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Astrobiology and the Search for Extra Terrestrial Life
The University of Edinburgh
Lecture Summary: Life on Earth

The Basics of Life
What is Life?
We should consider that there is no fixed definition for Life or what constitutes Life entirely
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How the Greeks saw Life: Materialism
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Life has a soul
The soul is made up of a certain atomic material
→ Empedocles asserted that the soul was made up of either air/water/Earth/fire
→ He believed that the soul was made up of atoms of fire because Life seems
lively

How the European Enlightenment saw Life: Vitalism
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Life contains a vital force
When this vital force is added to something non-biological, it becomes biological
Life has a characteristic making it categorically different
This led to the idea of spontaneous generation

These early thoughts are important because it shows that right from the beginning of early
definitions, people believed that there was something special about Life; something that
separates us from the non-living
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Francesco Redi
Dr
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Vitalism is: Non-living + Living = Living
Vitalism is: Meat + Flies = maggots
Conclusion:
The meat didn’t spontaneously give rise to Life, it had to be accessed
...
Francesco Redi, Louis Pasteur believed that the non-biological material needs
to be accessed in order to give rise to Life
...

The result of these experiments:
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Ended the idea of spontaneous generation
Also showed that micro-organisms were responsible for Life emerging in different
materials
This is why it is said that Life is a working definition

It’s important to keep an open mind that alien Life may not fit our definition of Life, but the
definitions we do have allows us to search for Life elsewhere and study the structure of Life
we’re familiar with
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Micro-organisms were first discovered in the 17th century by Antonie van Leeuwenhoek
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In order to do that, he wanted to look at the fibers in his fabric to see what
quality they were and how it could be improved
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He was curious about the outside world, and so he went about looking around his garden,
taking samples of water and soil, and he came upon a few discoveries
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He found tiny animals that he called animacals (micro-organisms)
He found them to be spherical, rod-like and in spiral formation
He showed that microorganisms can move around too

Since then, microbiologists have gone on to define the limits of life in extreme environments:
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High temperature
High radiation
Extreme acidity

As we go to greater extremes, so we find that microorganisms are the only organisms able to
live in these extreme environments
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Definition:
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Biospace

A space bounded by physical and chemical extremes within which life on this
planet lives

However, the boundary of the biospace is not to be transgressed
Once you do, it moves into physical conditions that are far too extreme for life to be
able to tolerate
→ Most of these extremes are dominated by micro-organisms
→ These microbes are Extremophiles – organisms that love extreme
environments
The estimated number of microbes on the Earth is 1000 billion, billion, billion microbes

In order to pursue the possibilities of life beyond Earth, as well as the search for life on other
planets, we need to understand:
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The limits of life on Earth
Where it lives
The depths of Earth in which it might grow and reproduce
How high it may survive in the atmosphere

The Biology of Earth
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A tiny veneer of life on the surface and in the subsurface of our planet
The further and deeper down, the higher the temperatures
The estimated upper temperature limit of life is about 122 degrees centigrade
The limit that defined the bottom of the thin layer is set by the high temperature inside
the crust of our planet
The upper limit is set by the ability of life to survive in the sky – like in clouds
However, clouds are unfavorable because:
→ There is no food
→ Its very cold in clouds
→ High exposure to radiation
➢ Upper and lower limit of the biosphere is undetermined
➢ The estimated thickness of the biosphere is 10 to 20 km thick
➢ The estimated limit of survival of microbes in the atmosphere is 3km

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Some physical and chemical environments on other planets seem to overlap with the
biospace of life on Earth

And so, by knowing the limits of life on Earth, and exploring the outer boundaries of our
biospace, we can improve our search for life on other planets

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