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The Atmosphere

How did Life Begin on the Earth?

What is the Origin of Life on the Earth?

The first life forms appeared on Earth over 3·5 billion years ago.
The atmosphere at that time was very different from today.
The first life forms may have been bacteria which were able to
live on the methane and ammonia in the ancient atmosphere.

It is not known how life began on the Earth because
there is not enough available evidence from so long ago.
There are many theories of how life may have started.
Some people believe that life did not begin on the surface
of the Earth but may have started at a hydrothermal vent
in mineral rich waters under the sea at a mid-ocean ridge.

What did the Experiments by Miller and Urey show?

An experiment by Miller and Urey in 1952 tried to
recreate the conditions which may have been present in
the Earth's atmosphere about 3 billion years ago. They
wanted to see if life could have begun from natural events.

They used a sealed and sterile glass flask with the gases
ammonia, methane, hydrogen and water vapour inside.
They passed electric sparks (to simulate lightning) through
the gases for a week. When they analyzed the mixture
they found that many carbon compounds had formed inside
the flask. The carbon had come from the methane gas.
Some of these compounds were found to be amino acids
which are used by living organisms to make proteins.

This experiment does not conclude that life began naturally
on Earth. No one so far has been able to make
living things from simple elements and compounds.
The experiment does show that complex molecules like amino
acids can occur spontaneously in the right conditions. It is
possible that these conditions existed on Earth a long time ago.

Continued on the next page.

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