Origins of life | Biology | Khan Academy

- [Voiceover] We have many
videos on Khan Academy

on things like evolution
and natural selection.

We think we have a fairly
solid understanding

of how life can evolve
to give us the variety,

the diversity, and the complexity

that we've seen around us,

but it still leaves unanswered
a very fundamental question.

And this might be the
biggest question known to us,

and that is the origins of life.

How did life first
emerge, at least on Earth,

and that even starts to
lead to other questions

about is there life outside of this planet

and what could it be like?

And so let's start with
what we actually know

and I'm gonna start with a timeline.

So let's go one billion years ago,

let's go two billion years ago,

three billion years ago,

four billion years ago.

So this is now.

And once again, we're talking
about a billion years ago.

You'll sometimes see the abbreviation BYA,

billion years ago, which
is an unfathomable amount

of time going into the past.

But we know that Earth along with the rest

of the solar system was formed

around 4.6,

4.6 billion

years ago,

so that's when Earth was formed.

And right at 4.6 or
even, you wait a casual

100 million years after
that 4.5 billion years ago,

we believe that Earth wasn't very suitable

for even very simple life to form,

and that's because the solar
system was a crazy place.

You had collisions of all scales
happening all of the time.

The moon itself was
formed from the collision

of two planet-sized objects,
one we call it the proto-Earth

and another planet-sized
object and they collided

and then they started to spin around

and one part became the moon.

It was tidally linked with the Earth.

But you can imagine,
that's not an environment

where it would be easy for life to form.

And even once the moon was formed,

you had a heavy bombardment of things

in the solar system, the solar
system was a messy place.

It took a long time for the stability

that we now observe out there.

And so that continued, we believe,

until about 3.9 billion years ago,

which is the earliest
that we currently think

that Earth might have
been suitable for life.

Before that, there might have been pockets

where the bombardment
stops and maybe some type

of primitive life might have formed,

but then they would have gone away

with the heavy bombardment.

But who knows?

Maybe they could have
survived that somehow.

But that's the current mainstream belief.

The other thing we know is
that we see fossil evidence

for life 3.5 billions years ago.

And these are stromatolites.

This is fossil evidence, microorganisms,

they formed these structures
that actually continue

to be formed today,
these types of structures

continue to be formed today.

And although it might not
feel like microorganisms

are complex life, when you
think about what has to happen

within a microorganism, they're
actually incredibly complex,

and especially if you compare them

to very simple, non-living organisms.

So our current belief is someplace

in this region life must
have arisen on Earth.

But that still doesn't,
even if we were able

to answer that question, oh,
it was exactly 3.7 billion

years ago was the first
time that some RNA decided,

or not decided, ended up getting
in the right confirmation

so it could replicate itself in some way,

even if we know that date,
it still leaves unanswered

maybe the more interesting
question, which is the how.

The how is really, at least
to me, more important,

more interesting than the when.

And to the how question, there's
a couple of layers on it.

The first is, let's just
start with the most simple

molecules that we would
have expected to find

on early Earth.

Here are some examples
of it right over here.

This is H2O, or more
commonly-known as water.

Right over here is CO2, more
commonly-known as carbon.

That's a little hard to see,

let me do it with a lighter color.

So we have

carbon dioxide right over here.

Here we have molecular
nitrogen, you have some ammonia,

you have some phosphate, and
many other of the elements

that we see on Earth today,
they might have been available

in that early Earth, but
how do they form at least,

even the next step up, which
is the slightly more complex,

or actually a good bit more
complex organic molecules.

And when people talk about
organic molecules they might

be talking about things like this.

These are amino acids.

These are the building blocks
of proteins, amino acids.

You see over here nucleotides.

These are the building blocks
of RNA, DNA, other things.

And so the first question is,

and these aren't the only
simple organic molecules.

You could think about sugars
and all sorts of other things.

But the question is, is it realistic?

Do we at least understand how we can go

from these very simple molecules up here

to these more complex, often
called organic, molecules?

And the simple answer is we
now have a lot of evidence

that this is doable,

that you can go from these
things to these things,

abiotically, without the presence of life.

You'll hear that word abiotic a lot.

Think about it, antibiotic,
you're killing life,

you're killing bacteria.

Abiotic, that is without life.

And the points of
evidence that we now have

are we believe and we've seen evidence

that there's amino acids and
organic molecules related

to them on comets,
meteorites, on other planets,

that they formed spontaneously in space,

once again, without the
presence of life there.

We've even been able to form amino acids

and other molecules like this

from these more simple
elements in the laboratory.

The most famous experiment there

is the Miller and Urey Experiment.

This was in the 1950's,
where they were able to show

with some energy, they provided a spark.

You could imagine that in the early Earth

it could have been from lighting.

And they tried to set up a mix
of gases that they believed

was similar to the atmospheric
mix in the early Earth

which didn't have much oxygen
in the atmosphere then.

We needed life to
actually start to produce

some of that oxygen.

And even though today we think

that they probably didn't
have the mix of gases right,

they did do something significant.

They were able to show
that with that mix of gases

at least they thought
were in that atmosphere,

and some energy being
added to that system,

that they were able to form some

of these organic molecules.

So we should feel pretty good

that at least this first step is doable.

Now the next question is
these organic molecules

by themselves, that's not life.

In fact, these aren't even
the most complex molecules

that are, we believe, essential for life.

Proteins are where things start
to get really interesting,

and a protein,

a protein,

or proteins are one of the places,

a protein might have
thousands of amino acids,

thousands

of amino acids.

Things like DNA and RNA,
also we believe essential

for life, or at least life as we know it,

could be made up of tens
of millions of nucleotides

for one DNA molecule.

So for example, this is just a small part

of a DNA molecule, but
you can already see much,

much more complex than
what we see over here.

And there, too, we have
evidence that you can go

from the amino acids to the proteins,

or you can go from the
nucleotides to the DNA

without the presence of life,

that these things can happen spontaneously

if you have the right context,
the right energy available,

some people believe,
or it's been observed,

that if you have the right surfaces

that these molecules can be
organized in the right way

to form these more complex things.

Now, I know what you're thinking.

Alright, proteins are really cool,

DNA, RNA is really cool, but
then how does that become life?

At what point would we start going,

"That was a proto-life form?"

And this is where we
really get into the area

of the unknown because we don't know.

And there's a couple of
hypotheses out there.

One of them is called
the RNA World Hypothesis.

I'll write that down.

RNA

World

Hypothesis.

And this is the idea
that the first proto-life

was self-replicating RNA molecules.

And the reason why people tend to focus in

on RNA a little bit more than DNA is

that even in cells today,

RNA doesn't just store information,

it can actually play a role as a catalyst.

And when you think about things like tRNA

and you think about ribosomal RNA.

And so maybe some of that
first proto-life was RNA,

information that replicated itself

and catalyzed the replication of itself.

Maybe it somehow got organized

into membrane-bound structures
so it could separate

so you had environments
that were separated

from the outside world.

But the simple answer is we don't know.

Another mainstream hypothesis

is the Metabolism First Hypothesis.

Metabolism

Metabolism First.

And this is the idea that
a lot of basic pathways

that you might study
in a biochemistry book,

that these were first
just happening, well,

all of this could have been happening

in this primordial soup where you had

these organic molecules
in the right conditions,

maybe around heat vents and whatever else,

but the Metabolism First is
that some of these mechanisms

that we now study in biochemistry,

these might have happened
outside of a cell

or outside of life and they
just kept creating more

and more complexity, but
at some point these things

started happening in self-organizing,

membrane-bound structures.

Maybe there's some kind
of combination of the two.

The simple answer is we just don't know,

but there's some fascinating clues.

Even if we observe current biology,

and even in fact if we
see the commonalities

of things that happen,
central dogma of biology,

if we see how proteins,
which structures are common

to all life as we know
it, it might give us clues

or hints at what some of
that very earliest life

or proto-life was actually like.