That's a great question to ask! Scientists do not
know why life started on Earth, but they have
theories. The question is hard to answer is
because we only have one example to study - our
own planet! Life is found nearly everywhere on
Earth - hot or cold, light or dark - so any
theory of what life needs must include all the
types of life on Earth. That's why scientists are
searching for life on Mars, on Europa (a moon of
Jupiter) and on passing asteroids. If (and
hopefully when) we find life on another planet, we
can finally test our theories about how life on
Earth may have started.
Here are some things that scientists think make
Earth favorable for life. It's also what they look
for on other planets that might have life:
- Sunlight: All forms of life need
energy and sunlight is a good source. A planet
needs just enough sunlight to have a range of
temperatures that allow for life, so it needs to
be the right distance from its star. Too little
sunlight won't provide enough energy for life too
start but too much sunlight can heat up and
destroy delicate chemicals that might form life.
- Water: As a liquid, water dissolves many
chemicals and allows them to come together and
react. Water also separates from oils, which makes
it easier for structures like cells to form. Water
can be broken apart and reassembled for easy
energy. Water also can absorb a lot of heat so
temperatures don't change too quickly for life to
adapt (not too hot, not too cold).
- Lots of Carbon: All forms of life on
Earth are made with mostly carbon, because
chemicals containing carbon can have a wide
variety of properties. Carbon chemicals are stable
enough not to fall apart on their own but can be
changed with a little energy.
- Geological Activity: Volcanoes and other
geologic activity provide heat and energy. They
move land and water around, so that new
environments are created where life can grow.
- A Magnetic Field: A magnetic field
protects the Earth from radiation and asteroids
that might disrupt life on the surface of the
- Time: Life here started about a billion
years after the Earth was formed and has taken
another 3.5 billion years to become what it is
today. During that time, Earth was stable for long
enough periods for life to grow and adapt.
This article has more detail, if you are
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