Why we think it took comets or asteroids to bring water and the building blocks of life to Earth escapes me. Turn the clock back to when everything in this solar system began as a dust cloud cast off from several dead stars. Most of that cloud became our sun. Everything from Mercury to the Ort cloud formed from the remnants. There is nothing special about comets. The processes that formed water on comets formed water on Earth. The same processes that formed all the natural organics in comets formed natural organics in the aggregate that became our planet. When all these 'exciting life giving, water creating things' formed in comets they were forming in the aggregate that made our planet. And, just having all these organics get you no where near life. You need these building blocks for life in dynamic solution. A volcanic process on a planetary scale, not one dependant on the odd rock from space. I repeat, there is nothing in comets or like bodies that wasn't already in the Earth.
