish
have a gut pouch that isn't related to
the original lungs. Mud skippers can
trap air in their gill areas and have
special muscles that stop their gills
from collapsing in air. Betta fish have
a special high surface area pouch that
branches off from the interior of the
gill that fills with air. But the lungs
that we breathe into are based on the
same lung structure that evolved in the
original bony fish more than 400 million
years ago. Lungs were around for so long
before vertebrates got on land that even
though I think most people would
consider them the number one thing fish
would need, I'd actually put it pretty
much last on the list cuz it wasn't just
evolutionarily inevitable. There have
been fish with lungs for as long as
there have been fish with bones. Oh,
also this is why sharks don't have swim
bladders because swim bladders evolved
from lungs and lungs evolved in bony
fish and sharks aren't bony fish and
only fish with lung ancestors have swim
bladders today. The reason I got
obsessed with this is that there's
something really beautiful behind this
quirky little science fact that either
people are fish or fish don't exist.
Like if you're in sciency spaces, fish
don't exist is one of the best little
fun little science facts. It's just
three words long and it sounds really
fake, but it turns out that it's
basically true. And there are ways in
which this fact isn't even true. Like if
you instead of looking at ancestry, you
look at physiology or lifestyle. Fish
are just aquatic vertebrates. Though
people are generally pretty big
sticklers about whales not being fish.
So, like if whales aren't fish, then
fish don't exist. That's my opinion. You
could complexify the physiological
definition of fish and say that they
can't breathe air, but that doesn't work
cuz there's lots of air breathing fish.
I say, and I'm sorry for getting into
this. I could do a whole video on it.
Doesn't really belong in this
conversation. I think whales are
definitely fish. They're either fish
because we're using a physiological
definition, in which case, whales are
aquatic vertebrates, or they're fish
because we're using a taxonomic
definition that's based on ancestry, and
they, like us, are evolved from fish.
And so I'm comfortable saying that if
whales aren't fish, then yeah, fish
don't exist. But that's not my point.
That's not the thing that's like filling
me with this excitement of knowledge
right now. My point is that there's this
cute little fact that because we are
more closely related to bass than bass
are to sharks. Then the reality is that
humans and dinosaurs and frogs and
eagles are all just super weird fish
covered in keratin and sucking air into
their lungs. But there's something
really big there that every one of those
species and every one of us originated
in the ocean. And we can see that not
just in the salty water we lock inside
of our bodies to carry around our entire
lives, but also in our physiology, in
our eye lens that has the wrong
refractive index, in our lungs that
branch off from our digestive system,
and in our skin built out of a protein
that originally evolved not to hold
together a body, but to hold together a
cell. If you see this whole story the
way it really is, you realize that fish
don't exist isn't the fact. The fact is
you are a fish and you never really left
the water. You carry the ocean with you
everywhere you go. Physically in
yourselves, but also ancestrally in your
genes and your physiology. I am obsessed
with this and I hope that you are too.
We never left the water. Also, if you
have questions about any of this or
corrections or things you'd like to
argue with me about, please leave them
in the comment. I'll make a follow-up
video. And also because this was such an
information dense episode, I do actually
have references for this one. That's all
in the description. Again, the shirt
designed by Matias Ball is available for
the next two weeks. You order it during
the pre-order window, we close it down,
we have all the orders, we know exactly
how many we have to ship and there's no
waste. It looks really good and it is
cozy and it tells this story that I have
become so obsessed with. And I really do
honestly love that I get to commission
artists to make things that I want there
to be in the world. So check it out.
There is of course a link in the description.
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