but now the human brain is shrinking.... (make up your own humor).Mankind developed agriculture some 12 to 15000 years ago. Our brain grew faster in size than in any other period.
when fasting for 24 hours or more the brain switches to run on ketones for energy, but indeed the brain is a large energy consumer. The body's sugar (glycogen) stores are limited so we switch to burning fat when the sugar is depleted.The cause is that we started to grow wheat and other grains and these are a source of starch, which converts fast to sugar. And wheat could be stored for long periods, avoiding famine.
That happened again in the 17th and 18th century when sugar became available to most people. Before, sugar was too expensive and only available to pharmacists and rich people. This might have lead us to the industrial revolution.
The only organ in our body that runs on sugar is our brain.
JR