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 planet.

- 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.

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