[BLANK_AUDIO] 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 astrobiologits. 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
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 milometer 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. Micro-organisms were first discovered in
the 17th century By a rather remarkable individual, Anthony
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 better cloth. But van Leeuwenhoek was a very curious
individual and once he had built His microscopes, 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 at These he discovered tiny animals,
animalcules as he Called them, that he began to write papers
about. And you can see an image here of one of
the First figures in one of the papers he
wrote in the 1670s. And it's 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 Loenhoeks 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 micro-organisms 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,
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 the space
bounded by physical and Chemical extremes within which life in our
planet lives. Within which it inhabits the physical
spaces of our planet. And I've shown 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 and other extremes define 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 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 of life on Earth? Most of these extremes that 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, there are micro-organisms. Organisms called thermophiles, Heat lovers, that 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've said that there the most
abundant life form. Well, in fact, there are many, many
microbes. Ten to the 30 in fact and here is a slide. With that number written down, a 1000 Billion, billion, billion microbes on our
planet. At least, that's the estimate, but it's
probably something around that number. Whatever that number is, it's very, very
large and it gives you Some indication of how abundant micro
organisms are On our own planet and why it is That astro-biologists 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 The side, it's five times larger than the
real biosphere. I've increased the thickness of the line
by five 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 all 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 defines 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 micro-organisms. Next time, you look at 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 micro-organisms can survive
high up in the atmosphere. But it can't really multiply and that
height Is probably something like three
kilometers in the atmosphere. Beyond that, it is very difficult for
micro-organisms 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 the current stage
that The biosphere is something like ten to 20
kilometers thick. But even if that number changes quite a
lot Even it was to double or even multiply by Five 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 Of this tell us about search for life
elsewhere? Why would I want to know this if I were
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've Shown you earlier, I think it's true to
say that one Of the most remarkable 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 micro-organisms growing in that
environment. And we also know that the planet Mars has cold, salty environments. And the question is, do those environments
contain life?
But 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 learned in this talk? Well, hopefully what you've learned is
that life 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 define the limits
of the biosphere. Although life is abundant, 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. 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 leaned 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 love Extreme environments, and later on in the
course, we'll have a look, we'll Have a look more at these extremophiles
and some of the environments they inhabit. [BLANK_AUDIO]
