If the emotion behind the word was anger or frustration the words were physically easier to produce. When I interviewed Andy Serkis last week, I asked him whether he was coached to get the correct breathiness? 'No, I instinctively arrived at the voice, Caesar's urgency to express his thought, plus the expulsion of air through the larynx created the voice. Chimps have a vocal air sac, for example, but humans don't. Of course, humans and chimpanzees have distinctive anatomies for vocalisation, which presents further challenges. Wars can only be won or lives defended if those in the ranks are united through shared information. When the former San Francisco mayor Dreyfus (played by Gary Oldman) addresses a crowd of human survivors we see this benefit come into play, but we also see it when the chimpanzee Caesar holds court in his ape community. It has been theorised that speech evolved because of the survival benefits of imparting information to large numbers of individuals at once. 'Evolution' is emblazoned across the publicity for the rebooted Planet of the Apes prequels, but is that label justified by their plots? One challenge for screenwriter Mark Bomback was to show why the apes needed to develop the ability to speak.

Symbols of planet of the apes