My answer to the question on Quora.
As others have told you, the problem appears to be in your understanding. An evolutionist* would tell you that humans and other apes currently alive are descendants of a now extinct ape or hominid. Humans evolved from a species of ape and so did currently living species of apes. If it helps, recall that there are at least three species of great ape besides us: orangutans, gorillas and chimpanzees. In this analogy, apes are not my grandparents but my cousins.--
A month ago, I would spoken less sympathetically about who is 'smartest' and 'dumbest'. However, I have considered the problem further since then and think you might be confused by some of the analogies normally used. The typical rebuttal by analogy goes, "If Americans are descended from Europeans, why are there still Europeans." Modern formulations would add Asia, Africa, ... and all the wide variety of places people have left to become Americans. The problem with the analogy is that people today change from European to American, while apes are not currently changing into humans.
So why did some apes become human while others did not? I'm going to use another analogy and ask a question. What are the most successful animals on Earth today? let's look at success as a combination of numbers and mass. There are around eight billion humans and we average forty kg (88 lb). Well, chickens have similar numbers - around 24 billion though much lighter. E-coli bacteria in total would have similar mass. Ants, though not a single species, would also be in the running.
The point is, there are many routes to success. There is no reason to believe that the evolutionary line that led to humans immediately jumped to the success we have now. As I understand it, our line became more savanna based while other apes moved deeper into the forest.
In short, there is no reason to expect that all apes would evolve into humans nor that our evolutionary line was successful right away.
*I am an evolution proponent. An evolutionist would be a scientist who studies evolution and would be under the greater umbrella of 'biologist'. I have a degree in biology but do not work in that field.
And, "Will Chimps evolve into Humans?"
Randomness is an oft-misunderstood pillar of evolution. In natural selection, a number of mutations occur in random locations that may cause difference in how the animal looks, acts or lives. Ones that benefit the animal -in longer lifespan and/or increased number of offspring - are more likely to be 'selected' and passed on to those offspring. It might be possible to backtrack one beneficial mutation, for evolution to reverse one position - but more likely if such a reversal were selected, the gene would be broken. I've written it very abstractly so here is a sort of example. When a group of fish in Mexico began living their whole lives in caves, having eyes actually became a liability (as a entry point for infection with no sensory value). The eyes did not vanish but were covered over with flesh. The eyes are still there, just covered over and broken.
Once two groups take a few steps away from each other, neither one can retreat to the original intersection and follow the other's path. As Ariel Williams (another responder to this question) noted, chimps might become more intelligent and capable of swimming and long distance running but still would not be human - they not look much like humans either