"For the selection pressure to continue—for brains to keep getting larger—there must have been huge rewards for being smart and having a bigger head. One of those rewards was better social skills. The increased complication of childbirth would require mothers to have help from others; individual females who were more socially adept would get more help, and therefore they and their infants were more likely to survive. This positive feedback loop drove the evolution of larger brains as a means of having greater social influence.
Underlying this social brain hypothesis is an internal arms race to develop the higher cognitive skills to enable greater social control. Clearly, with the emergence of H. erectus, H. heidelbergensis, and eventually H. sapiens, the positives outweighed the negatives. In my latest book, Cradle of Humanity, I discuss how this was driven by rapid environmental changes in East Africa, increased competition for resources within the species as the population expanded, and competition with other species.
We humans emerged from Africa with an extremely large, flexible, and complex “social brain.” This has allowed us to live relatively peaceful lives around millions of others. This does not, however, mean that we live in harmony with each other; instead, we have swapped physical conflict for social competition. We are constantly strengthening our alliances with friends and partners while working out how to keep up and if possible surpass our peers in terms of social position. And that is why it is so stressful simply being human." (https://www.sapiens.org/biology/human-brain-evolution-social/)