The disadvantage of a big brain is that it has a high metabolic cost. It's the intellectual equivalent of having a car with a big V-8 (or V-10 or V-12) engine. Powerful but expensive to operate. And that's not even counting the cost of having young who might decide to become philosophy majors.
A paper in this week's nature attempts to model both the unusual size and unusual growth pattern of human brains (complete by age 10, long before body growth finishes, unlike our fellow apes, who have brains that develop in concert with their bodies.) Besides modelling the metabolic costs, the paper attempts to compare favorite theories of the large human brain: ecological requirements, cooperation with other humans, or competition with other individuals or other groups of individuals.
The author's tentative conclusions: ecological factors play the biggest role, followed by cooperation and then competition between groups.
We find near-perfect adult fits across Homo species (Fig. 4a and Extended Data Figs. 6–8). A near-perfect adult fit for H. sapiens occurs with a large proportion of ecological challenges (approximately 60%), a moderate proportion of cooperative challenges (around 30%), a small proportion of between-group competitive challenges (around 10%), and an approximately complete absence of between-individual competitive challenges (around 0%) (Figs. 3e, 4a and Extended Data Figs. 6, 9). In the resulting reconstruction for Homo, ecological challenges increase brain size whereas social challenges decrease it (Extended Data Fig. 4), the proportion of ecological challenges tends to increase from early to late Homo species, and a steep increase in encephalization quotient from Homo ergaster to Homo heidelbergensis is due to a transition from strongly to weakly decelerating EEE* (Fig. 4a). The adult best-fit eco-social scenario for H. sapiens also yields a predicted life history that closely matches the species’ life-history timing (Fig. 4b and Extended Data Fig. 10). The resulting ontogenetic fit is high for body size, but lower for brain size early in ontogeny (Fig. 4b), perhaps caused in part by our use of a power-law relationship between resting metabolic rate and body mass that underestimates resting metabolic rate early in the ontogeny24. With the adult brain size resulting from the best-fitting scenario for H. sapiens , the predicted adult skill level for energy extraction is 3.92 terabytes (TB), which can be calculated with an equation transforming brain mass to skill level24 ( where is the asymptotic skill level in adulthood; equation (5) in the Methods). By comparison, current rough estimates25 suggest a human-neocortex storage capacity of approximately 600 TB*EEE - Energy Extraction Efficiency. I think that a "weakly decelerating EEE" means that we keep learning how to better collect food until relatively old.