Architecture, not size
People, chimpanzees and bonobos separated from a common ancestor about 6-8 million years ago. About 3 million years ago, the brain of the future man began to grow rapidly and increased from 350 grams to more than 1,300 grams. But size is not the most important thing - the human brain is much smaller than that of an elephant or a whale.
There are about 86 billion neurons in our brain: 69 billion in the cerebellum, which is responsible for the basic functions of the body and movement, 1 billion in the brain stem and 16 billion in the cortex, which is responsible for self-awareness, language, problem solving and abstract thinking. And the elephant has 251 billion neurons in the cerebellum and only 5.6 billion in the cerebral cortex. For comparison, orangutans and gorillas each have 9 billion neurons in the cerebral cortex, while chimpanzees have 6 billion.
Although our brain accounts for only 2% of body weight, it absorbs 20% of all body energy at rest (in chimpanzees only 10%). And, of course, for the development of such a voracious body, it was necessary to sacrifice something. According to one of the versions, our intestines sharply decreased in the course of evolution and one of the most important reasons for this was the invention of cooking - ready and hot food is much easier to digest.Also, for the sake of the brain, a man sacrificed and musculature - he became two to three times weaker than monkeys.