The Meme Machine

70 THE MEME MACHINE
There is still much argument about which hominid line produced modern
humans, and what happened to the Neanderthals. However, sequencing of
mitochondrial DNA suggests that they were not our ancestors (Krings et al.
1997). So did we kill them off, as we have killed off so many other species, or
did they become extinct for some other reason?
A rather odd fact is that for most of the past 5 million years there have
always been several species of hominid living at the same time, as there are
several species of other primates now. Today there is only one kind of human
with rather minor differences around the world. ‘What happened to all the rest?
These are fascinating issues but we must return to our main argument. Most
relevant is that brain size increased dramatically during the relatively short
period of 2.5 million years that separated the last australopithecines from fully
modern humans. By about 100 000 years ago all living hominids would
probably class as H. sapiens and had brains about as large as ours.
This massive increase must have been very expensive in energy terms. First,
the brain is expensive to run. It is often said that the brain consumes 20 per cent
of the body’s energy but consists of only 2 per cent of the body weight. This
figure is slightly misleading because it refers to a body at rest. When large
muscles are lugging you and your suitcase as fast as you can go across the
platform as the train whistle blows, the brain’s energy use is small by
comparison. Nevertheless, your muscles often rest, but the brain does not, even
in sleep. It uses roughly the energy consumed by a light bulb, all the time.
The brain consists primarily of neurons that conduct impulses along their
axons. These impulses consist of a wave of depolarisation which sweeps along
the axon as charged ions flood across the axon’s membrane. Much of the energy
the brain uses is consumed in maintaining the chemical differences across these
membranes so that the neurons are continuously ready to fire. Also, many
neurons keep firing at a low frequency all the time so that incoming signals can
pass on information by either increasing or decreasing the resting frequency.
The body’s energy budget must have to find a large surplus to keep all this
going. A smaller brain would certainly save a lot of energy, and evolution does
not waste energy for no reason. As Steven Pinker (1994, p. 363) said ‘Why
would evolution ever have selected for sheer bigness of brain, that bulbous,
metabolically greedy organ? . . . Any selection on brain size itself would surely
have favored the pinhead’.
Second, the brain is expensive to build. The neurons are surrounded by a