and welcome to Crash Course Big History

where today, we're going to get a life.

Or at least, the earth is going to get a life.

But first, today we have to start with a disclaimer.

The origin of life is in many ways a blank spot

in the pages of history.

Like the mystery surround the Big Bang or dark matter,

the origin of life is still pretty puzzling to us.

Like, thanks to scientific research,

we have a general idea of what needed to happen

to bring about life, but we're pretty fuzzy on the details.

Mr. Green, Mr. Green!

I mean, if we don't know,

then why are we studying it as history?

Maybe we should just, like, let scientists

figure all that stuff out and then they'll get back to us,

like, after this class is over.

Well, me from the past, I'm sure the thousands of scientists

working on that question appreciate your patience,

but even when we have blank pages in the annals of history,

it's still history.

Like, there are still competing ideas and theories

about the presidency of Franklin Delano Roosevelt,

but the fact that there are open questions doesn't mean

it didn't happen.

Sometimes we don't have a clear narrative of events

and it's up to us to collect more evidence

and refine those theories, but first,

we have to know about the current evidence

and the current theories.

I mean, ultimately, that's what history is.

>> I'm Hank Green.

And this is still Crash Course Big History.

Last time, we left off with a newly born Earth

that was molten hot, pelted by asteroids.

Then, millions of years of torrential rainfall

cooled the surface and created the first oceans.

We know that life emerged in the oceans

between 3.5 billion and four billion years ago.

We have solid fossil evidence for life 3.5 billion years ago

and many scientists are pretty confident

that life was around 3.8 billion years ago.

It's pretty clear that life is a different thing

from the rest of the universe,

but what makes up that difference?

I'm kind of surprised that this turns out

to be a super puzzling question that we have yet

to come up with a 100% satisfying answer to.

But some of the major characteristics

of most life are, it adapts to the environment,

it has a metabolism that processes energy

to keep itself going-- like humans do with pizza--

and it reproduces, whether it be a cell splitting in two

or two animals... doing their thing in nature.

Even these simple criteria have their problems, though.

Some animals, like mules are born unable to have offspring.

Some microorganisms can shut down their metabolisms

for long stretches of time, but neither are exactly dead

or not life.

Given the incredible variety of species,

definitions for life are, by necessity, very broad,

but one such definition by big historian Fred Spier is,

and I quote, "A regime that contains a hereditary program

"for defining and directing molecular mechanisms

"that actively extract matter and energy from the environment

"with the aid of which matter and energy are converted

"into building blocks for its own maintenance and,

if possible, reproduction."

(sighs)

In other words, what makes you different from the stars that,

while a stars burns down till it dies and doesn't actively float

around the cosmos looking for more fuel,

a living organism does actively seek out pizza

to keep itself going, preferably long enough to,

you know, have some babies.

But how do we know what we know?

How do we know that life is just a different kind

of molecular mechanism and not something more profound?

Well, we can test these claims, and we do, using science.

Because life looks so radically different

from the inanimate universe, people once thought

that life was made of completely different stuff.

Then, in 1828, a German chemist, Friedrich Wohler,

used inorganic chemicals to synthesize an organic chemical.

This was a big deal just as Newton's theory of gravity

showed that the heavens and earth followed

the same physical laws, Wohler's experiment proved

that life and non-life follow the same chemical laws,

which implied that life could emerge from non-life.

Even this idea wasn't completely new.

For centuries, the Aristotelian idea that life

just spontaneously emerged from non-life was widely believed.

For example, if you put some rotten meat out in the sun,

eventually the meat would transform itself into maggots.

You could probably work out the weaknesses in this theory.

17th century scientists took meat and various other objects

thought to spontaneously generate life,

boiled them to kill off any eggs previously laid by insects,

sealed them in jars and nothing happened.

Oh Aristotle, first you told us that snot was our brain

coming out of our noses, and now you made all those

nice people waste their steak dinner.

This however did not rule out some form of life force

in the air.

Some invisible force in the earth's atmosphere

that could enter an object and literally breathe life into it.

But spores from plants can also travel in the air,

as can microorganisms.

So in the mid-19th century, Louis Pasteur boiled

some organic broth friendly to life and placed it

in a flask with a swan neck to trap plant spores

and smaller particles.

If a life force was in the air, it could enter freely

while spores and other particles would get trapped in the U-bend.

And what happened?

Nothing.

A century and a half later

those flasks are still devoid of life.

The conclusion: The ancients were wrong.

After a dose of claim testing, it became clear that life

must emerge from the inanimate world by chemical processes

that are discoverable by science.

But what did early life look like?

Well, for a whopping 2.1 billion of the 3.8 billion years

of the evolutionary epic

history was made by tiny, single-cell organisms

called prokaryotes.

That's roughly 55% of the entire story of life.

Now some of those prokaryotes evolved about 1.7 billion

years ago into slightly bigger single-celled organisms

called eukaryotes.

And then, you know, that kept happening and eventually us.

But for now, let's just talk about prokaryotes.

Prokaryotes lived in the seas and ate chemicals

in their surrounding environment.

Now these microscopic prokaryotes might not sound

very impressive, but they do make up

the vast majority of your family tree.

They're also distant relatives of the modern bacteria

that are everywhere.

Crawling around the room that you're in right now,

crawling all over you, crawling inside of your intestines.

That's right, somewhere right now there's a bacterium

that will give you food poisoning

in an undercooked hamburger, and it is your cousin.

But the thing is, even in its earliest stages,

single cell life was massively complex compared

to the inanimate universe.

I mean I know these are tiny little specks,

but compared to everything else that had happened on earth

until them they were an immense tangle of chemical networks

and building blocks.

But how did an object as ridiculously complex

as a prokaryote first emerge?

Well, first of all, it's very difficult to think

of how life would form in an oxygen-rich atmosphere

like present-day Earth's.

Oxygen is kind of a nasty, highly reactive chemical.

In fact, if the oxygen levels in this room were

substantially higher and I would just rub my hands together

really fast, I could burst into flames.

And while that would make for a nice viral YouTube video,

I would rather not be on fire than get lots of views.

3.8 billion years ago, the free oxygen content of the atmosphere

was at negligible levels,

which had some not so pleasant consequences.

For millions upon millions upon millions of years,

life dwelled fairly deeply in the ocean,

eating chemicals and staying where

the earth's heat kept warm.

Eventually, some prokaryotes floated near to the top

of the ocean and started using sunlight, water,

and the carbon dioxide that was abundant

in the earth's atmosphere to sustain their own complexity

using this sweet chemical process they'd come up with

called photosynthesis.

The waste product of this chemical process is oxygen.

And these photosynthesizing prokaryotes pumped a lot of it

into the atmosphere.

By around 2.5 billions year ago, the amount of free oxygen

in the atmosphere was up to about 3%.

Oxygen can be nasty and so scores and scores

of tiny single-celled organisms couldn't handle it

and died off in a massive wave sometimes known

as the oxygen holocaust.

So many species of single-celled organisms,

each with the potential to evolve into more complex life

were wiped out.

Even at this early stage, our evolutionary ancestors

were squeezed through a bottleneck.

And this will not be the last such disaster

that nearly wiped everything out.

Next time you have a bad day remember that it is amazing

that you are alive at all, much less a member

of a self-aware species living at the height

of human technological progress.

Speaking of ancestors, somewhere between 1.6 and two billion

years ago, the eukaryotes evolved.

And because you, your dog and the chicken you ate last week

and the mushroom you ate the week before all descended

from them, they really put the "you" in eukaryotes.

And eukaryotes contained organelles like cellular organs

that enhanced their abilities.

About 1.5 billion years ago, eukaryotes invented sex.

Up until that point, single-celled organisms

split in two or cloned with no need to find a partner

for romance and DNA exchange.

Sexually reproducing eukaryotes possibly obtained

these abilities through cannibalism,

just eating each other,

which may have led to some accidental exchange of DNA.

After that, the evolutionary advantages of sex

probably resulted in it catching on.

Having a partner means having two sets of genes

and thus a wider range of genetic diversity

from which evolution can pick and choose.

Sex is a huge deal.

It enhanced evolution, and therefore deserves to be classed

as one of the most revolutionary advances in the history

of life on earth.

And a huge leap forward in the rise of complexities

since the very beginning of the universe.

So where did these complex single-celled organisms

come from in the first place?

Well, Charles Darwin's own hypothesis was that life

evolved in some "warm little pond suitable

for fostering life."

Other scientists postulate that life may have formed

from organic chemicals next to the warmth

of underwater volcanoes.

And still others champion the idea of panspermia,

which states that life may have evolved elsewhere

in the solar system and then been transported here

by an asteroid, which seeded the earth.

Like I said, this is a blank spot where many different

historical theories are seeking evidence

to clarify what happened.

It's possible actually that this problem could be solved

in our lifetime, which is pretty exciting.

Anyway, whatever physical forces were at play,

primitive organic chemicals eventually came together

into balls with protective membranes.

They would have reproduced and proliferated

much as life does today, but the earliest blobs

or organic chemicals would have reproduced clumsily,