Evolution and Genetics, which puts us at the halfway mark

of CrashCourse Biology.

So far we've learned about DNA, genetics, natural selection,

how cells multiply, populations, speciation, replication,

respiration, and photosynthesitation.

I'm so proud of you.

But I couldn't let this section end without discussing the

iscussion that everybody can't help but discuss these days:

Evolution.

It's a thing.

It's not a debate.

Evolution is what makes life possible.

It allows organisms to adapt to the environment as it changes.

It's responsible for the enormous diversity and complexity of life

on Earth, which not only provides organisms with sources of food and

some healthy competition.

It also gives us some truly awesome stuff to marvel at.

And even though evolution makes living things different from

one another, it also shows us how we're all the same.

All of life, every single thing that's alive on the Earth today,

can claim the same shared heritage, having descended from the very

first microorganism when life originated on this planet

3.8 billion years ago.

There are people who will say that this is all random-

It's not.

And that this clumsy process could not be possible for the majestic

beauty of our world.

To them, I say, well at least we agree that our world is beautiful

but, well you're probably not going to enjoy

the rest of this video.

To me, there are two sorts of people in the world,

those who are excited about the power and beauty

and simplicity of the process of evolution,

and those who don't understand it.

And somehow, I live in a country where only 40% of the population

believes that evolution is a thing.

The only possible reason for that that I can accept is that

they just don't understand it.

It's time to get real, people.

First, let's understand what we mean when we talk about

the theory of evolution.

Evolution is just the idea that gene distribution changes

over time, which is an indisputable fact which we observe all the time

in the natural world.

But the THEORY of evolution is a large set of ideas that integrates

and explains a huge mass of observations from different

disciplines including embryology, paleontology, botany, biochemistry,

anatomy and geophysics.

In every day language, the word "theory" means "hunch"

or even "hypothesis."

But in science, a theory is an idea that explains several

phenomena at once.

Thus, The theory of evolution is a bunch of ideas that explain

many things that we, as humans, have observed for

thousands of years.

It's the theory that meticulously and precisely explains the facts,

and the facts are indisputable.

So let's spend some time going through the facts, and how

evolution explains them all so well.

First, fossils:

The fossil record shows that organisms that lived long ago

were different from those that we see today.

Sounds obvious, but two hundred years ago it seemed

a little bit crazy.

When scientists first started studying

dinosaur fossils in the 1820s, they thought that all dinosaurs

were basically giant iguanas.

That's why the first fossil dinosaur was named Iguanodon.

It wasn't until the fossils of two-legged dinosaurs started

showing up in the 1850s that scientists had to grapple with

the idea that organisms of the past were somewhat similar to ones today

like, dinosaurs were reptiles, but many of them took on a diversity

that's barely recognizable to us.

And of all those ancient not-really-iguanas

were all extinct, either dying out completely or evolving into

organisms that survive today, like birds.

Fossils make it clear that only evolution can explain the origin

of these new kinds of organisms.

For instance, fossils taught us that whales used to walk.

Whales are cetaceans, a group of mammals that includes porpoises

and dolphins, and biologists long suspected that whales descended

from land mammals.

Partly because some modern whales still have the vestigial remnants

of a pelvis and hind-limb bones.

But it wasn't until recently, the 1990s and 2000s, that

the pieces really came together.

First, paleontologists discovered fossils of DOR-oo-dons,

cetaceans that had different skulls from modern whales but

still had the same vestigial leg bones.

Then they found even older fossil remains of another cetacean that

actually had hind legs and a pelvis.

The pelvis wasn't fused to the backbone like ours is,

so it did swim like a whale, but more importantly,

it still had ankle bones

And they were ankle bones that are unique to the order that

includes bison, pigs, hippos and deer.

So by following these clues left behind in fossilized bones,

paleontologists were able to track the origin of whales back to the

same origin as bison and pigs.

This leads us to another series of facts that evolution explains:

Not how animals were different, but how they are incredibly similar.

Last week we talked about Carl Linnaeus and how he

classified organisms by their structural similarities.

Well he didn't know anything about evolution or genetics,

but when he began grouping things in this way,

he hit upon one of evolution's most prominent clues:

homologous structures.

The fact that so many organisms share so many finely detailed

structures shows us that we're related.

Let's go back to the whale.

Like my dog, Lemon, and me, the whale has two limbs at the

front of its body, its front flippers.

And so does this bat, its wings.

Inside our limbs we all have the very same structure: one longish

bone on top, connected to two thin bones at the joint, followed by a

cluster of small bones called the carpals, and then our fingers,

or digits.

We each use our forelimbs for totally different purposes:

the bat flies, the whale swims, Lemon walks and I...

you know,

jazz hands!

Building limbs like this isn't the most efficient way to swim

or fly or walk.

Our limbs have the same structure because we descended from

the same animal, something like this more-gan-uh-cah-don here,

which, yeah, has the same forelimb structure.

In the first stage of our existence, every

vertebrate looks almost exactly the same.

Why?

Because we're all descended from the same initial vertebrates.

So our structures are the same as other mammals and

other vertebrates, sure, but it also turns out that our molecules

are the same as, like, everything.

In fact, if we were ever to find life on Mars or something,

the sure fire way of knowing whether it's really

extra-terrestrial is to check and see if it has RNA in it.

All living things on our planet use DNA and/or RNA to encode the

information that makes them what they are.

The fact that we all use the same molecule itself suggests that

we are all related, even if very distantly.

But what's more, by sequencing the DNA of any given creature,

we can see precisely how alike we are.

The more closely related species are, the more of

the same DNA sequences they have.

So the human genome is 98.6% identical to that

of the chimpanzee, our closest evolutionary relative,

and fellow primate.

But it's also 85% the same as a mouse.

And I wonder how you're going to feel about this, about half of our

genes are the same as in fruit flies, which are animals, at least.

So, just as your DNA proves that you descended from your parents,

your DNA also shows that you descended from other organisms

and ultimately, from that one prokaryotic microorganism

3.8 billion years ago that is the grandparent of us all.

Now when it comes to species that are very similar,

like say, marsupials, their distribution around the world

or their biogeography, is also explained extraordinarily well

by the theory of evolution.

Animals that are the most similar, and are the most closely related,

tend to be found in the same regions, because evolutionary

change is driven in part by geographical change.

As we talked about in our speciation episode,

when organisms become isolated by physical barriers, like oceans

or mountains, they take their own evolutionary courses.

But in the time scales we're talking about,

the geographical barriers are much older,

and are often even the result of continental drift.

So, marsupials.