Monkeys do eat bananas in the wild, but they are not the primary part of their diet. Bananas are native to Australia and South Asia, and monkey species living in the Americas or Africa had no access to them prior to human farming of the banana plant in those areas.
Monkeys are mostly fruit eaters, though they also consume insects, roots, parts of plants and even other monkeys. Their natural diet is extremely rich in nutrients and highly varied, depending on the region and season. The banana is consumed by monkeys when it is available, but the kind they eat in the wild is smaller and less sweet than the kind sold in supermarkets for human consumption. Domesticated bananas are considered unhealthy for monkeys because they contain too much sugar, which encourages obesity and diabetes.