I mean a species of ape that is morphologically similar to a chimp: an ape such that if a specimen were found alive today, it might be taxonomically categorized as a third species of chimp.
I might have said, more simply, "chimp", but this traditionally leads to confusion (among creationists) between the ancient species of chimp and the modern species of chimp. So taking the word "chimp" to mean "modern chimp", I used the more accurate term "chimp-like ape" to refer to the putative last common ancestor shared by chimps and hominids.
Why would only some evolve? What do you think the chimp-like ape thought when it gave birth to a human? How did the human survive with a chimp mother?