Evolution and complexity

I'm new here so I don't know if this has been discussed already. Is there a simple answer to the question of why there are different degrees of complexity in species out there? I mean, there are fish, monkeys, and humans out there; I know they at some point took different paths in evolution but why do some evolve more than others? We have a common root with fish at some point in the past. Why are not fish intelligent (like human inteligence I mean)? This is probably a dumb question but I'd really like to know the answer because I think this is one of the "arguments" against evolution. Thanks!