we've explored all of the fundamentals of biology,

from the tiny little machines that make us work, to the systems that

power the most awe-inspiring and most complex organisms on Earth.

Now I want to take our learning to the next level

simply by broadening our perspective.

With a solid understanding of biology at the small scale

under our belts, it's time for the long view.

For the next 12 weeks, we'll be learning how the living things

that we've studied interact with

and influence each other, and their environments.

It's the science of ecology.

The idea of an organism interacting with its environment may sound

simple, but it's actually pretty marvelously complicated.

Nothing escapes the long arm of ecology.

Life influences the chemical makeup of the atmosphere,

the geology of the planet, the climate.

Nothing impacts life on earth today more than humanity does.

And so we're going to be exploring that relationship,

between mankind and our environment,

in a lot of detail, in the coming weeks.

Life is powerful.

And in order to understand how living systems work,

you first have to understand how they originated, and developed,

and diversified over the past 4.5 billion years of Earth's history.

The principles that you hear about here today

will form the framework of our next 12 weeks together.

Get ready for the epic drama that is the history of life on Earth.

Earth is like a cheap rental house in a college town:

there's always somebody living there, but tenants are always moving

in and out, new tenants moving into old ones' rooms, and sometimes

the electricity gets turned off because nobody paid the power bill.

It's pretty much just chaos.

And like a rental house, the Earth isn't invincible.

I mean, a good semester of back-to-back frat parties can

tear a house up pretty fast.

And then, you know, everybody gets evicted,

and some new tenants move in.

Which gives you a little bit of perspective on humanity's position

as the current dominant species on the planet:

There have been others before us,

and there will probably be others when we're gone.

That's just how the planet rolls.

But it's taken eons for the earth to get the way that it is today.

Earth formed about 4.6 billion years ago, and for a while,

it was just a chunk of rock, circling the sun, suffering collisions

with other chunks of rock generating unreal amounts of heat.

Eventually the constant smashing slowed down a little bit, and the

outer layer of Earth cooled, but the core of the planet remained hot.

Like, really hot. As hot as the surface of the sun.

It gives you some idea as to how much energy and radiation

contributed to the earth's formation when you consider that

four and a half billion years later,

there's still molten stuff in the middle of our planet.

And it's that inner heat that makes the earth's surface so dynamic.

The planet's insides have been continually bubbling up,

destroying what's there, renovating and rearranging furniture.

It's this constant renewal that's part of what makes life here possible.

And in the scheme of things, it didn't actually take life

very long to show up on that hot rock.

About 4.4 billion years ago, the planet was still getting pelted

with giant chunks of rock and ice left over from the formation

of the solar system, there was no oxygen in the atmosphere,

and volcanoes were firing off all over the place.

But it was finally cool enough for some of the water

in the atmosphere to turn from vapor into liquid.

The first seas formed, and in them,

and in the atmosphere, a soup of chemicals.

Nitrogen, nitrogen oxides, carbon dioxide,

methane, ammonia, hydrogen, hydrogen sulfide.

All those things that with the help of heat and lightning and who

knows what else eventually created small organic molecules.

And some way, some how, life happened.

We don't know how it happened, I wish we did, but we don't.

The first life on Earth probably wasn't even life as we think of it:

It was just a collection of chemicals surrounded by a membrane,

because phospholipids, as you may remember,

spontaneously form bilayer membranes in water.

Over time some of the chemicals inside these membranes developed

into amino acids and eventually RNA, the nucleic acid

that was probably the first genetic material.

These collections of chemicals trapped within membranes,

called protobionts, most likely began to grow and split

and replicate themselves until some crazy copying error

gave way to DNA nucleotides, which is a way more stable

repository for genetic information,

because it's double-stranded and not single-stranded like RNA.

And once these little guys had DNA, they were on their way.

They might have only been one cell,

and they were probably living in the equivalent of

a hydrothermal vent, but they were making the most of it.

So the first living things were prokaryotes,

single-celled organisms with no nuclei, that were probably

pretty similar to the archaea that we find today living

in hydrothermal vents, sulphur hot springs, and oil wells.

And I apologize for pronouncing archaea wrong...

for the entire Biology series. My bad.

These prokaryotes probably made their grand entrance

between 3.9 and 3.5 billion years ago,

during what's known as the Archaean Eon.

This is the first of what scientists would identify as 3 eons

in Earth history: the Archaean, the Proterozoic and the Phanerozoic

The Phanerozoic being what we've been experiencing for

the last half billion years or so.

An eon, as you might expect, takes freaking forever.

The first one went on for 2.5 billion years.

So, we break up the eons into Eras, Periods, and Epochs, which are

organized by the big-deal events that we can see in the fossil record.

So, from 3.5 billion years ago until about 2.1 billion years ago,

these prokaryotes were all alone on Earth.

But then something weird happened: The amount of oxygen

in the atmosphere suddenly shot up basically from none

to 10% in a very short period of time, geologically speaking.

This oxygen was most likely produced by a brand new prokaryote

called cyanobacteria, which had figured out how to make

its own food through photosynthesis.

The more cyanobacteria were out there,

the higher the atmospheric concentration of oxygen became.

This "oxygen revolution," as it's called, probably spelled D-O-O-M

for a lot of prokaryotes out there that had evolved without oxygen.

And this was also one of the first real game-changers for life on Earth

because it was the first major instance of living things

bringing massive change to their own environment.

In fact, it may be the earliest example of Ecology at Work:

Cyanobacteria changed the atmosphere, judo-chopped the competition,

and made way for the evolution of

living things to take a new, specific direction.

Then about 2.1 billion years ago, a new kind of organism

made its big debut: eukaryotes.

And these, as you know, are a big deal

because they include all plants and animals.

Eukaryotes probably evolved by a process called endosymbiosis,

where one prokaryote parasitized another prokaryote,

or maybe just ate it but didn't digest it,

and the result was actually awesome

for both the host and the parasite slash undigested prey.

It formed a single-celled organism with organelles,

specifically mitochondria and plastids, which probably evolved

from those eaten or parasitic prokaryotes.

This allowed for much more complexity,

and by 1.5 billion years ago,

we start seeing multicellular eukaryotic organisms in

the fossil record, the very first of them probably being algae.

But it wasn't until around 535 million years ago

that the eukaryotes went berserk.

And that's known as the Cambrian explosion, a super-major biological

Golden Age when the diversity of all animal life on Earth exploded.

Nobody's entirely sure what started it, but suddenly life created

innovations that the planet had never seen: Creatures used minerals

in the seawater to build skeletons and shells, some acquired weapons

like claws, while others developed defensive plates.

The evolutionary arms race between predators and prey was underway.

This heralded the dawn of the Phanerozoic Eon,

the one we're in right now.

That's right, the Earth spent the better part

of two eons under the rule of a bunch of archaea and bacteria

and some, like, soft-bodied worms.

Until the Cambrian exploded and we started to see a lot of

animal phyla that we actually are hanging out with today.

After the Cambrian, the party got so hot in the oceans

that by the Ordovician Period around 500 million years ago,

plants, animals and fungi started colonizing the land,

probably as a strategy for escaping predation.

Now there were whole new ecosystems to explore and adapt to and create.

During the Devonian Period about 365 million years ago,

tetrapods, 4-legged vertebrates that probably evolved from

lobe-finned fishes, showed up on land,

and so did arthropods, like insects and spiders.

From here, we begin to see ecological systems that we

recognize today because organisms were changing their environments

by consuming oxygen in the atmosphere and releasing carbon dioxide.

And you know who likes carbon dioxide? The plants.

The Carboniferous Period that extended from 359-299 million years

ago was when the plants entirely went nuts.

The forests were so dense and so widespread that they made

all our fossil fuels, all the coal and oil that we now use

to power all the things with, were made over the course of

about 60 million years.

This time, it was plants that had changed both

the climate and the geology of Earth.

These forests cranked out so much oxygen that the atmosphere

contained around 35% oxygen rather than today's 21%.

All this oxygen started cooling the planet because there wasn't

enough carbon dioxide to maintain the balmy temperatures

that the vast Carboniferous jungles needed to survive.

So the whole system crashed, and all that carbon from

these forests sunk into swamps and eventually got locked in rocks.

Of course, now we're releasing all that carbon by burning fossil

fuels, which is certainly helping to keep the planet toasty now.

That right there: some good Ecology.

In the Permian Period, 299-251 million years ago, all the landmasses

of the world joined to form one giant continent that we call Pangea,

altering global climate and ocean currents,

and animals and plants evolved in response.

We start seeing gymnosperms, the first plants with seeds

like modern pines and spruces and firs, and archosaurs,

the granddaddies of dinosaurs and modern birds showed up.

But you've probably picked up enough of a pattern here

to predict that this party didn't last forever.

About 252 million years ago, something happened,

or maybe a lot of things happened in quick succession,

but whatever they were, movie executives take note:

the Permian-Triassic extinction event would make the most

awesome disaster film of all time.

Because up to 96% of all marine species

and 70% of terrestrial vertebrate species bought the farm,

and it's the only known mass extinction of insects.

About 57% of all taxonomic families

and 83% of all genera became extinct.

It was the most significant extinction event on the planet, ever.

It's been hard to pinpoint the reason for this extinction event

because most of the evidence has been wiped out, of course.

It may have been kicked off by an asteroid that released the energy

equivalent to the detonation of a few million

nuclear weapons all at once.

And then insult added to injury when a whole bunch

of volcanoes erupted, methane was released from the sea floor,

there were probably some gas explosions in what's now Siberia,

and then a whole bunch of climate changes, sea-level change

and changes in ocean salinity probably occurred.

Nobody's sure exactly what happened, but we do know it took

a long time for life on earth to make a comeback.

But look on the bright side: as a result of

the Permian-Triassic event, we got dinosaurs!

They were able to evolve during the Triassic because

there wasn't much competition for resources, so they evolved

to fill an available niche, that is, a combination of