How is Natural selection supposed to explain the diversity of life on earth?
You must understand that natural selection is not goal-oriented and is dictated by chance a lot more than people think. Complex multicellular forms of life evolved (gradually, step by step of course) by chance like all other mutations, and survived because those organisms managed to find a new niche to fill. Thus they prospered. This process is repeated over and over again, building to higher levels of complexity; again, entirely by chance. There were still “simple” organisms that continued to diversify to take advantage of new niches, on their own unicellular microscopic terms. In fact, if microorganisms had all died out (which, as you have pointed out, would have been extremely unlikely given their superb adaptive qualities), there would be probably be no way for the complex organisms to survive, as they are dependent on simple organisms, or the other creatures that eat them, for food. However, going back to “advanced” organisms, if those adaptations would not allow complex creatures
