Evolutionary Development: Chicken Teeth – Crash Course Biology #17


When little kids say they want to
grow up to be a scientist, here’s what they actually mean: a. They want to blow things
up in a laboratory setting. b. They want to get bitten by a
radioactive monkey which will turn them into a terrifying
humanoid battle monkey. c. They want to make a fly with
eyeballs on its butt, or like, chickens with fangs. Most of the time, scientists don’t
get to do that stuff. Like, you may blow something up,
but it’s either going to be in a really controlled setting or it
will be an accident, in which case, it’s BAD. Like, the lab where
I first worked- The first lab I ever worked in
had a blood stain on the ceiling. BUT, if you’re a scientist
specializing in the amazing new discipline of evolutionary
developmental biology, you might just get to make a fly
with eyeballs on its butt, or even a chicken with teeth. But no battle-monkeys! So evolutionary developmental
biology, or Evo/Devo for all of us cool kids, is a new science
that looks deep into our genes to figure out how exactly they
give instructions to make different parts of our bodies. And as the name suggests,
it’s giving us some hot leads into the nature of,
and mechanisms behind, evolution. One big thing it’s showing
us is that animals, all animals, are way more
similar than we ever even imagined. You know how you always hear about
how humans and chimps are 98.6% genetically similar? It kind of makes sense, right? Because chimps and humans you can
see that we kind of look alike. Like if you walk into a
coffee shop and there’s a chimp sitting in a chair,
and it’s like, maybe wearing a fedora
or something, you might briefly mistake that
chimp for a human. You might not even notice it’s
sitting there. It could happen! But what about a mouse? You are
not going to mistake a mouse for a person. How genetically similar do
you think we are with mice? How about 85% similar? OFF SCREEN: Shut up! HANK: No, I won’t shut up.
Humans and mice are 85% genetically identical! So why then are mice little and
skittery, covered in white fur and have beady little eyes,
while I can walk upright in a non-skittery way
and have beautiful, deep, mysterious eyes? I’ll give you the long answer
in a minute. But for now the short answer is:
It’s all because of incredibly weird and amazingly
powerful genes called developmental regulatory genes. Mostly when we’re thinking of
genes we think of the things that code for some useful enzyme
or protein, like the ones that determine what our ankles are
going to look like. But those ankle genes don’t just
come on and off at random, they have to be turned on and off.
That’s what these developmental regulatory genes do:
they activate the genes that put the body parts together. They don’t tell them how to do it,
mind you, they just tell them when, or if, it’s time to get to work. And since they’re the ones pretty
much calling the plays, regulatory genes start working rather early
in embryonic development. For instance, a kind of regulatory
gene called gap genes are responsible for telling the
blastula, that little hollow ball of cells that forms during
the early stages of development, “make a mouth here and let’s put
an anus over on this other end.” But probably the most amazing kind
of regulatory genes are the homeobox genes, or hox genes,
which kick into gear after the embryo is more developed. Hox genes literally control the
identity of body parts, setting up how an animal’s
body is organized. Like, here’s where you put the leg
and here’s where you put the tail. And like I said, these hox genes
don’t give instructions for how to create legs and tails, there
are a bunch of other genes that are in charge of the actual
craftsmanship of the body parts. You can think of the hox genes as
the head architects in the construction of a building:
they’ve got the master plan, but they don’t actually do any of
the construction themselves, that’s WAY beneath them. And under this top tier of
regulatory genes, there are scads of other genes that act as
subcontractors. If a hox gene tells its direct subordinates to
“make an eye here” and the subordinates then turn around and
activate other regulatory genes that give more specific
instructions, like “This is where to put the collagen
for the outer shell of the eyeball” and “make some nerve tissue
for a retina right here.” Again, these second-tier genes, and third tier and fourth tier
and on down the line don’t actually do any of the work,
they just send instructions down the chain of command,
adding more specific information to the instructions as they go. It’s a really rigid hierarchy: No gene in your body, aside from
that very first one, does anything until it’s told when and
how much to do it. So, because I know that you’re
such an intelligent and curious student,
I know what you’re wondering right now. What activates that first
regulatory gene? And how in the name of Bill McGinnis do they
tell each other to do stuff?! Well, since Evo-Devo is a
relatively new discipline, we don’t really know all
of the stuff that I wish we knew. That’s for YOU to figure out
when you become a biologist. But scientists are starting to
think that a lot of the human genome that has until recently
been considered “junk DNA,” because it apparently doesn’t code
for anything, might actually be regulatory genes. For instance,
just in the past few years we’ve learned that humans have about 230
separate hox genes in our genome, and they appear on every one
of our chromosomes, even the sex chromosomes. How regulatory genes are inherited
is also still being studied. From what scientists have been
able to deduce so far, most regulatory genes are
inherited in very much the same way as all
your other genes. But for some really early-stage
regulatory genes, the proteins that they’re coded to produce,
called gene products, have already been made and are
sitting in the egg before it’s fertilized, waiting to tell
the embryonic cells what to do to get the ball rolling.
Another thing that your mom did for you that you
probably never thanked her for. So here’s the really cool thing:
Even though most regulatory genes are inherited, each individual
within a species tends to have the exact same DNA
sequence in those genes. There aren’t even
different alleles. And when you think about it,
they kind of have to be the same, since all individuals of a
species should be built from the same basic blueprint. Like,
you don’t want people walking around with thumbs sticking
out of their heads. Now, this gets me back to me
and my beady-eyed friend the mouse. Hox genes and other regulatory
genes that are at the very highest tier,
the ones that say “head here” and “eye here,” not only tend to be the same
within a species, they’re also very similar across
different animal groups. Like between all mammals
or even all vertebrates. The differences between my
regulatory genes and a mouse’s regulatory genes are way down
the chain of commands, where the instructions
are the most specific. But the big-picture stuff,
like you’re a vertebrate, you have four limbs,
you have hair and breast tissue and ear bones and all that
stuff that all mammals have, all of those general
instructions are the same. And that’s why 85% of humans’
genetic makeup is the same as mice. Mice’s. Mouse-mice-meese’s? OK, you’ve been very patient,
my students, so I’ve got a surprise for you. We’re gonna make some BUTT EYEBALLS! In 1995, in a very cool and also
totally messed up experiment, a team of researchers in
Switzerland took a hox gene from a mouse embryo, one that
said “EYE GOES HERE,” and inserted it into the DNA of a
developing fruit fly embryo. BUT, they activated the
mouse-eyeball gene in a region of the fly that would
become the fly’s back leg. And so what do you think happened? I’m not going to tell you yet
because I want you to guess. WRONG! The fruit fly DID NOT
grow a mouse eyeball next to its back leg!
It actually developed a fruit fly eye next
to its back leg. Remember, the gene didn’t say
how to make an eye, it just gave the
instruction to make an eye. If it had said how to make it,
you’d get a mouse eye on the fruit fly’s butt. Instead it told the fruit fly
cells “make an eye here!” and those fruit fly cells had
their own instructions, regulated by another whole set
of regulatory genes. And once they got the order to
make the eye, they made it in the only way they knew how. That is pretty freakin’ messed up,
but also FREAKING AWESOME! Now, in addition to getting me in
touch with my inner mentally-unstable child scientist,
this kind of experiment is where Evo-Devo has begun to revolutionize
our understanding of evolution. Because we’ve known that evolution
can take place over a really long time, but we haven’t
really been able to figure out how it sometimes happens
really fast. Traditionally one of the main
ways scientists have explained evolution is through
genetic mutations. But an organism would have to do
a lot of mutating to evolve from, say, a dinosaur into a bird. It used to be thought that a 50%
change in form would require a 50% mutation in genes. Which would take a long time,
way longer than the pace at which we see things
actually evolving. But it turns out that a small
change in a regulatory genes up at the top of the chain of
command can have huge effects on how an organism is
actually assembled. To understand how this works,
let’s look at why birds don’t have teeth. So, birds evolved from
theropod dinosaurs, which are these freakin’ sweet dinosaurs
like velociraptors, which look a lot like birds,
but way more awesome and with big, razor-sharp teeth. But you may have noticed that
birds don’t have razor-sharp teeth. They have beaks. Under the old
way of thinking about evolution, the loss of the teeth would have
had to happen very slowly as the genes that make enamel and dentin
gradually mutated to make less and less of each of those things
until they made none at all. And for a long time, that’s just
how we thought dinosaurs evolved into birds. But there was
one problem: It would’ve taken way longer for all of those
mutations to occur than it actually took for the dinosaurs
to evolve into birds, based on the fossil record. Fortunately, Evo-Devo is
offering us an explanation. A single mutation in the
regulatory genes could have shut off the enamel and dentin production, and another mutation
in another regulatory gene could have upped the keratin production
from the level of “make some scales” to the level
of “make a beak”. So birds actually still do have
genes for teeth from their dinosaurian ancestors, they’re
just not expressed, because the regulators don’t turn them on. How do we know? Well, in 2006,
a biologist at the University of Wisconsin named John Fallon,
who studies birth defects, was looking at some mutant chicken
embryos and noticed that they had formed little teeth,
little baby reptile teeth. It turns out that mutations
affected the chickens’ gene regulation, allowing the teeth,
a feature lost to birds around 60 million years ago, to
just pop back up again. The same sort of crazy throwback
features have been observed in snakes born with legs like their
ancestors once had, or blind cave fish suddenly born with eyes. If you turn those genes on,
those ancient, repressed features come back. CRAZY! I know! That’s so cool! It’s all fairly new science so
this is still in my head just really fantascinating. That’s a word I made up! Thank you for watching this
episode of Crash Course Biology, I hope that I blew your mind or
that you learned something or that you do well on your test or
why-ever you came to watch this episode. If you go over there, you can
click to catch up on things that you may have missed or just
re-watch the whole episode because you’ve got to emphasize
it in your mind otherwise, you don’t remember these things. Thanks to everybody who
helped put this episode together. If you have any questions,
you can catch us down in the YouTube comments below
or on Facebook or Twitter, and we will endeavor
to answer them. Goodbye.

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