Plenty of species have thrived on this planet without much of a brain at all.

From The Dunbar Number, From the Guru of Social Networks by Drake Bennett.

It was the monkeys’ grooming habits that really interested him. For geladas, as for many other primates, grooming is only partly about cleanliness. It’s also a form of bonding. Gelada life is rife with intrigue—there are cabals and coups and uneasy alliances—and the monkeys cement friendships by picking through each other’s fur for parasites and kneading the skin beneath. In an early paper, Dunbar showed that the amount of time geladas spend grooming is not a function of body size, which would suggest a solely hygienic purpose, as bigger bodies take longer to pick over. Instead, it’s a function of group size. The bigger the troop, the more time its members spend trying to curry favor with each other through massage. Dunbar began to wonder what other characteristics might correlate with group size.

In 1992, Dunbar published his answer: brain size. Scientists have long been intrigued by the question of why primates have such big brains. It’s nice to be smart, of course, but big brains demand an enormous amount of energy and require years to grow to full size, and the larger skulls that protect them make childbirth much more dangerous. Plenty of species have thrived on this planet without much of a brain at all.