Later This site tells the story of the history of the universe. Click Earlier and Later to follow the story.
Some people think that life was carried to Earth from another planet .
But that just puts back the problem of how life began to that other planet. It is simplest to assume that life began on Earth. Indeed, the Earth has the right conditions for life to exist. It is at just the right distance from the Sun for water to be a liquid, and its orbit is almost circular. This is probably unusual among planets round other stars, so it seems sensible to assume Earth was the place where life began.