Welcome to Crash Course Big History.

Today we're going to be traversing

the evolutionary epic-- the great story

of magnificent beasts, terrifying predators,

quite a lot of extinctions, and countless varieties

of evolutionary forms.

It's the ultimate epic-- millions upon millions

of species playing out a drama

that has so far lasted 3.8 billion years

with 99% of the actors having already left the stage forever.

And you thought finding employment

in this job market was tough.

You've already won the lottery, my friend.

The keystone of our story is:

So, in the 1830s, a young Charles Darwin traveled

around the world on theH.M.S. Beagle.

Inarguably, by the way, the most important beagle of all time.

I apologize, Snoopy, but it's true.

Darwin had the rare and amazing opportunity

to study a great variety of the world's wildlife,

and upon returning to England, he discovered

that a variety of finches he had collected

on the Galapagos Islands had beaks that were subtly adapted

to their different environments and food sources.

Darwin later combined this idea with the observation

of how populations tend to overbreed

and strain their resources.

I mean, if there's competition for resources in an environment,

then animals with useful traits would survive

and pass those traits on to their offspring.

Those who didn't survive long enough

to reproduce would have their traits wiped out

from the evolutionary tree: natural selection.

We talked some on Crash Course Big History about good science,

and Darwin was a good scientist.

He worked on his ideas for two decades,

systematically finding new evidence

to support his case, and then finally in 1859,

he publishedOn the Origin of Species,

and it sent shockwaves around the world.

The book offered an explanation for why so many species

that seemed perfectly adapted to their environment

could have been formed by a blind but elegant law of nature.

Darwin's theory was so elegant yet so effective

that his colleague Thomas Huxley exclaimed,

"How extremely stupid not to have thought of that!"

Side note: if you ever read On the Origin of Species,

try to get a first edition because in later editions,

Darwin made a bunch of revisions in answer to some critics,

but he got it actually more right the first time.

Speaking of which, one of the phrases only included

in the later editions

and commonly attributed to Darwin was:

But that phrase was actually coined by Herbert Spencer,

father of the more troubling Social Darwinism,

which tried to apply nature's rather harsh laws

to human social orders.

I prefer Darwin's original phrase:

Everything from cuckoo birds that lay their eggs

in the nests of other birds to giraffes

whose long necks are good for reaching food in high trees,

to humans whose brains make up for their fragile bodies

are selected for naturally.

An even better phrase, though, would be:

While all genetic mutations are generated

by a random copying error or a random variation

completely beyond the animal's control,

the selection of those traits is not random.

Successful variations that allow you to survive

and reproduce are determined by the very specific circumstances

of your environment where elimination-- death--

might not be far away.

So the selection of your traits is done by a very specific

and sometimes brutal list of criteria.

This is why people who say that they don't understand

how all animals could have "evolved by chance"

don't really understand how evolution works.

Here's another phrase that doesn't get it right:

In everyday speech, "theory" means "guess,"

but in science, a theory is something

that was tested time and time again,

explains many different observations,

and is backed up by a mountain of evidence.

Evolution is a theory like gravity is a theory,

and you don't go jumping out your window

because gravity is just a theory.

Why are we so certain?

Emily knows.

Evolution is one of the most tested, most utilized,

and widely accepted theories in science.

It's backed up by literal tons of fossil evidence,

which can show us shared traits with species

that no longer exist and help us map out lines of descent

for creatures around today.

DNA sequencing further tells us about lines of descent,

and you can measure the commonality of the DNA possessed

by two animals to tell how closely related they are

and when they may have split off from a common ancestor.

Radiometric dating allows us to assign dates to various fossils,

further helping us map out lines of descent.

Then there's the simple fact that extinct species

are always found in the same rock layers

you'd expect to find them.

Which is why you don't see a bunny skeleton

in Cambrian rock layers from half a billion years ago.

That's also how we know that Dimetrodon is not a dinosaur.

Closely related species are often geographically distributed

near one another.

That's not to mention that:

Whether it be the discovery of a new species

that recently moved into a different environment,

the development of newly adapted bacteria into superbugs,

the evolution of new breeds of rapidly reproducing insects,

or the almost constant changes in gene distribution

in animal populations all over the world.

So, remember the prokaryotes and the eukaryotes?

Gradually some single-celled eukaryotes began

to work together in a thing called symbiosis,

where one cell did something in exchange

for another cell doing something else,

thus aiding the survival of both.

Some eukaryotes became so cooperative and even interwoven

that one cell could not possibly live without the other.

Symbiosis was particularly handy in times of disaster.

Around 650 million years ago,

the earth was completely frozen over.

Snowball Earth was not a great place for life.

Many underwater bacteria survived

under the ice in oceans.

Photosynthesizers may have survived

in small hot spots where there was liquid water.

In such constrained conditions, it's likely

that individual cells started to work together more and more.

Now is where we start to blaze

through the evolutionary epic of:

In nature, species compete in niches.

It's also calledniches, depending on where you're from,

but I call them niches, as:

When niches are full, competition is heavy,

traits become finely tuned,

and evolution generally slows down a little.

But, when a disastrous extinction event wipes out

the majority of the animals living in a niche,

the surviving species have room in a lack of competitors

to evolve new traits very fast to fill the niche again

in what we call an adaptive radiation.

The evolutionary epic is dotted with periods

of niches filling up, being swept clean by disaster

and filling again by new rapidly evolving species.

Example: for the longest time dinosaurs ruled the earth

and mammals were a puny, timid race

of small shrew-like creatures that stayed out of their way.

Sometimes we burrowed in the ground or only came out

at night or confined our diet to tiny bugs.

We could not compete with dinosaurs in their niches.

Then the dinosaurs were wiped out and mammals were able

to rapidly fill all the empty niches,

creating apes and elephants and horses and even whales.

So after Snowball Earth,

the Ediacaran era gives us the first extensive fossil evidence

for multicellular organisms.

There were various ancient forms

that resembled today's worms, corals, mollusks,

various underwater plants.

But then in the Cambrian era,

adaptive radiation really got underway

and multicellular life filled thousands upon thousands

of niches unlocked by their new traits.

A lot more is just possible for multi-celled organisms

than for single-celled ones.

Like, not to brag or to bring up my astonishing strength again,

but I can bench much more than a eukaryote.

Some of the most famous creatures that got their start

in the Cambrian were trilobites, these bug-like creatures

with exoskeletons that existed in a variety of species

and forms, occasionally in swarms of thousands.

And they didn't go extinct for nearly 300 million years.

That's over a thousand times longer

thanHomo sapienshave been on the planet.

Also, as my four-year-old son can tell you,

the Cambrian era had predators, like anomalocaris,

which reached sizes of nearly a meter long

with razor sharp teeth and grasping limbs.

By the time of the Ordovician period,

photosynthesizers were making their first tentative steps out

of the sea into a new niche, the land.

Plants colonized coastlines first and then gradually,

over millions and millions of years,

moved further and further inland.

In the oceans, life continued to be abundant

with fish and sharks multiplying into a variety of forms.

And there were all kinds of crazy life forms,

like underwater scorpions that were two-and-a-half meters long.

I mean, for the first 100 million years

of complex evolution, a mind-boggling diversity

of creatures was emerging.

But that also meant all the niches

on the planet were getting very full

and many competitors in the same niche made it difficult

for a new species to enter it with ease.

And then came extinction.

I feel like extinction is going to be a thing, Stan.

Is there anyway we can make a thing for extinction?

Yes!

Ordovician Earth went through first a major freezing period,

killing off many warm water species,

and then a radical heating period,

killing of many cold water species.

Many ecological niches were swept clean

and this removal of competition meant

that new species could enter empty niches

and evolve rapidly in one of those adaptive radiations.

There was also incentive to move out of the seas

and onto the land.

In the Silurian period, one of those groups

that evolved rapidly by filling terrestrial niches

was the arthropods, those exoskeleton species

and the ancestors of many of today's bugs.

Since plants continue to colonize the land

and more and more of the Earth's surface was becoming forested,

that increase in the number of photosynthesizers

increased the percentage of oxygen in the atmosphere

to between 30% and 35%.

Today, it's approximately 21%.

Arthropods came out of the sea, started filling niches on land,

and their metabolism took advantage of this

all-you-can-respire oxygen buffet,

growing to enormous sizes, like a dragonfly

with a meter-long wingspan or a scorpion 1.8 meters long.

Again with the scorpions!

In the early Devonian period,

the forests of the earth were composed

of mosses, ferns, and short shrubs.

Some plants eventually evolved a woody covering,

which provided some back support and allowed them

to grow taller and taller and compete with others

in their niche by grasping higher and higher for sunlight.

In the first episode, we did promise

to explain the existence of trees.

Bingo.

Also, by the Devonian, our vertebrate ancestors

had arrived on land.

Unlike arthropods, vertebrate skeletons are on the inside

and our skin is more porous, making it easier

for water to escape.

This limited our ability to fill land-based niches.

At first, we were amphibious.

From this amphibious ancestor, all tetropods gain

their characteristic skeletal structure--