|MadSci Network: Evolution|
You've asked a tough question, Steve. We know that humans and chimps had a common ancestor about six million years ago (6-8 mya is the range I usually hear), and humans and bonobos may have had a common ancestor more recently than that. The recent fossil record of apes in Africa is notoriously sparse, and even more so for the chimpanzees and gorillas than for humans. There has almost certainly been some evolution in 6-8 million years of chimpanzee life, as modern chimpanzee skeletons differ from the skeletons that we have found that are close to that common ancestor.
The current species Pan troglodytes (common chimpanzee) has three recognized subspecies: Pan troglodytes troglodytes, Pan troglodytes schweinfurthii, and Pan troglodytes verii. These subspecies have some physical differences, and based on DNA analysis, some people have suggested that Pan troglodytes verii should be recognized as a separate species. I believe there is only one subspecies of Pan paniscus (bonobo). Apart from these living species, no species of chimpanzee has gone extinct in recent times -- but it is possible that there were other species that became extinct before we started keeping track of them.
If you want to read more about chimpanzees, take a look at Discovering Chimps, the web site maintained by the Jane Goodall Institute's Center for Primate Studies. They have little evolution information, but a lot of information about chimpanzees, especially the famous chimpanzees of Gombe studied by Jane Goodall, and they have links to other chimpanzee sites. Below is a picture of a chimpanzee family from Gombe.
Try the links in the MadSci Library for more information on Evolution.