Hi, I want you to meet my friend Shoshanna.
She's a Zebra Finch and she is very good at it.
She's here to help me talk about comparative anatomy,
which is the study of similarities and differences
between the anatomies of animals.
We study comparative anatomy because it helps us learn more
about our evolution and our shared ancestry.
Organisms have their evolutionary history written all over them,
if you know what to look for.
For instance, which of these two living organisms would you say
I'm more closely related to?
Shoshanna the finch? Or Gordon the plant?
This isn't a quiz, but....
Sure, it is a quiz.
It is the easiest quiz that you will ever take in your life.
Gordon is green and can make his own food with just sunlight,
water and carbon dioxide.
While Shoshanna can't make her own food,
she has to move around to find stuff to eat,
escape predators, find mates and poop on park benches.
Just like me...except, not the pooping on park benches,
I mean the moving around.
So yeah, shocker, I am more closely related to a bird than to a plant.
You get a gold star.
So that one's obvious, but as the relationships between organisms
get closer, the questions get a lot more interesting.
So what IS an animal?
I mean, I know you know what an animal is, but when you're looking
at Shoshanna and me here, what clues you in to the fact that we
are members of the kingdom Animalia?
Two things:
For starters, we're both moving.
Locomotion is a really good sign that an organism is an animal,
unless you're a sponge.
Now, I know what you're thinking though:
Protists, bacteria and archaea move around using
flagella and cilia.
But they also only have one cell.
It's the multi-cellular locomotion that's so peculiar and specific
to animals.
So animals move because of the second trait that
we have in common: We're heterotrophs.
We get our energy from eating other life forms.
Locomotion also helps us avoid predation and
seek out mates for reproduction.
Plants can mate by dispersing their seed to the wind or
having an insect come by and fertilize them.
But if land animals did that?
Things would get like, really messy and gross.
Some aquatic animals actually do just release their sex cells into
their surroundings and cross their fingers
and presumably close their mouths,
and hope that somebody gets pregnant.
So, since animals have to eat and move around,
they've evolved anatomical forms that help them do those things.
But obviously those forms aren't the same on all animals.
For instance, in order to move, Shoshanna and I both have to be
able to apply force to the ground or the air to propel ourselves.
Here's me pushing off the ground with my feet.
And now, here's Shoshanna applying force to the air with her wings,
which keeps her afloat and moving.
And if I had a shark in the studio with me, which thankfully I do not,
so I'll just pretend to be a shark,
my fins would apply force to the water,
which would propel me forward.
Now, you have to be careful with this stuff,
because even though similar body structures,
like fins or wings or feet, can mean animals have
a close common ancestor, it can also mean the animals
just evolved similar forms just because that's the best
structure for the job.
When this happens, it's called convergent evolution.
For example, a tuna, a penguin and a seal are all animals that
spend all or a lot of their time in the water.
One's a fish, one's a bird and one's a mammal,
but all three of them have a suite of similar features,
the most notable being a really sleek,
fusiform body that can move through the water like nobody's business
and fins for propelling those bodies.
But of course those three animals have very different
evolutionary origins.
Each of these three marine animals have independently "converged"
on similar body shapes because they live in the same environment
and need to do the same sorts of things.
So instances of convergent evolution can make
linking physical structure of an animal to its evolutionary history
a little bit tricky.
Which is why, for a long time, nobody really put much stock in
comparative anatomy as proof as evolution.
That is, until Thomas Henry Huxley came along.
[BIOLO-GRAPHY]
Thomas Henry Huxley was the Father of Comparative Anatomy and the
Father of Modern Paleontology.
And he invented the word "agnostic" to describe his spiritual views.
And he was the first person to conclude that birds evolved from
small carnivorous dinosaurs!
Phew! I'm glad I'm sitting down for this.
Plus, we have much respect
for his facial hair.
Huxley was born in England in 1825, and though he started out
as a doctor, after serving as a ship surgeon on a voyage
to Australia in his 20s, he took to studying marine invertebrates.
During his voyage, he sent all his papers back to England, and when
he got home he found that he had become a kinda famous marine
invertebrate expert and he was admitted into the Royal Society.
Huxley made friends with other hot-shot natural scientists,
including Charles Darwin, and a few years later,
when Darwin outlined his theory of evolution
in On the Origin of Species, Huxley is reported to have said,
"How extremely stupid not to have thought of that!"
In fact, he became such a huge Darwin supporter,
that everybody started calling him "Darwin's Bulldog"
because he threatened to cut the fool who badmouthed evolution.
This is a good one: Huxley said, when talking about
On the Origin of Species, "Old ladies, of both sexes,
consider it a decidedly dangerous book."
You just got Huxslapped.
With this new tool of the theory of evolution,
and in part to help promote the theory of evolution,
Huxley connected paleontology and biology together by looking
for similarities in anatomy in the fossil record,
where he found all kinds of interesting stuff.
Like some really obvious similarities between prehistoric
horse fossils and modern day horses,
as well as between dinosaurs and birds,
though nobody really bought his insights into the resemblance
between birds and dinosaurs for another hundred years.
And just in case you were still on the fence as to whether
intelligence is heritable, Thomas Henry Huxley is the
grandfather of Brave New World writer Aldous Huxley and of
Sir Andrew Huxley, who won the Nobel Prize for Physiology
or Medicine in 1963.
Because all animals come from the same evolutionary origin,
in addition to sharing some anatomical structures
like Huxley studied, we're also built from the same
rudimentary blueprint.
Our cells work pretty much the same no matter what sort
of animal we are.
So while animals have different strategies for moving around
and acquiring food, once the food is gotten,
all animals break it down, turn it into useful energy,
distribute nutrients, and eliminate waste
in pretty similar ways,
unless you're a sponge.
Each of those functions is performed by collections of cells
that group together in the body to form tissues.
There are 4 primary tissue types in the human body:
epithelial tissue, connective tissue,
muscle tissue and nerve tissue.
Epithelial tissue is formed by cells that
bind very closely together.
A layer of it covers every organ and lines the digestive tract to
prevent crazy acids and poop and stuff from going where it's not
supposed to go.
Epithelial tissue can also produce the slippery fluid to let your
organs slide over each other like the membrane that lines the
inside of your ribs so that your inflating lungs don't build
up friction as they expand.
Most types of connective tissue are made up of fibrous strands of
collagen protein, and it adds support and structure to your
body and holds your parts together.
Some examples of connective tissue include the inner layers
of your skin, your tendons, ligaments, cartilage, and bone.
But oddly enough, connective tissue isn't defined by it's ability
to connect, but instead by the presence of an extra-cellular
matrix, meaning that part of the tissue extends outside of cell.
And so, somewhat confusingly, blood and fat are also considered
connective tissues.
Muscle tissue is made up mostly of two specialized proteins:
actin and myosin, which can slide past one another and
allow for movement.
It also includes a bunch of other proteins,
including that longest word-in-the-world one, titin.
And finally, there's nerve tissue, which generates and conducts
electrical signals in the body.
These electrical messages are managed by the nerve tissue in the
brain and transmitted down the spinal cord to the rest
of the body.
Nerve tissue is made up of two types of cells the neurons,
which do the electrical work, and the glial cells,
which insulate and support the neurons.
These tissues are then organized into organs,
which perform different functions in the body,
and these organs work together in organ systems.
For instance, most animals have a digestive system made up of a
mouth and an esophagus and a stomach and intestines
and an anus.
And a lot of animals have a skeletal system made up of bones,
tendons, ligaments and cartilage.
We're going to be talking about each of these systems in a lot
more detail in a few weeks.
These organ systems, like many different kinds of anatomical
structures, are shared by lots of different kinds of organisms,
unless you're a sponge.
Because about 1.6 billion years ago, an organism developed
that had a digestive system and a muscular system, and suddenly that
organism was in it to win it.
That organism was the common ancestor for all animals today,
and it's the reason me and Shoshanna here are gonna hang out
at the animal family reunion.
Thank you for watching this episode of Crash Course Biology.
If you want to check out any of the stuff we talked about,
there's a table of contents over on the side.
Or you can just re-watch the whole video
and we'll love you extra much.
If you have any questions for us, please leave them
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We'll see you next time.