The Meme Machine

THE BIG BRAIN 71
fatty sheath of myelin which insulates them and increases the speed at which
impulses travel. Myelination occurs during fetal development and early
childhood and must be quite a drain on the infant’s resources. Homo erectus
may have begun eating more meat than the australopithecines (and making tools
to cut it up), primarily to provide for the greater demand of the increasingly
greedy brain.
The brain is also a dangerous organ to produce. The fact that large brains
came about in a species that was already bipedal may be a coincidence, but it
means that we are especially ill-suited to giving birth to our big-brained babies.
Various adaptations have made it possible, including the fact that human babies
are born extremely premature as compared with most other species. They are
helpless and unable to fend for themselves, and are born with soft skulls that
only harden up later. A baby’s brain is about 385 cc at birth and more than
triples in size in the first few years. Even with these adaptations, birth is a
hazardous process for modern humans. Many babies and mothers die because
the skull is simply too big for an easy birth. All these facts suggest that a
powerful and consistent selection pressure for larger brains was at work, but we
do not know what it was.
I have so far talked about the increase in brain size as though it were just a
simple enlargement, when in fact it is more complicated than that. Higher
vertebrates in general have more cerebral cortex than other animals while the
older parts of the brain that control breathing, feeding, sleep-waking cycles and
emotional responses are more similar. However, the most interesting
comparisons are between actual human brains and what might be expected of a
typical ape of our size. Although we are highly visual animals our visual cortex
(at the very back of the brain) is relatively small while the prefrontal cortex, at
the very front, is most enlarged. This difference may well be because our eyes
are a normal size and the amount of cortex needed to process the complex visual
information coming in, is relatively constant for any ape. The prefrontal cortex,
by contrast, does not directly take sensory information but is fed by neurons
coming from other parts of the brain.
The frontal cortex is itself a kind of mystery. There is no clear answer to the
question ‘What does the frontal cortex do?’ This is particularly frustrating
because if we knew precisely what this part of the brain did, then we might be
closer to understanding the selection pressures for the larger brain – but we do
not know. People can function surprisingly well with gross damage to this part
of the brain, as is known from the famous 1848 case of Phineas Gage, the
railroad foreman whose frontal cortex was pierced right through with an iron bar
in an explosion. Although his personality was completely changed, and his life