How did humans evolve larger and more sophisticated brains?In general, evolution depends on a special combination of circumstances: part genetics, part time, and part environment. In the case of human brain evolution, the main environmental influence was adaptation to a 'shore-based' diet, which provided the world's richest source of nutrition, as well as a sedentary lifestyle that promoted fat deposition. Such a diet included shellfish, fish, marsh plants, frogs, bird's eggs, etc. Humans and, and more importantly, hominid babies started to get fat, a crucial distinction that led to the development of larger brains and to the evolution of modern humans. A larger brain is expensive to maintain and this increasing demand for energy results in, succinctly, survival of the fattest.
A great book on the proposed theory of human intelligence which involved our consumption of fish and other high fat (EPA & DHA) content foods near aquatic environments (as humans or precursor beings). The book is very well referenced and easy to read/understand.