Primates are, above all, social animals.
So Dunbar first establishes the starting point of his argument. We, human beings, and the other species of primates are characteristically social animals. What is really interesting is the observation that there is a close relationship between brain size and group size, at least in primates. Certainly based on etological observations the group size always conforms to the a number of interrelated ecological and species specific factors, such as the need for defence against predators, availability of food resources. The predicted and hypothesized cognitive capacity with its limits is one of the key points in Dunbar's argument.