There are many types of primates, and we evolved from one particular branch. The other branches progressed in different manners (not necessarily cognitive intelligence).
There is this mistake people have with evolution, mistakingly seaching for a “missing link” between apes and humans. The truth is that evolution shows that we evoloved along side the apes from a common ancestor, not from them. A better question is how we could evolve the way Darwin explained without factoring in Lemarckian evolution. That could explain all the different chromosome counts in the different species.
A simple way to put it is that being a monkey still works pretty well. Animals disappear when they are no longer suited to a particular environment, for any number of reasons. As long as a particular organism has what it takes to cope with its environment, then there’s no reason that it would vanish. Others will emerge alongside it as genetic mutations cause variations, but that in itself won’t make the old model obsolete.
It’s easier if you think of evolution as a tree and not a rope. A tree has many branches and some of those branches are just branches of branches (hope that’s not confusing), but with a rope each section is an extension of the previous section (not the case with evolution).
@robmandu: I was trying out the cheebdragon variation so I was. Imagine some poor sod coming along to fluther and this is their first question, none of this will make any sense to the poor thing.
@Litghtlyseared Off topic, but my random brain sometimes finds the juxtaposition of your screen name with the jellyfish avatar unsettling. In the Fluther world, liightly seared jellyfish equals cannibalism, right? :)