"Suppose a species turns into another one. Why did chimpanzees, gorillas, and other large apes "remain" in their past form, and then the intermediaries to humans died off?
If the large apes had the same circumstances as the ones that branched off and became humans, why didn't THEY die off?
I am missing something here?"
Yes, you are. You're making the incorrect assumption that evolution is a steady, progressive process, which is not necessarily the case. Evolution is driven by two factors: Mutation and natural selection. Mutations happen all the time. The simple fact that no two animals-including humans- are alike, bears this out. Natural selection simply favors a creature whose mutations make it better able to cope with its environment, survive, and then pass on its mutated genes to the next generation. So evolution affects different species in different ways. Some species evolve rapidly-like Homo Sapiens Sapiens, and some evolve or change hardly at all- like sharks.
All species of modern apes, which includes humans, have undergone evolutionary change and many "intermediate" species of all great apes have died off. (I put "intermediate" in quotation marks because all species living today and in the past are in fact intermediate. Biological evolution does not have a finish line.) It is interesting that of all the hominids only humans remain today, (even though there is a movement afoot to declare chimpanzees and bonobos as hominids). However this is easily explained, given human nature. These other hominids, like Homo Erectus and Homo Sapiens Neanderthalensis, simply couldn't compete. They were probably assimilated or simply wiped out by the superior Homo Sapiens.
CyrusThePersian