i haven't read this thread so dunno if this came up already, but there is a chance that more than a certain time and a certain number of people are required, to force them, to develop language.
an analogy is the development of writing. writing came up with the higher degree of organisation of society that is implied by farming, as opposed to the lesser organisation of hunter tribes. the ability (inventing signs) was there before, but no need for this super systematical approach that "writing" is.
maybe monkeys, any number of them, didn't evolve language, because they have never been forced into a situation, wherein in order to survive, they had to become organised in a way, that can't be done with body language and grunting. and beside whatever other ingredients are needed for such a situation, most likely a higher number of monkeys would have to be involved. language allows to keep track of the past, part of that is creating traditional tabus, that can't be communicated without language. "stay away from the north, there are aliens in the north, they will use you for dirty experiments and you will never be seen again"