Why did life develop on Earth and not on the other planets?
Earth is the only planet on which water can exist in liquid form on the surface. Earth had the right temperature to allow water to circulate in all three states— solids, liquid, and water vapor—conditions that helped produce life. Scientists believe that early life was primitive single-celled organisms that lived in the oceans. The reason is simple: life needed a filter to protect it from the incoming ultraviolet energy from the Sun, and the deep ocean waters gave life that protection.