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About 3-6 million years ago, the general shape of the brain of Australopithecus was about the same as that of modern great apes. The difference was in the parietal lobe of the brain. Because Australopithecines were erect, their hands were free, and they worked with their hands all the time. The parietal lobe of the brain, among other things, is responsible for the sensitivity of the hand and coordination of movement. But in principle, the brain of an Australopithecus was no different!
About 2 million years ago, there was a key moment in the evolution of the brain at the homo habilis stage. They first started making tools, but this is not the cause of brain enlargement, but only a consequence.
The size of the brain began to increase dramatically. As I said, the reason for this is not the manufacture of tools, but the transition to omnivorous food.
In general, the reason for a sharp change in the structure of an organism in evolution is often a change in climate, as in our case. At this time, climate change occurs, open savannas appear in large numbers, large ungulates and large predators die out. They've tried to take including the late Australopithecines (the first people), they also hunted, partly collected carrion in Savannah, but more profitable food was meat food because it is more nutritious; food of plant origin have a long time to chew, you need a large jaw, large teeth, healthy gums muscles need to spend rehashing a lot of time and get a few calories.
When switching to omnivorous food, the jaws may decrease, the thickness of the cranial vault will also decrease, the ridges for attaching the masticatory muscles will disappear, the muscles themselves will become smaller, the teeth and jaw will decrease.
Accordingly, with a decrease in bone density, the volume of the brain can increase with the same head size. Since the head is pressing on the spine, and the body mass has not decreased, the head should be of the same mass. Accordingly, the jaws decreased slightly, and the brain grew quickly.
In addition, to get meat food, you need to use more intelligence than for plant food, meat food runs away, in order to eat meat, you need to use more intelligence than for plant food. So, the brain started to grow pretty fast. At the same time, the brain size increased first, and then the shape.
Consider the change in the brain in the process of evolution by lobes: were the fastest growing of the frontal lobe responsible for thinking in the broadest sense of the word; growing at a rapid pace and parietal fraction responsible for the sensitivity of the brush and the gun human activity; increased at a good pace and the temporal lobe (its role is memory, emotion, smell, detection of live objects, the perception of speech and language abilities).
So, the key to the transition to a developed brain was eating meat. Man became man when he began to eat meat.
In the comments to this answer, there are additions on why exactly we have become so intelligent, and not other animals