Learn about the origin and evolution of life and the search for life beyond the Earth.
Life on Earth
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Do curso por The University of Edinburgh
A Astrobiologia e a Busca por Vida Extraterrestre
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Life and Its Origins
What is life and what are the definitions of life? What do we know about the origin of life and what are the current hypotheses for how it originated on the Earth?
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Charles CockellProfessor
Astrobiology
Before we look at some of the details of life on Earth and
think about how we might apply that to the search for life beyond the Earth.
Let's take a brief tour and
look at some of the basic facts about life on our planet.
Some of the things perhaps we should know about as astrobiologists.
Well, most of us are familiar with what we might refer to as big life,
multicellular life.
Here are some examples of the sort of creatures that you and
I are familiar with on a day to day basis.
But the first thing we should understand is the majority of life on Earth.
And when I say, the majority, I mean, most of the biomass,
the biological mass of life on Earth.
As well as most of the genetic diversity of life is actually made up in
the microscopic world like these bacteria.
A thousandth of a millimeter long.
Tiny organisms you can't see with your own eye, but you can see under a microscope.
These dominate life on Earth and they have dominated life on our planet since they
first evolved, and we'll look at more of these throughout the course.
Microbes are important, because they're actually the most likely forms of life
that we find on another planet.
Intelligent life is likely to be much rarer.
Microorganisms were first discovered in the 17th century by
a rather remarkable individual, Antonie van Leeuwenhoek.
Van Leeuwenhoek was a Dutch fabric maker and he had a problem.
He wanted to make high quality fabric that he could sell to his clients.
And in order to do that, he wanted to look at the fibers in his fabric
to see what sort of quality they were and how he could improve them.
So, he went about inventing tiny microscopes and
you can see one of his microscopes here.
They look very crude.
You hold up a little lens to your eye.
And with this device, he was able to look at the quality of his fibers and
improve the quality of them, so that he could make better cloth.
Van Leeuwenhoek was a very curious individual.
And once he had built his microscope, he went about looking at the outside
world to try and understand what might be out there in his garden.
He started to look at samples of soil and samples of water.
And as he looked as these, he discovered tiny animals,
animacals as he called them that he began to write papers about.
And you can see an image here,
one of the first figures in one of the first papers that he wrote in the 1670s.
And if rather remarkable, because it shows the first image of microorganisms.
Some of them are rod shaped.
Some of them are little spheres.
Some of them seem to be spiral shaped and he even showed a microbe moving around.
It's remarkable to us now that with such a primitive microscope.
He was able to make out the different shapes of microorganisms that we're now so
familiar with.
But van Leeuwenhoek did not understand that he had opened up a whole new science
of microbiology.
And he had, in fact, discovered the most abundant and
the most diverse creatures on the Earth.
This science of microbiology would eventually lead us to searching for
life in extreme environments.
And in fact, provide the underpinning for what we now know as astrobiology.
Since van Leeuwenhoek's time, microbiologists have gone to all sorts of
extreme environments and we find environments with extremes,
such as high temperature, high radiation, extreme acid and so on.
And these extremes seem to define the limits of life.
And as we go to greater and greater extremes, so
we find that microorganisms are the only organisms that are able to live in
these sorts of extreme environments.
They become less and less diverse in general, the more and
more extreme the physical and chemical conditions become.
These extremes define the boundaries of life on our planet.
They define, if you like, the boundary of the biosphere and
you can think of the Earth as like a zoo.
A zoo surrounded by a fence of physical and chemical extremes, and
these physical, and chemical extremes define what we might call a biospace.
A biospace we might refer to as a space bounded by physical and
chemical extremes within which life in our planets live,
within which it inhabits the physical spaces of our planet.
And I'm showing here a very simple conceptual two-dimensional diagram of
the Earth's biospace.
And you can see this boundary of life, high temperature, pH,
desiccation, other extremes to find this outer layer of the biosphere.
And provided you remain within these boundaries, you can be anything you like.
And here's a weird collection of organisms that I've shown here,
that live on planet Earth within this biospace.
But you cannot transgress the boundary.
Once you transgress the boundary, you move into physical conditions that are far
too extreme for life to be able to tolerate.
Defining these extremes, understanding where the limits of the biosphere are.
One of the jobs of astrobiologists, and it's one of the tasks that
astrobiologists have to do is to find out what are the limits to life on Earth.
Most of these extremes as I've said are dominated by microorganisms.
Here's a hot spring in Yellowstone National Park.
And within this hot spring at a temperature of 80 or 90 degrees,
they're microorganisms.
Organisms called thermophiles, heat lovers.
We'll see later on in this course and
these microbes inhabit these very hot volcanic environments,
that may represent some of the earliest environments on our own planet.
And so by looking in these extreme environments, by looking at the limits of
life on Earth, we can learn something about the possibilities for
life in the early history of our planet.
And possibly, volcanic, extreme environments elsewhere in the universe.
You might wonder how many microbes are there on the Earth.
I said that they're most abundant life form well, in fact,
there are many, many microbes.
10 to the 30, in fact and here is a slide with that number written down.
1,000 billion, billion, billion microbes on our planet.
At least, that's the estimate.
But it's probably something around that number.
Whatever the number is, it's very, very large and it gives you some indication of
how abundant microorganisms are on our own planet.
And why it is that astrobiologists tend to focus on microbiology,
microbes when they're looking for life on other planets.
So, where is this life on our planet?
Where does it reside?
This is a slide that shows you a geological cross section through
our planet.
There's nothing particularly special about this.
You can see the core of the Earth, the liquid core and then the mantle and
the lithosphere.
The crusty surface on which we live and
what I am now going to do is take away that geology, and
leave only the biology on our planet and here is the biology of planet Earth.
A tiny veneer of life on the surface and
in the subsurface of our planet that inhabits planet Earth.
Now in fact,
this line that you can see in this slide is 5 times larger than the real biosphere.
I've increased the thickness of the line by 5 times.
Otherwise, you probably wouldn't be able to see it on the slide at all.
So, you can see that life is a very thin layer on our planet.
It's not everywhere and you might wonder, why is that the case?
Why doesn't the biosphere look like this?
Why isn't the Earth filled with life?
Why doesn't life live right the way through to the core of the Earth or
why doesn't it look like this?
Why isn't the biosphere a thick layer growing on the surface of the Earth and
deep down into the crust, and mantle of our planet and even below into the core.
Well, the simple reason is that as you go down into our planet, it gets hotter.
And the deeper you go, the hotter it becomes and we think that the upper
temperature limit of life at least at the current time is 122 degrees centigrade.
So as you go deep into the Earth and you exceed that temperature,
you get to places where life simply cannot live.
And so the lower limit for life on our planet,
the limit that defies the bottom of that thin layer of life that I showed you is
set by the high temperatures inside the crust of our planet.
What about the upper limit of that thin layer of life?
Well, that's set by the ability of life to survive in the sky.
Clouds contain microorganisms.
Next time you look at a cloud, you can think of it slightly differently as a bus
carrying microbes around the Earth.
But in fact, it's a very difficult place to live.
Clouds are very cold.
They're exposed to solar radiation and there's not much food there.
As you go higher up into the atmosphere, it becomes more and more difficult for
life to grow.
Certainly some microorganisms can survive high up in the atmosphere, but
it can't really multiply and
that height is probably something like 3 kilometers in the atmosphere.
Beyond that, it's very difficult for microorganisms to grow.
So the lower limit and
the upper limit of the biosphere are not exactly known at the moment.
And scientists continue to study life in the atmosphere, and
many scientists are studying life in the deep subsurface to try, and
understand the lower depths of the biosphere.
But probably, we can say at the current stage that the biosphere
is something like 10 to 20 kilometers thick.
But even if that number changes quite a lot even if it was to double or
even multiply by 5 times.
It doesn't really change the fact that life on Earth is a very thin veneer
on the surface and near surface environment of our planet.
So you might be thinking, what does all this tell us about the search for
life elsewhere?
Why would I want to know this, if I was searching for life on Mars?
Well, understanding the limits of life on our planet.
Understanding where it lives.
The depths of the Earth in which it might grow and reproduce, and
how high it might live in the atmosphere.
And the physical extremes that it can tolerate tell us something about
the possibilities of life beyond the Earth.
It helps us search for life on planets such as Mars.
If we go back to this conceptual diagram that I had shown you earlier.
I think it's true to say that one of the most unremarkable advances in
astrobiology over the last one or two decades.
Has been the realization that some environments on the Earth,
environments that are very extreme.
But nevertheless, harbor microorganisms have similar physical and
chemical conditions to environments that we find on other planets.
Just one example is very cold and salty environments on the Earth.
For example, if you go to Antarctica and you find a very cold salty lake,
you can find microorganisms growing in that environment.
And we also know the planet Mars has cold salty environments and the question is,
do those environments contain life?
And at the moment we don't know, but the point is this, that some physical and
chemical environments on other planets seem to overlap with the biospace of life.
The space in which life might be able to persist.
And so, by knowing the limits of life on Earth,
by exploring the outer boundaries of our biospace,
we can improve our attempts to search for life on other planets.
So, what have we learnt in this talk?
Well, hopefully, what you've learned is that life is abundant on the surface and
just underneath the surface of the Earth.
But you've also learned that life is bounded by physical limits and extremes.
And it occupies a biospace, and understanding the limits of that biospace
is the job of astrobiologists, to find the limits of the biosphere.
Although life is abundant it,
it also means that it occupies a very small part of our planet.
And we would expect that to probably be the case on other planets as well.
We've also learned that some physical conditions that are suitable for
life may overlap with conditions on other planetary bodies,
driving an interest in the search for extraterrestrial life.
And so, by understanding the limits of life on Earth,
we can assess better the habitability and the prospects for life elsewhere.
And finally, you've learned that microbes are the most abundant form of
life on the Earth and usually occupy the most extreme environments on our planet.
These microbes are called extremophiles from the Greek philos, to love.
Extremophiles, microbes that love extreme environments.
And later on in the course, we'll have a look more at these extremophiles and
some of the environments they inhabit.
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