The ability to talk had evoluntionary advantage for humans, and the necessary mutation was introduced into our gene pool. At least one of these is not true of monkeys and gorillas.
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Gorillas chose a plant based diet so they have evolved a body that requires low protein which leads to a small brain and no need for speech since eating plants does not require any social interaction. The chimpanzee is our closest relative which has shown signs of culture, learned behaviors for tools, and one famous one actually learned sign language. Chimps are fruit specialists but also eat insects and are known to hunt other monkeys, but they have evolved to be the best a finding fruit which is shown in their social interactions. Humans however where "protein-specialists" and as we evolved better methods of hunting through communication with others we evolved speech to better our hunting skills. The protein also aided in our large brains which helped even more with more hunting and more speech skills. Its about evolution.
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Most animals have their own way to communicate like people from different countries have different languages. Some animals like horses prefer to talk to each other mainly by body language and you can learn to kind of understand them for example they "talk" alot with their ears. If they are forwards they are usually alert, excited or happy, backwards and they are upset and right back flat usually something like angry. There is alot of what they can say through their "body language". Animals can also learn to understand us somewhat dogs can understand certain words and tones if you say something to your dog in an excited tone that it doesn't know it will think this word is exciting no matter what word it is. Our language is something that we gained through evolution. Monkeys and gorillas evolved over a very long time in a different way to us and so I guess thats why you don't hear of many talking gorillas lol but it doesn't mean they can't understand each other. Prehaps it is because they haven't physically evolved to talk in the same way as us and so couldn't speak any of our languages (although other than zoo monkeys how can you test if they can or not).