Why water? Why is (has) water become
necessary for life?
There are probably lots of reasons. Here are
some good ones.
Water makes up 80-90% of all living organisms.
Water maintains a relatively constant temperature
for the Earth's surface (water is a temperature buffer).
Moderate temperature range required for living
organisms, even extremophiles.
There is lots of water on the Earth's surface so
the day/night temperatures tend to be similar.
It is hard -- thermodynamically -- to vaporize
water.
It is also hard to freeze water.
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