Animal Development: We’re Just Tubes – Crash Course Biology #16


You’re a miracle. Did you know that? Today we’re going to talk about
animal development and the miracle of life. The process that animals go
through to turn, like, a sperm cell and an egg cell
into a multi-cellular organism is incredible. It’s not just incredible, it’s unbelievably, transcendentally magnificent, man. Magnificent! And dudes… the thing is: we’re all just, like, tubes! OFFSCREEN: Dude, no edge.
HANK: I know. So, animals. They come in all sorts of shapes
and sizes and smartnesses and things. And in our infinite
wisdom, humans have come up with a system of classifying animals
based on how similar they are to each other. Today we’re going to be talking
about some differences between animals at the phylum level, here,
which happen at the earliest stage of development. That’s because a bunch of really
big decisions are made within a few moments of the sperm
fertilizing the egg, and how this early embryonic
groundwork is laid makes a big difference when it comes to what
kind of amazing multicellular being you’re going to end up being. Or, you know, not so amazing. Hello, sea sponges! Animal phyla range from the very
simplest, like sea sponges, to the more complicated. Signs of an animal’s complexity
include how symmetrical it is, how many organs it has,
and how specialized its cells are. A sea sponge, for instance,
is a total freakin’ mess symmetry-wise and doesn’t really
have any organs to speak of. In fact, if you were to blenderize
a live sea sponge and then leave the sponge smoothie to settle
overnight, you’d wake up the next morning to find that the
surviving cells had found each other and were reforming
themselves into a sea sponge again. Try doing that with any
other animal and- -actually, NO. DO NOT TRY DOING THAT
WITH ANY OTHER ANIMAL. My point is, most animals are more
complicated than sponges, and an animal’s complexity has everything
to do with what happens in the first couple of hours
of its development. And here’s a neat rule of thumb:
the more complex an animal is, the more it resembles a tube
with some different stuff layered around it. And now’s when you’re like,
“Uh, Hank… uhh, what?” Okay, so here’s the deal: A really important clue indicating
that you’re dealing with a complex life form is how many layers of
tissue it makes in its very early stages of development. Sea sponges
make just one, things like jellyfish and corals make two,
and all the more complicated animals make three. So, the early stages of development
are similar for most animals. Remember, sperm cells and egg
cells are both gametes, haploid cells that only carry
one set of chromosomes. Once the sperm fertilizes the egg,
the two haploid cells fuse their information together and form
a zygote, one beautiful diploid cell with two sets of chromosomes
that contain all the instructions needed to create a
new living thing. Which is, of course,
totally far out. Fast forwarding to like an hour
and a half after fertilization: The zygote has started dividing
and cleaving through mitosis, resulting in 2, 4, 8, 16 cells
until it creates a solid ball of 32 cells. This is
actually a morula. Or more-ah-luh.
At least according to this guy. LAPTOP: more-ah-luh, or more-u-luh. And the morula actually looks a lot
like a raspberry, or a mulberry, which is what it’s
named after in Latin. Mmm, juicy! Morula pie! Oh god. They’re going to
ban us from schools! As more cells are created,
the solid wad of cells begins to secrete a fluid that forms a
space in the center, resulting in a hollow
sphere of cells called a blastula. Okay, so pay attention, because
here’s where we get down to the real business. Most animals that you just sort
of think of off the top of your head have a mouth, right?
And by the same token, most of them have an anus. Yeah, go ahead and get your
giggles out because I’m going to be saying anus
a lot in this video. For example, right now! ANUS. So most animals have
a mouth and an anus. Wait for it: Unless you’re a sea sponge! Sponges don’t have a
mouth OR an anus. And there are also other animals
like your sea anemones, your jellyfish, your corals,
that have just one hole that serves as both mouth and anus. Aren’t you glad we’re a little
bit more complicated than that!? It’s worth noting that these
animals have radial symmetry. All their junk kind of radiates
out from a central point that is their mouthhole-slash-poohole,
and that is a little bit more sophisticated than having no
symmetry at all, like a sponge, but just barely. I mean, their anus and their
mouth are the same thing. But more complex animals,
with the notable exception of the echinoderms like starfish
and sand dollars exhibit bilateral symmetry: We have
two-sided bodies that look the same on both sides. Something else we have in common
is we have an anus that is, get this, in a different
place than our mouth. This separation is pretty key,
because it means that we as animals are basically
built around a tube, a digestive tract, with a mouth at
one end and an anus at the other. The process of forming this tract
is called gastrulation, and it’s kind of a big deal. So, when we left our little
blastula, it was still just hangin’ out, a little, round,
hollow ball of cells. Gastrulation begins when an
indentation starts to form at a single point on the blastula. This place on the blastula that
starts to invaginate, or fold in on itself, is called
the blastopore. Now for animals whose mouth and
anus are the same thing, this is where the development
stops, which is why they only have one hole for
all their business. But in everything else,
the invagination continues until the indentation makes its
way all the way through and opens on the other side, creating what
is essentially a hollow bead made of cells. Now we have a gastrula! Now, two different things can
happen at this point, depending on what kind of animal
this is going to be. It can either be an animal whose mouth is the
orifice that’s formed by the blastopore, called a protostome,
or one whose anus is the structure created by the blastospore,
a deuterostome. Guess which one you are. Write it down. I want to see your guesses. Chordates, that is to say all
vertebrates and a couple of our relatives like starfish,
are deuterostomes, meaning we were once just a
butthole attached to a little wad of cells.
And that includes you. And me. Congratulations! And hopefully you’re getting the
idea here: The formation of the digestive tract is the first thing
that happens in the development of an animal, and it happens to
every living thing, whether it’s going to be a tardigrade
or a polar bear or a T Pain. THE MIRACLE OF LIFE! Now so far the little hollow bead
of cells is basically two layers of tissue thick: an outer layer
called the ectoderm, and an inner layer called the endoderm.
These are called your germ layers. For those organisms that stop
developing at this point, with that classy mouth-anus combo,
they only get two germ layers. They’re called diploblastic,
and they were born that way, it’s totally OK. But for us more complex animals,
whose mouths are separate from our anuses- Yes! we develop a third layer of tissue,
making us triploblasts. Here, the ectoderm is going to end
up being the animal’s skin, nerves, spinal cord and most of its brain,
while the endoderm ends up forming the digestive tract: the esophagus
and stomach and colon and stuff. And in addition, some of the cells
start breaking off between the endoderm and the ectoderm and
form another layer called the mesoderm. These will eventually end up as
the muscles, circulatory system, reproductive systems, and
in the case of vertebrates, most of the bones. So what’s our embryo looking
like now? Awesome. From here, this little guy is going to go on
to fulfill his destiny as a ladybug or walrus or whatever. And now this seems to me like a
great time take a look at a completely disproven theory that
biologists hold in the highest contempt, but which is actually a
kind of useful way to think about the way an animal embryo develops
into fully-formed animal. Plus, it makes for a great BIOLO-GRAPHY! Back in the mid-1800’s a German
zoologist named Ernst Haeckel tried to prove what we now refer
to as Recapitulation Theory. Basically, and this is not basic
at all, recapitulation theory states that “ontogeny
recapitulates phylogeny.” Eh? In other words, ontogeny,
or the growth and development of an embryo, recapitulates,
or sums up, phylogeny, which is the evolutionary
history of a species. So this means, for instance,
that a human embryo over the course of its development,
will go through all the hundreds of millions of years
worth of evolutionary steps that it took for a single-celled
organisms to evolve into a fully-tricked-out person. Haeckel was a contemporary
of Darwin, and On the Origin of Species made a giant
impression on him, especially a section of it that notes how
cool it is that all vertebrate embryos look pretty similar
to one another, regardless of whether they’re a mammal
or a bird or a reptile. Darwin, however, cautioned that
this probably wasn’t a very good way of reconstructing the
history of evolution, he just thought it meant that the
embryological similarities were evidence of common
ancestry between species. Well, Haeckel was kind of a spaz,
and he definitely heard the first part of Darwin’s idea,
but not the rest. So Haeckel jumped on this idea
and very quickly wrote a couple books about how the development
of an embryo mirrors the evolutionary development
of adults of a species. Which is exactly what Darwin
said was not happening. Anyway, Haekel did spend a lot
of time looking at embryos and observed that the slits
in the neck of the human embryo resembled gill slits of fish,
which he took to mean that we must have at one point
had a fish-like ancestor. He drew tons of figures of
different animal embryos in different stages of
development to prove his theory, and his illustrations of embryos
started to make their way into textbooks all over the world. Haeckel is exactly the sort of
person who really ticks other scientists off because real
science-loving scientists like to sit and think about
stuff and find out all the problems with an idea before
they start publishing books about it. And here Haeckel was,
firing off volume after volume, and before long all the “data”
he had “collected” convinced a bunch of other people,
including Darwin actually, that he was onto something. But in the end it turned out that
Haeckel was kind of fiddling with his drawings of embryos to make
the data fit his Recapitulation Theory,
instead of, you know, making the theory
to fit the data. But by that time,
everybody already knew about the theory,
and if there’s anything harder than teaching people something,
it’s unteaching them something. So, here we are, almost 150 years
later, and we’re still talking about the Recapitulation Theory. But that might have less to do
with the stubbornness of a bad idea than it does with the
fact that it actually makes a kind of sense when you don’t
take it literally. At some point in our embryonic
development, humans actually do have gills slits like a fish,
and tails like a dog or a pig or a jaguar, and webbed fingers
and toes like a frog. So while it’s not true that
every zygote re-enacts all of animal evolution,
the way that an animal develops does reminds us
that we are, in fact, related to other Chordates. And we all start off
as just a tube. A mouth at one end,
an anus on the other. Which is pretty freakin’ amazing. Thank you for watching this
episode of Crash Course Biology. If you’re confused about any of
the anus-anus that we covered today you can go back
and watch that now. If you have any questions for us,
you can leave them down in the comments below or
on Facebook or Twitter. Thank you for watching,
and we’ll see you next time.