I haven't read the posts which follow yet so this may have been addressed, and probably more adroitly than I can manage. But I see this statement crop up a lot from people who are either laymen in the field of evolution or simply know nothing about it (and that isn't meant to be offensive to you, Jeanie - there are lots of subjects which I am ignorant to and, consequently, sometimes ask ignorant questions about: like computers, which I am embarrassingly illiterate with, heh!).

Anyway. Imagine we have a species of animal called X and they live in the desert with a biological makeup suited for the desert. A group of them (read: not all, a group of them) migrate to further grounds in search of food, which eventually leads them to a different climate. After spending tens of thousands of years that generation (we'll call them X.2) has evolved to match its new environtment. Given enough time to evolve they may look remarkably different from their original counterparts but their original counterparts (group X) has not needed to change very much since there have been fewer changes in their climate.

One may as well ask why alligators haven't changed if evolution proves they are related to the chicken. I think the more reasonable question is: why wouldn't other primates exist if we evolved from/with/alongside them? We cannot think of evolution as one single line. This is precisely why there are multiple species of monkeys (and many other animals)