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.