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Origin of Cells
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From the course by Princeton University
Imagining Other Earths
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From the lesson
Is Earth Special? Origin of Life
This lecture describes our efforts to understand the origin of life on Earth. While it is relatively easy to synthesize amino acids, we do not yet understand how this building blocks are assembled into complex cells. The lecture ends with an exploration of the speculation that life originated outside Earth.
Meet the Instructors
David SpergelCharles Young Professor of Astronomy on the Class of 1897 Foundation and Chair
Department of Astrophysics
So now let's start thinking about the origin of cells.
because we want to form not just a self replicating molecule but
we'd like to get it encased inside a cell wall so
it has a kind of a separate existence as an organism
that could have an individuality that could be selected for and can evolve.
So the picture I want you to have in mind is this picture from a paper of
Jack Szostak, one of the pioneers in thinking about the origin of life.
And you see this cell wall surrounding some RNA
and we want to find a situation where the cell can grow and
they've done some experimental work showing that in the right
environment these cells will grow and then potentially bifurcate.
And at the same time if you can have RNA making copies, you can start to have
a cycle of growth, bifurcation, reproduction.
So, when did cellular life arise.
We know it arose very quickly.
And we know that it must require certain basic ingredients.
In order to have a cell, we're going to have to have a cell wall.
We're going to have to find a way of making and enclosing liquid membrane.
And what's intriguing is that simple membranes have formed in lab experiments.
And these membranes will serve the role of separating the RNA, or
whatever the reproducing molecule is, from their environment.
Now, early cells have to be much simpler than
the cells we have today as I already stressed.
These protocells ought to be quite permeable to let things in and
out, so the cell can bring in material for its interior to make use of.
Today's cell's walls are quite nearly impermeable.
And if you want to bring in appropriate ions,
you need to make use of sophisticated transport mechanisms in the cell wall.
We've evolved a complex biochemistry
that makes use of the fact that the cell walls are quite impermeable.
Early cell walls were probably quite different,
they didn't have this complex machinery.
They were likely made of fairly simple fatty acids.
Things like capric acids, which are fairly short chemicals.
And experiments have shown that these acids could assemble
into what looks like protocells and intriguingly these vesicles,
these bound fatty acids that close up on themselves,
taking advantage that we've got a hydrophobic tail here and
a hydrophilic side here that make it energetically favorable.
To form these nice, enclosed surfaces.
One intriguing result is that if you take these in the presence of clay, that
that turns out to be a good environment to bring RNA into the vesicles.
So I find that particularly intriguing, that the same sites that might help
synthesize RNA, might also help incorporate the RNA into the cell wall.
And this suggests that we might have
a plausible cycle where these protocells in
the presence of the right fatty acids could
grow Become larger.
You then form these elongated structure.
That if shaken, perhaps by wave action on the clay,
on some part of an early Earth, might divide and then grow again.
And once we could have a process like this,
you could imagine evolution acting or evolution will act.
And whichever one of these proto-cells
is best capable of growing, reproducing and
surviving, it will become early life.
So, what can we learn from what progress has been made experimentally and what
we know about the basic characteristics of molecular biology on life?
Well, we know that life on Earth today is very complex,
it's very interconnected, much richer than any other chemical system we know.
Another thing we know is that all life on Earth seems to share the same
biochemistry, even down to a lot of the details, life seemed to have arisen once.
When we think about applying this to other planets we notice that in order to get
the ingredients we need things to be carbon based and water dependent.
As we've stressed early on it's very hierarchically structured,
made up of simpler systems.
So however life has been assembled it found a way to take these simple pieces
and assemble them.
Into more complex forms.
So based on what we've learned about life on Earth,
do we think life has arisen elsewhere?
Well we don't know.
In fact the big challenge is we don't really know what the bottlenecks are in
the process we've talked about so far.
We have a scenario from getting from proteins and
lipids to cells and life.
But we know on Earth it seemed to have happened only once.
And We don't know whether there's other roots that could have worked.
Are there other biochemistries that could happen elsewhere?
Once life gets started, regardless of the underlying biochemistry,
we can expect that the basic principles of evolution will hold.
And as we'll talk about in the next lecture, we can take our ideas about
evolution and apply them without knowing the underlying biochemical processes.
So when we think about life on other planets,
we can think about evolution happening.
Regardless of understanding the biochemistry, but
to get at this really fundamental question of is there life elsewhere,
it's very hard at this stage forr us to answer that.
Yet from first principles, we don't know enough about the origin of life and
the probability of life arising to know how often it will occur.
We can draw some lessons from what happened on Earth
after cellular life first appears.
After it appears it actually took a long time for
life to go from simple prokaryotic life to more complex life forms.
First it splits into archaea and bacteria, it took a long time before.
We start seeing things like cyanobacteria, the photoautotrophs.
Life doesn't really kick off into its complex rich form we see today
until about two billion years after it arises, close to two billion years.
When oxygen really starts to rise.
And eukaryotes, the kind of richer life forms that we are,
took several billion years to evolve.
And it's really only been in the last 25% of the history of
life on this planet, that we've had rich multi cellular life.
So it seems as if, at least on the Earth, taking the step to get simple life
may be easier, at least in terms of the time it took,
in the time it took to go from simple life to complex life.
And there seems to be several difficult steps it takes to get from
that first prokaryotic cell to us.
So even if we knew that life arises on other planets and
that other planets have simple life.
We also don't know the answer to the question, given a planet with simple life
how long will it take to get from simple single cell prokaryote life
like life to complex life, multicellular life like we see on Earth.
And we certainly don't know how long it would take to get from
multicellular life of some form.
To life forms that are capable of having say, interesting conversations with us.
So what is this implications for thinking about life on other planets.
Well there seems a whole bunch of different hurdles that you have to go
through to get from simple biochemistry to us.
You need to first assemble proteins and nucleic acids.
That we think we can do pretty often.
We have to get from proteins, nucleic acids, fatty acids to cells.
We can see a glimmering of how to do that and some of the progress made in the lab.
But we don't know how rapidly and often this happens.
We need to be able to begin the process of photosynthesis,
chemosynthesis, creating food, creating its own food from the environment.
Developing the complex inner organelles that
enable the cell to carry out more complex function.
Oxygen respiration, starting to bring some process of multi-cellular
cooperation either by having individual cell prokaryotic cells work together or
bringing them together into the more complex multi-cellular form.
The fact that each one of these steps took hundreds of millions or
sometimes billion years between one step and the next.
Suggests that these might be hard and
it could be that we are the planet where this happened fastest.
And life here is more advanced than life elsewhere.
Or it could be that we're one of the slow pokes and
there are other planets where some of these steps took not
a billion years but ten million or 100 million years.
And there are other planets where complex life arose billions of years ago.
And whatever life is on that planet is far more sophisticated
than what we see here today.
So what will life be like on other planets?
Will life look similar to terrestrial life?
We've been informed by our science fiction movies.
In Star Trek, most of the aliens look basically humanoid.
They're mostly humans dressed up in costume.
They are bipedal, they have symmetries.
That the left and right side look the same, they have arms,
they have a head, they communicate with a mouth.
Life need not look like that.
We see interesting things happening though on Earth
that suggest perhaps we'll see similarities in form.
We see this phenomenon of biological convergence that on large scales,
sometimes two branches of life that are in many ways quite different.
Bats and birds, both develop wings.
Because there is a kind of best solution for
the problem of flying and while the way the wings form are quite different,
evolution selects for something that has the same broad, coarse outline.
Eyes are thought to have evolved independently multiple times.
They seem to be a biologically kind of effective solution
to the problem of learning more about new environment.
So one possibility is that life on other planets
could be based on a very different biochemistry.
The underlying processes at the cellular level could be quite different.
Yet, there might be some surprising similarities in
body form, perhaps how societies are drawn up.
Perhaps the way reproduction happens.
Perhaps the same evolutionary pressures that led to sexual reproduction on Earth,
will mean that a similar phenomenon operates in a completely different planet.
On the other hand, divergence might happen on all
scales from the underlying biochemistry to the physical kind of macro form.
Of the life forms, to how they interact,
to how they are embedded in their ecosystem.
I think one lesson that we can learn from what we've found
out about the diversity of planetary systems,
is that we should expect to be surprised that there is a lot
out there that goes beyond what our minds can imagine.
And the universe is remarkably rich, just
in the physical properties of the planets, let alone what biology we might discover.
And as we have explored life on our own planet,
looked at the range of possibilities that life has taken advantage of,
the diversity of these extremophiles we've looked at.
I think one of the lessons is to expect diversity,
expect richness, expect surprises.
And that when or if we discover life, I hope when,
perhaps we should just say if for now.
When we discover life, I think we will be amazed
by the richness of what is out there beyond our own planet.
So let me go back and ask you some basic questions here.
Just want you to go back and think about this basic simple cell,
proposed in the Szostak paper, of a lipid membrane and RNA.
And think about what basic
building blocks do you
need to make this cell?
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