Comparative Anatomy: What Makes Us Animals – Crash Course Biology #21


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 in the comments below or
you can get in touch with us on Facebook or Twitter. We’ll see you next time.

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