That's a paper I'd like to see someone do at some point: given the scaling information about human-level brains in the very interesting recent paper "The remarkable, yet not extraordinary, human brain as a scaled-up primate brain and its associated cost", Herculano-Houzel 2012 (quotes from it are in my essay linked in OP), and something like OP and my African points, estimate how close to the break-even point we are: how few calories/day of brain consumption are we away from being able to support civilization development on any timescale at all?