Evolution has repeatedly and independently transformed various unrelated crustaceans into crab-like forms through a process called carcinization, suggesting the crab body plan offers significant evolutionary advantages.
Mind Map
Click to expand
Click to explore the full interactive mind map • Zoom, pan, and navigate
Evolution has made crabs five times. Not
five species of crabs, but five separate
times. It has turned completely
different animals into crabs. Like a
lobster looking thing evolves for
millions of years and boom, crap. Then a
different lobster looking thing that is
totally unrelated does its own thing for
millions of years and boom, also a crab.
This keeps happening. Scientists call it
carcinization, which is just a fancy way
of saying nature won't stop making
crabs. And the weirdest part, we don't
exactly know why. We have guesses, sure,
but the fact that evolution keeps
hitting the crab button over and over
suggests something about being a crab is
just superior. Crab supremacy. Are they
better than us? We're going to find out.
So, we need to talk about what's going
on here because this whole situation is
honestly pretty bizarre. And by looking
into this, I am now afraid that humans
might turn into crabs. So what is going
on? So basically carsonization means the
process of turning into a crab, which is
already kind of funny because most
scientific terms describe something that
happens once, maybe twice. This one
describes something that keeps happening
[music] over and over, which should tell
you something right there. Here's what
carcinization actually looks like. You
start with some kind of longbodied
creature. Maybe it looks sort of
shrimpish or maybe sort of lobster-ish.
It's got a long body and a big tail that
sticks out behind it. Then over millions
of years, that body gets flatter, wider,
and the tail shrinks and folds up
underneath. Suddenly, you've got
something that looks a whole lot like a
crab. Flat shell, tucked tail, and walks
around on these spled out legs. The
thing is, this isn't just happening to
one group of animals. It's happening to
completely separate groups throughout
prehistory. That's what makes it weird
because usually when you see the same
body shape evolve multiple times, it's
in really different animals. You know,
birds have wings, bats have wings, but
nobody's confused about whether a bat is
a bird. They're obviously doing
different things. But with crabs, you've
got animals in the same family doing
this, crustaceians, and they keep
landing on the exact same design.
Scientists actually have specific
criteria for what counts as being
carcinized. Your shell has to be flatter
than it is long. You need these distinct
edges on the sides. And your underside
has to be this broad fused plate. And
your abdomen, that's your tail part. It
has to fold completely under your body.
So you can't even see it from above. If
you picture a crab, that's exactly what
you're seeing. No tail hanging out, just
this discshaped body with legs. Now,
here's where this gets interesting. [music]
[music]
Not every crab is actually a crab. Yes,
this has been a crab conspiracy from big
crab government. They don't want you to
know that not every crab is actually a
crab. So, you got to hit the like and
subscribe button right now to stay
informed against the rebel crab
government. See, there are true crabs,
which scientists call brakura. And then
there are false crabs. And false crabs
aren't false because they're lying or
anything, but they're false because they
evolved the crab look, but they're not
in the true crab family. They're in this
other group called the anomeura. So
you've got hermit crabs, king crabs,
porcelain crabs, all these things that
look like crabs but technically aren't.
And both groups did [music] this
independently. True crabs evolved from
some non crab ancestor. False crabs
evolved from some other non crab
ancestor separately, which means the
crab body plan is so effective that
evolution found it at least twice just
between these two groups. And that's not
even counting all the times it happened
within each group. You can actually tell
true crabs and false crabs apart pretty
easily if you know what to look for.
True crabs have four pairs of walking
legs that you can see. False crabs look
like they only have three pairs because
that fourth pair is tiny or hidden
somewhere. But both of them have that
same general vibe. Flat, wide, hard
shell, no visible tail. Because it's a
really clear example of something called
convergent evolution. That's when
different animals evolve the same
solution to the same [music] problem.
Except usually conversion evolution
happens across really different groups.
But this is all happening with
crustaceans, which is much weirder cuz
it means there's something about that
crab shape specifically that keeps
working. Something about having a flat
body and a tucked tail solves problems
that long body crustaceians run into.
And look, evolution doesn't have a goal.
It's not sitting over there going,
"Let's make crabs." Or maybe it is. We
should be scared. It's just random
mutations and the ones that help you
survive stick around. But when you see
the same thing happen five separate
times, you start to think maybe it's not
that random. Maybe there's something
about the way crustaceian bodies are
built that makes turning into a crab
kind of inevitable if you're in the
right situation. Because here's the
thing, these crabshaped animals are
everywhere now. Oceans, rivers, beaches,
and they've been succeeding for hundreds
of millions of years. So whatever
advantages the crab shape gives you,
they're not small. They're big enough
that evolution keeps finding the shape
over and over in different time periods,
whether it's right now or in ancient
prehistory in different parts of the
world with different starting animals.
The scientist who coined the term back
in 1916, his name was Lancelot Boradale.
Of course, his name was Lancelot because
why not? Anyways, he described
Carsonization as basically taking a long
buddied crustation and squashing it down
and widening it out until it looks like
a crab, which is pretty much exactly
what happens. The back end shrinks, the
front end spreads out. You end up with
something that can scuttle around
sideways and hide under rocks. So,
that's what's happening. Evolution keeps
turning things into crabs because
apparently being a crab works really
well. But saying it happened multiple
times doesn't quite capture how many
times it has actually happened. And when
I say multiple times, I mean it's
honestly kind of absurd. Five times at
least, but probably more. Scientists are
still arguing about the exact number,
which is kind of funny because you would
think how many times did this happen
would be a straightforward question. But
evolution doesn't exactly leave
receipts. So, they're working backwards
from what they can see now and what they
can find in fossils. The first group is
true crabs. Rachura. These are your
classic crabs. The ones that showed up
first in the fossil record around 200
million years ago in the Jurassic
period. Somewhere back then, some
long-bodied crustaceian ancestor
flattened out and tucked its tail under
and boom, true crab. Every single modern
true crab, and there are thousands of
species of these, comes from that one
original shift. So, that's one time.
Then you've got king crabs. And king
crabs are fascinating because they're
enormous, right? They're the ones you
see in seafood restaurants that cost a
ridiculous amount of money. But they're
not actually true crabs. They're false
crabs, and they evolved from hermit
crabs. Yes, those little guys that carry
shells around. At some point, a group of
hermit crabs got too big for shells,
stopped using them, and their bodies
hardened up and flattened out into
something that looks a whole lot like a
regular crab. Except here's the weird
part. If you look at a king crab's
anatomy, it's still asymmetrical inside.
Most of their organs are lopsided
because hermit crabs have asymmetrical
bodies so they can coil into the spiral
snail shells. And king crabs never fully
lost that. So they're scuttling around
looking like crabs, but internally
they're still kind of hermit crabs,
which is honestly pretty strange when
you think about it. Third, you've got
porcelain crabs. They are small,
delicate, flat little things you find on
rocky shores. They look like crabs. They
act like crabs, but they're actually
more closely related to squat lobsters
than [music] to true crabs. They show up
in fossils around 150 million years ago,
which means they independently became
crabshaped during the age of [music] the
dinosaurs, completely separate from true
crabs. Fourth, there's this group called
stone crabs. And I'm not talking about
the ones that you eat. I'm talking about
Lis Herta, which is this hairy crab from
the Indo-Pacific that looks so much like
a true crab that people called it a
stone crab. But it's not. It's another
false crab, another independent
evolution of the crab shape. And fifth,
you've got coconut crabs. Now, coconut
crabs are technically hermit crabs, but
they're hermit crabs that said, "Forget
the shell." As adults, they don't use
shells. They just walk around with their
hard, flat bodies, climbing trees, and
cracking open coconuts with their claws.
They can even weigh up to 9 lb. And
they're the largest land arthropod on
Earth. And they're basically crabs now,
even though they start life as
shellwearing hermits. They also have
been known to eat humans. But let's not
talk about that for too [music] long or
else big crab government is going to get
me. So that's five clear cases where
evolution looked at a non- crab and
turned it into a crab. But there's more
because scientists have found extinct
groups of crustaceans that also went
crabshaped. There were these things
called cyclia [music] in the triacic and
Jurassic periods that looked strikingly
crabish even though they weren't even in
the same lineage as modern crabs. And
within the groups I just mentioned,
there are subgroups that might count as
separate instances, too. Some
researchers think true crabs might have
evolved the crab shape twice within
their own family tree. Some think there
are more false crab evolutions we
haven't fully documented yet. The fossil
record isn't complete and genetic data
is still being analyzed. And every few
years, someone finds a new weird crab
fossil that changes the count. But the
point is, evolution has run this
experiment at least five times that
we're sure about. Five separate lineages
of crustaceians starting from different
ancestors ending up with the same
general body plan. Flat shell, tuck
tail, sidewalking legs. And these didn't
happen all at once. They have been
spread over hundreds of millions of
years. Which means this isn't some fluke
or one-time accident. This is a pattern.
Nature keeps making crabs because
something about being a crab means you
get to live longer. So when did exactly
all of this start? Well, way earlier
than you would think. The first real
crabs show up around 200 million years
ago in the early Jurassic period.
Dinosaurs are just starting to become a
thing on land. And down in the ocean,
something's busy flattening itself out
and tucking its tail under. Just quietly
becoming a crab while the rest of the
world is focused on giant reptiles. For
a while, not much happens. Crabs exist.
They're doing their crab thing, but
they're not exactly taking over. Then
you hit the late Jurassic about 150
million years ago and suddenly crabs are
everywhere, specifically in reef
environments. Coral reefs are expanding
and there are all these little cracks
and hiding spots and crabs just explode
in diversity. Scientists call this the
first big crab radiation, which sounds
dramatic, but it really just means a
bunch of new crab species showed up at
once. But the Cretaceous period is
really when [music] things start to get
genuinely wild. This is 145 to 66
million years ago. And paleontologists
actually call this the Cretaceous Crab
Revolution, which is a great name for a
band, but it is also an accurate
description of what happened. Something
like 80% of modern crab families
originated during this time. Most of the
crabs you see today can trace their
family tree back to the Cretaceous. This
is also when crabs started developing
their really nasty traits. Strong claws
that could crush shells. Some of them
became specialized predators that would
hunt down snails and clams and just
crack them open. There's this fossil
crab called Mega Xantho, and it had
these toothy ridges on its claws
specifically for breaking shells. So,
crabs weren't just scuttling around
anymore. They were actively haunting
things and doing it pretty well. And
then crabs started leaving the ocean,
which seems ambitious for an animal that
breathes through gills, but they made it
work. They moved into fresh water, and
some of them started hanging out on
beaches. A few really bold ones went
into the forests. And we know this
because in 2021, scientists found a crab
preserved in amber. Yes, amber, which is
fossilized tree resin. You find
mosquitoes in amber. You find ants in
amber. You don't find crabs in amber
because crabs don't hang out in trees.
Except apparently 100 billion years ago
they did, which is a thought that should
terrify you. This tree crab is called
Cretuspara Antha. Do not clock me on
that name. I don't know how to say it.
And it is tiny. But the fact that it
exists at all means crabs were exploring
land way earlier than anyone thought.
We're talking 50 million years earlier
than the previous estimate. So, while
T-Rex is still tens of millions of years
away from even existing, crabs are
already climbing around in forests
getting stuck in tree sap. Now, my
favorite prehistoric crab is this thing
called Cali Chimeriia Perplexa. And
perplexa is right because this crab is
genuinely perplexing. It's 95 million
years old. And when scientists found it,
they didn't know what to do with it
because it doesn't look like a crab.
It's got these huge eyes with no
sockets, paddle-shaped legs for
swimming, and a long tail that sticks
out behind it, which is one of the
things crabs specifically don't have.
Tell me this thing does not look like
Paris the Pokémon, like without the
mushrooms. It's like a one to one. I
feel like scientists actually called it
the strangest crab that ever lived. Its
name means perplexing beautiful chimera,
which is a very fancy way of saying we
have no idea what the we're looking at
because Cali Chamaria kept a bunch of
features that you normally see only in
baby crabs. It's like it just never grew
up and it was only the size of a quarter
and it swam around in open water, which
is not standard crab behavior at all.
What this tells us is that the
Cretaceous wasn't just crabs perfecting
the crab shape. It was crabs trying out
every possible variation. Some worked,
some didn't, but they were
experimenting. And then the dinosaurs
went extinct 66 million [music] years
ago, and crabs just kept going. They
didn't even slow down. Fast forward to
today, and you've got crabs that are
genuinely absurd. The Japanese spider
crab has legs that span 13 ft. 13 ft. It
is the largest arthropod on the planet,
and it is just a crab that decided to
get really, really big. It lives in deep
water near Japan and can live for a
hundred years, which is a long time to
be a spider crab. On the opposite end,
you've got pea crabs are the size of,
well, who could have guessed, a carrot?
No, it's a pee. They're the size of a
pee. Uh, they're a few millimeters
across. They live inside oysters and
muscles and just steal food from their
hosts. Their bodies are soft because
they don't need armor when they're
living inside someone else's shell. So,
you've got the full range, giant crabs,
tiny crabs, and everything in between.
Then there's the coconut crab, which I
mentioned earlier, which is nine pounds,
lives on land, and climbs trees, and has
claws strong enough to crack open
coconuts and also strong enough to hurt
you pretty badly if you bother it. There
are stories of coconut crabs stealing
camping gear, which is both impressive
and deeply annoying if you're the
camper. And in 2005, scientists found
the yeti crab deep sea covered in hair.
Actually, it's bristles, but they look
like hair. So, everyone calls it the
yeti crab. And it doesn't eat the hair.
It eats the bacteria that grows on the
hair. It's farming bacteria on its own
arms, which is a weird way to make a
living, but it works. The range of crabs
today is honestly kind of ridiculous.
Mountain streams, deep ocean trenches,
coral reefs, beaches, rainforests, your
local motel. Some are carnivores, some
are filter feeders, some are herbivores.
They figured out how to survive
basically everywhere. Which brings us
back to the question we started with.
What is it about being a crab that makes
this possible? Scientists don't actually
know for sure. And I know that's a
disappointing answer, but it's the
honest one. They have theories, good
theories, but nobody's proven exactly
which one is right or if it's all of
them at once. The first theory is
protection. Crabs have a thick shell on
top and their tail is folded underneath
the shell. So, there's no soft exposed
belly for a predator to bite. Compare
that to a lobster where the entire tail
is just hanging out in the open. It
makes for an easy target. Crabs fix that
problem by tucking everything away.
You're basically a rock with legs.
you're pretty hard to eat. The second
theory is stability. Crabs are wide,
flat, and low to the ground. You can
wedge yourself into a crack in a reef
and just stay there. Predators cannot
flip you over, can't pull you out, and
you're firmly planted, and if you need
to move, you're not going to tip over
because your center of gravity is
basically on the floor. The third theory
is the sideways thing. Most crabs walk
sideways. There's actually a reason for
that. If a predator comes at you, you
can dart left or right without having to
turn your whole body around. Your eyes
stay pointed at the threat while you're
escaping, which is genuinely smart if
you think about it. Most animals have to
turn and run, which means they're not
looking at the danger anymore. Crabs
don't have that problem. And the fourth
theory, and this one's interesting, is
that the crab body plan is versatile.
Once you get rid of that big tail, your
legs can evolve to basically do
anything. Some crabs have paddle legs
for swimming. Some have digging legs for
burrowing. Some have thick walking legs
for land. The crab shape is a base model
that you can modify. And evolution loves
things that it can modify because that
means you can adapt to new environments
without starting from scratch. There's a
researcher named Javier Lu who studies
crabs. Yes, he's the crabs guy. And he
pointed out that having a big lobster
tail actually limits what the rest of
your body can do. You're hauling this
huge tail around. So that constrains
your options. Get rid of the tail and
suddenly you've got room to try new
things. The fifth theory is hiding. A
flat body fits under rocks. It fits in
coral. Fits in places where a
long-bodied animal can't go. And if
predators can't reach you, you don't get
eaten. Simple. But here's the thing that
nobody wants to admit. These are all
guesses. Educated guesses, sure, but
still guesses. Because the real answer
is probably some combination of all
these things. And it probably varies
depending on which group of crabs we're
talking about. Maybe one group became
crabs because of predator pressure.
Maybe another group became crabs because
it let them invade new habitats.
Evolution doesn't follow a script. It
just does what works in the moment. Or,
and what the elite won't tell you is
that it's big crab lobbying the
government to make more crabs. They
don't want this leaked, but I have to
tell people. There's also this idea that
maybe being a crab isn't the advantage
by itself. Maybe it's what being a crab
allows you to do next. You become a
crab, and now you can evolve better
claws or faster legs or whatever. The
crab shape opens doors. It's not the end
goal. It's just a really useful starting
point for other changes. What we do know
is that when something evolves five
separate times, there's a reason.
Natural selection doesn't keep picking
the same option unless that option is
working. And crabs are everywhere.
They're in every ocean, every kind of
habitat, and they're thriving. So,
whatever the crab shape is doing for
them, it's enough. But scientists are
still studying this. They're looking at
crab DNA to find patterns. They're
comparing Carsonized crabs to their non-
crab relatives to see what's different.
testing whether crabshaped animals
actually survive predators better or
reproduce more or whatever. The research
is ongoing and every few years someone
finds a new fossil that changes part of
the story. So the answer to why does
this keep happening is we don't know
yet, but we're working on it. In the
meantime, evolution is probably busy
making another crab somewhere. But now I
should mention that evolution doesn't
just turn things into crabs. Sometimes
it does the opposite. There's actually a
term for when crabs stop being crabs.
decarcenization, which is exactly what
it sounds like. A crab-shaped animal
evolves back into something that's not
crabshaped. And this has happened at
least seven times that scientists know
about. Take frog crabs. These are true
crabs, proper members of the brachiier
family, but they've gone and made
themselves longer and narrower again.
They borrow into sand on the ocean
floor. Apparently, being flat and wide
wasn't working for that. So, over time,
they stretched back out. Their shells
got longer and their tails unfolded a
bit. They're still crabs technically,
but they're crabs that decide to undo
some of the crabness. Mole crabs did
something similar. These are the little
guys you find on beaches that burrow
backwards into the sand right when the
waves wash up, and they basically turn
themselves into torpedoes. Smooth,
streamlined, barely any legs, no big
claws. They don't look like crabs at
all. They look like someone tried to
design the least crab thing possible
while still technically being a crab.
And it works for them because they need
to dig fast and filterfeed in the surf.
But the really interesting case is the
whole hermit crab situation. Hermit
crabs originally gave up being crabs so
that they could live in shells. They let
their abdomen stay soft and coiled so
they could fit into snail shells. That's
decarcinization. Then later, king crabs
evolved from hermit crabs and went back
to being crabshaped. That's
recarcenization, I guess. So, you've got
crabs that become not crabs that became
crabs again. It's always crabs. It's
crabs all the way down. And you could
still see the evidence of this in king
crab anatomy because, as I mentioned,
their insides are asymmetrical. Their
reproductive organs are lopsided because
their ancestors spent millions of years
twisted up inside spiral shells. King
crabs never fully corrected that.
They're wearing the evolutionary history
of their hermit crab phase on the
inside. So, why would any animal stop
being a crab if being a crab is so
great? Well, turns out being a crab is
great for a lot of things, but not
everything. If like if a crab runs into
its natural predator, a cooking pot,
it's going to die. That's a case where
you would not want to be a crab. If
you're trying to burrow straight down
into sand really fast, being flat and
wide is actually bad. You want to be
narrow and streamlined. If you're
carrying around a borrowed shell for
protection, you don't need a thick hard
shell of your own. You can save the
energy and just stay soft. Evolution
doesn't care about being consistent. It
just does whatever helps you survive
right now. If being a crab helps, great.
If unbeing a crab helps, also great.
Neither one is permanent. Meanwhile,
scientists are still trying to map out
the full crab family tree, and they're
running into problems because about half
of the branches are basically unknown.
They don't have genetic data for a lot
of groups. Fossils are incomplete, and
every time someone digs up a new
prehistoric crab, it changes part of the
picture. In 2021, a team of researchers
published this big analysis trying to
nail down exactly how many times
carcinization happened and when. are
using DNA sequencing now trying to find
genetic patterns that show up every time
something becomes a crab because if
there are specific genes that get turned
on or off during carcinization that
would explain why it keeps happening.
Maybe it's just an easy switch for
crustaceian bodies to flip. People
always ask if everything is eventually
going to become a crab. And the answer
is sadly no. Don't worry, you're not
going to wake up a crab as much as I
would like you to. This only happens to
crustaceians because their bodies are
built in a modular way. They've got
segments that they can arrange. Humans
don't have that. Neither do most
animals. You can't turn a mammal into a
crab. The body plan doesn't work that
way. Well, not yet, that is. But within
crustaceations, yeah, we'll probably see
more carcinization in the future because
if it keeps working, there's no reason
to think evolution is done making crabs.
Give it another 50 million years, and
some shrimp lineage that hasn't tried it
yet probably will. Right now, there are
thousands of crab species all over the
planet. Deep ocean vents where the water
is boiling hot. Antarctic waters where
everything is frozen. Coral reefs,
rivers, forests, deserts. Somehow, some
crabs can hold their breath and climb
trees. Some crabs live their entire
lives in pitch black caves. Some crabs
are transparent. There are crabs that
have symbiotic relationships with sea
enemies that carry them around in their
claws for protection. I actually own
some in my fish tank behind me. They're
really cute, but they tried to pinch me
once. One species of crab, and I'm not
making this up, farms algae gardens on
its back. It plants algae in its shell
and tends to it and then eats it later.
That is agriculture. Yes, crabs invented
farming. It's not advanced farming, but
still, humans didn't invent that. Crabs
did. The crabs are rising up. They are
learning. The range of what crabs can do
is genuinely impressive when you lay it
all out. And that's probably the real
answer to why carcinization keeps
happening. The crab body plan is a Swiss
Army knife. It works in enough different
situations that evolution keeps coming
back to it. Not because it's perfect,
but because it's good enough for most
things and flexible enough to be tweaked
for specific things. Scientists are
going to keep studying this. They're
going to find more fossils, sequence
more DNA, and run more comparisons
between crabshaped and non- crabshaped
crustaceations. Eventually, we'll have a
more complete answer, but for now, we've
got a pretty good idea of what's
happening, even if we don't know every
detail of why. So, in the end, are
humans turning into crabs? Not yet, that
is. But hey, maybe in a 100 million
years they can come back to this video
and say, "Damn, look at this dummy. He
thought we weren't going to be crabs."
But meanwhile, it was always crabs. It
was crabs all along. If you enjoyed
watching this video, make sure to give
it a like and comment down below what
your favorite part was. And if you would
like to learn any other topics that
relate to prehistory. If you don't want
to miss any of our future uploads, make
sure to hit the subscribe button and hit
the hype button to make sure this video
gets seen by more people. And if you
want to watch another one of my videos,
Click on any text or timestamp to jump to that moment in the video
Share:
Most transcripts ready in under 5 seconds
One-Click Copy125+ LanguagesSearch ContentJump to Timestamps
Paste YouTube URL
Enter any YouTube video link to get the full transcript
Transcript Extraction Form
Most transcripts ready in under 5 seconds
Get Our Chrome Extension
Get transcripts instantly without leaving YouTube. Install our Chrome extension for one-click access to any video's transcript directly on the watch page.
