I already know that bacteria evolve, that wasn't my question. My question was why they 'led' to more complex organisms when life at the bacterial level seems pretty resilient and successful already. For example, sharks are more or less the same as they were millions of years ago last I heard. It seems that these organisms are successful by essentially any measure, so why would they evolve into more complex organisms? Is it as simple as the idea that a given mutation was 'partially successful' and that nature has made it rare rather than extinct? It simply strikes me as odd since I thought efficiency was the name of the game in natural selection.