How are the organisms on Earth related, and how is there such variety of life on the planet today?

It is theorized that all life begun by microorganisms being able to take advantage of underwater volcanic vents. Furthermore, it is theorized that all life (except plant life) came from the ocean. Variety in earth has come about from variation, where different species have different characteristics for their survival. Also note that gravity has led to the development differences between earth creatures and ocean creatures. Ocean creatures have more mass than land. Usually this is because of the pull of gravity.