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Hi, I'm John Green,
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,
inaccurately with many useful adaptations getting lost.
Essentially, these molecular mechanisms
were badly programmed.
In 1950s, James Watson, Francis Crick, Maurice Wilkins,
and Rosalind Franklin discovered how living cells replicate
using DNA or deoxyribonucleic acid.
DNA is a double-stranded molecule that contains
a list of orders for how it wants a living cell
to be constructed.
And then a single strand, RNA, reads those program orders
and sets in motion the production of the proteins
necessary to accomplish them.
All life on earth has DNA, which is one of the reasons
we know that all living things on earth-- from farmers to fish,
from moles to microbes-- have a common ancestor.
It's why you share 98.4% of your DNA with a chimpanzee,
and why you share nearly half of your DNA with the banana
that it likes to eat.
Not quite cannibalism,
but we do eat a lot of our distant cousins.
But where do DNA and RNA come from?
Another mystery.
How could such complex programming evolve
from simpler organic forms?
One leading contender is the RNA world hypothesis,
which postulates that there might have been
an earlier version of just RNA, which was capable of both
coding and self-replicating and out of which separate
and more complex structures evolved... DNA.
DNA and RNA operate in extremely complex ways themselves,
which is what you'd expect with something with as many
connections and varied building blocks as life.
By the way, we're not expecting you to come away from this video
with a complete understanding of how DNA works.
There is a link in the description to ourCrash Course
biology video on DNA though if you want this
mind-blogging concept to come down a few boggles
on the boggle scale.
Remember this as well:
when looking at a historical narrative, it's always useful
to know how things work.
But it's still more useful to know why they work.
Because they can influence the future sequence of events.
Like you don't have to know exactly how to design,
build, assemble, and fire a 15th century long bow
to understand the French and English conflict
in the 100 Years' War.
All you need to know is that long bows made things
pretty unpleasant for a lot of French people.
Like, "There's a piece of wood sticking out of me" unpleasant.
It copies a living organism with stunning precision.
But even this impeccable copying process can occasionally be
somewhat peccable.
Once every billion copies or so there is an error.
These errors result in a slight mutation.
These can have no effect, they could be very good,
or they could be very bad.
If useful, it allows an organism to be more successful
and likely to pass on its genes.
If not so useful, things go poorly and the gene
does not get passed on.
They allow the tiny layer of fragile organic material
sitting atop the hulking geological structures
of the earth to be shaped and reshaped like Play-Doh
from prokaryotes, to eukaryotes, to trilobites, to dinosaurs,
to Abraham Lincoln.
As Charles Darwin put it at the end ofThe Origin of Species,
"There is a grandeur in this view of life,
"with its several powers having been originally breathed
"into a few forms or into one.
"And that, whilst this planet has gone cycling on
"according to the fixed law of gravity, from so simple
"a beginning, endless forms most beautiful and most wonderful
have been and are being evolved."
More on that next time.