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!

Thanks for you comments guys. I think the bottom line was nicely summarized by crashfrog: So for me the conclusion is: more complex != more evolvedI do not think as evolution as a one-brached three with humans on top (with their little crown), although that's a common mistake among many pleople (myself included perhaps). Showing that picture and telling me (or anybody else) that that's the way I probably think is kind of rude. It's ok to be aggresive when you try to make a point but don't ridicule ignorance.Anyways, perhaps a related question then is how two (new?) species can coexist? Let me try to explain. When a mutation occurs and a new species appears by natural selection, doesn't the predecesor ceases to exist after a certain time? How is it possible then to have branching in the tree of evolution?Edited by ircarrascal, : No reason given.

Thanks again everybody for responding. First of all, I know evolution is right and it works the way most of you know it works. I was just curious about these "arguments" against evolution and wanted to find simple answers because I haven't done much research about this yet. I am a scientist and I work with things* that have 'lived' billions of years and 'evolve' very slowly so the misconceptions about *quick* evolution I have are there because I'm human, and human time-scales are very very short. Boy I need to go read a good book about evolution, any suggestions?* stars - that's why i use quotes for life and evolution here =)

Oh men, I ask a couple of simple questions and some people treat me like a total fool. I thought I'd get a better welcome here.