Human intelligence is a complex trait that has evolved over millions of years. While brain size has certainly played a role in the evolution of intelligence, it is not the only factor involved. In fact, some studies have shown that there is no correlation between brain size and intelligence in humans.

One of the most important factors in the evolution of human intelligence is the development of cognitive circuitry. This refers to the way in which the different parts of the brain are connected and communicate with each other. The more efficient and interconnected the cognitive circuitry, the more intelligent the individual.

Another important factor is the development of working memory. This is the ability to hold information in mind temporarily and manipulate it in order to solve problems. The larger the working memory, the more complex the problems that an individual can solve.

Finally, the evolution of human intelligence has also been influenced by social and cultural factors. The more complex the social environment, the more pressure there is on individuals to develop their intelligence in order to compete for resources and mates.

In conclusion, human intelligence is a complex trait that has evolved over millions of years. While brain size has played a role in this evolution, it is not the only factor involved. Cognitive circuitry, working memory, and social and cultural factors have all also played a role.