First of all, just to set things straight, I'm not a biologist, nor do I play one on TV. I'm a computer programmer/admin, and simply like talking about things that I don't know much about.
"I wonder why we are the only creatures to develop such an amazing brain... just by chance?"
There are plenty of creatures out there with amazing brains. Dolphins, for example. Depends on your definition of amazing.
As for "why are there no primates/saurians/whatever that are kinda like us but not?", well, we killed'em off. Current theory is that we killed off the neanderathal, for example, who had that "similar yet different" angle you're looking for. (I know, some other theories state that neanderathals weren't as much killed off as assimilated. Not terribly pertinent for this discussion.)
Btw, this subject really gets close to racism at certain points (example, eastern-european Jews are prone to certain diseases), so we need to be carefull here.
"And if it is by chance why do all humans have near identical DNA?"
Heck, chimps are pretty close in that department too. So? Human bodies are amazingly complex things. For that matter, verterbrates are complex. Heck, even examining one-celled organisms and detailing all the parts is a pain in the rear.
The difference between "near identical" and "identical" is a big deal when you're talking about billions of comparisons.
-Jeff
P.S. Regarding "billions" - I don't remember the various numbers. Find one you like.
