Having a Bigger Brain Won't Cost You an Arm and a Leg
Over the course of human evolution human brains have gotten larger in size. This being said, the brain requires a large amount of energy to function properly. Until recently scientists have believed in the 'expensive-tissue hypothesis'. The hypothesis suggests that due to an increase in brain size over the course of human evolution the size and efficiency of the human gut has decreased. The thought is that since the size of the human brain has gotten bigger it requires more energy. So the brain took energy from the gut to allow growth. However that hypothesis has been challenged by the scientists at the Univeristy of Zurich.
Recent findings suggest that there is one main reason why the brain could adapt without affecting the other organs. Somehow, over the course of the evolution process humans found ways to decrease the amount of energy they used and transfered that to allow for a larger brain. This happened in a number of ways:
Bipedalism was developed
A better diet of meats over vegitation
These three things changed the amount of energy that humans use or taken in. Bipedalism was a big step in allowing this shift because walking on two legs uses up a lot less energy than knuckle walking. Since less energy is spent on moving around those who walked on two legs could afford having a bigger brain. Secondly, the meat diet increased the amount of energy that humans took in, thus allowing them to have a larger brain without decreasing the size of any of their other tissues. The third reason is that humans use communal childcare. Since more than one adult helps to raise one child in the big brained species more children can be born more often. If only one parent helps to raise a kid then they will only be able to have one child every five years or so. This effectively decreases the amount of energy it takes to raise a child, allowing for more childbirth of more offspring with bigger brains.