III. The brain
The “expensive” brain
The brain is a very expensive organ for a living organism because it consumes a lot of energy. Nevertheless, in mammals, primates, monkeys, great apes and finally hominids all the way up to humans, a steady increase in relative brain size can be observed.
In the beginning, two statistics related to energy consumption should make the costs of the brain clear: 1. The brain is responsible for approximately 20% of the total energy consumption of the body. 2. More than 50% of the energy that the human foetus absorbs is used to build up its brain.
Now three authors who have weighed in on the issue of the costs of the brain in the context of human evolution will have their say. Robin Dunbar, from whom I used the first percentage, wrote:
Brain tissue is unusually expensive to grow and maintain. It needs about ten times more energy than one would expect, based on its weight, and it is the most expensive tissue after that of the heart and liver.
Although the average brain of an adult is only about 2% of his total body weight, this organ under the skull consumes about 20% of the total body energy when the person is at rest (and even more when the individual becomes active).
(Robin Dunbar 2010: Warum die Menschen völlig anders wurden, in: Fischer/ Wiegandt: Evolution und Kultur des Menschen, Frankfurt/M., pp. 244-269, p. 254)