The Spirit in Human Evolution - Waldorf Research Institute

with erectus, to have even larger brains. Our hips are wider and therefore we are not as
efficient movers as erectus people were. What is clear, is that a long nursing period would
have been essential to provide the growing infant with sufficient energy-rich nutrient to
support rapid brain growth.
At birth a human baby’s brain consumes 60% of the available energy. At 7
years old this is still 40%. This high demand must have put hominid mothers under
considerable strain, even allowing for smaller brains. The Taung Child, an A. africanus
fossil thought to have been around 3 years old, based on the presence of milk teeth, had
a brain estimated to be 92% of its eventual adult size. This means the africanus mother
must have provided the child with a high level of nutrition, presumably by nursing it,
before it met a premature death in the claws of a large bird of prey, as a recent theory has
it.
Keeping a Cool Head
Another aspect of brain evolution is beginning to become clearer. The brain is
very sensitive to changes in temperature and it cannot take too much heat. A rise of
only twelve degrees Celsius can disturb brain functions and a rise of four degrees can
cause dangerous sunstroke. The brain, therefore, needs an exact temperature control
mechanism and, especially in hot climates, an effective cooling system.
Animals, such as large carnivores, dogs and big cats, that exert themselves in
hot places cool themselves by panting. An arterial network known as the “rete mirable”
extends through the snout, and the blood flow network is cooled by the evaporation of
fluid from the mucal lining of the nasal cavity (much the same heat exchange system as
a refrigerator). This cooler blood then flows to the brain, lowering the temperature. 32
Being upright, of course, reduces the body area presented to the direct rays of the
sun; in fact at noon, 60% less heat is absorbed by an upright man than by a quadruped of
equivalent size. Working in the opposite direction is the fact that being upright increases
the gravitational pull on arterial blood passing from the heart up into the head. On top
of this (literally) we present our head with its heat-sensitive brain to the full effect of the
sun, though we can assume that hominids at least had the benefit of dense, tightly curled
hair typical of many tropical peoples today.
As a thermo-regulatory adaptation, hominids have evolved a network of emissary
veins fed by the vertical plexus that lead in and out of the brain, out through the cranial
bones and under the surface of skin on the top of the head. The brain transfers heat to
the blood, which then flows out onto the surface of the skull where it cools. When the
brain overheats through exercise-related stress, the blood flow reverses and flows back
into the cranium, thus cooling the brain. This adaptation was essential once hominids
developed the ability to run over long distances, or at great speed over short ones in a
hot climate.
It seems that this system was absent in australopithecines. It first becomes
apparent in habilis and erectus, as revealed by the telltale impressions of blood veins on
the inside of the fossil crania and the small holes in the bone through which the veins
pass to the outer surface. Since this adaptation came some time after uprightness had
been established, this thermo-regulatory system must have both enabled hominids to
protect their delicate brains from heat stress and also removed a serious limit on the
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Fialkowski, K., 1987, A Mechanism for the Origin of the Human Brain, in Current Anthropology,
pp288–290.
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