Answer:
By the separation of the East African Rift, there were a difference among them.
Explanation:
Both Monkeys and Apes, have a common ancestor, in the late Oligocene, around 25 million years ago. But later, there were a branch in the East African Rift. This was a product of the crash of the Asian and Indian Continental plates, pushing the Tibetan Plateau. In an area that some day were flat, now some mountains, deep lakes and valleys appear. The new mountains, were a natural barrier for the sea wind, causing a dry weather and the lost of forest in this are. Since this is a topographical limits for some species, then the differentiation starts to Apes and Monkeys. One of the evidences is that the first hominids could develop the brain capacity, also the brain size.
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