There are a number of factors that enable life to exist on Earth. Here is a brief list of some of those factors:
- Distance from Sun: Earth is at the right distance from the Sun, the star it orbits. If Earth was closer, it would have been too hot for life. If it was more distant, temperatures may have been very low.
- Atmosphere: The presence of an atmosphere makes Earth livable. If Earth had no atmosphere, it would not have maintained the temperature and gases necessary for our survival and sustenance. Earth has the right density to keep its atmosphere; if it was lighter, the atmosphere would have escaped.
- Water: Presence of water is another key factor. Water allows many reactions and also works as a universal solvent.
- Time for evolution: Earth is lucky to have a star as long-living as Sun. This ensured billions of years of time for evolution of life, not to mention settlement of planet itself.
- Molten core: Earth's core allows the recycling of material and generates a magnetic field that protects us from radiation.
Hope this helps.