The Meme Machine (15 page)

Of course, the encephalisation quotient is only a crude measure and hides the different ways in which the body size to brain size ratio can come about. For example, a chihuahua has a very high encephalisation quotient compared with that of a Great Dane, but this is because chihuahuas have been specially bred for small bodies – not for large brains or superior intelligence! So could we have been selected for small bodies rather than large brains? Deacon (1997), who pointed out the ‘Chihuahua Fallacy’ explains that the higher encephalisation quotient of primates compared with other animals is a result of their having smaller, slower growing bodies. Primates’ brains grow at the same rate as other species’ but their bodies grow more slowly. However, when you compare humans with other primates the situation is different. Human fetuses start growing the same way as other primates’ but then our brains continue to grow for longer. So our brains do seem to have been selected for extra growth. Our high encephalisation has come about first from the slowed body growth of primates and second from the extra brain growth of humans.

When in evolution did this brain growth begin? About five million years ago the evolutionary branch leading to modern humans split off from that leading to the present day African apes (Leakey 1994; Wills 1993). After this, our early hominid ancestors include various species of
australopithecines and then of
Homo
– including
Homo habilis, Homo erectus,
and most recently
Homo sapiens.

The australopithecines include the famous skeleton Lucy, an example of
Australopithecus afarensis
found in Ethiopia by Maurice Taieb and Donald Johansen and named after the Beatles’ song Lucy in the Sky with Diamonds. Remains of
A. afarensis
range from four million to less than two and a half million years old. Lucy herself is thought to have lived a little over three million years ago, was about three feet tall and rather apelike in build with a brain of about 400–500 cc – not much larger than a modern chimpanzee’s. From fossil footprints and computer simulations of walking based on fossil bones, it is now clear that
A. afarensis
must have walked upright, though probably could not run. So we know that bipedalism came long before hominid brains began to grow significantly in size.

The increase in brain size probably began about two and a half million years ago, at about the same time (archaeologically speaking) as the beginnings of stone tools and the transition from
Australopithecus
to
Homo.
At this time, global cooling was transforming much of Africa’s lush forest into woodland and then into grassy savannah. Adaptation to this new environment is thought to account for some of the changes leading to
Homo.
The first species of
Homo
was
Homo habilis,
named the ‘handyman’ because of the primitive stone tools they made. Australopithecines may have used sticks or stones that they found as tools, as other apes do today, but
H. habilis
was the first to chip stones into specific shapes to use as knives, choppers or scrapers. Their brains were significantly larger than australopithecine brains at about 600–750 cc.

About 1.8 million years ago
Homo erectus
begin to appear in the fossil record in Kenya.
Homo erectus
was taller and had yet bigger brains, about 800–900 cc. They were the first hominids to travel out of Africa, the first to harness and use fire, and they survived in some parts of the world until as recently as 100000 years ago. More recently still, the fossil record becomes much richer but there are many arguments about the origins of fully modern humans. So–called archaic
Homo sapiens
are widely distributed and have brains around 1100 cc, with somewhat protruding faces and heavy brow ridges, but there are two main types. One type, which seems to have led to modern
H. sapiens,
appeared in Africa about 120000 years ago. The other lived at the same time and finally died out only about 35000 years ago – they were the Neanderthals,
Homo sapiens neanderthalensis.
They had large brow ridges and protruding faces. Their brains were possibly even larger than ours and there is increasing evidence of their use of fire, their culture, and the possibility that they too had
language. 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 100000 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 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 and ability to hold down a job were ruined, he could still walk and talk and, at least to some extent, appeared normal. The same is true of the victims of frontal lobotomy, a crude operation that destroyed parts of the frontal cortex and was once used to control serious psychiatric cases. They were never ‘themselves’ again but the changes were subtle considering the vast amounts of brain damage caused in this horrible ‘treatment’. There are numerous theories of the function of the frontal lobes but none is universally accepted. We cannot find out why our large brains evolved by appealing to the function of the part that was enlarged the most.

Apart from the massive increase in the frontal lobes, the brain has been reorganised in other ways. For example, there are two main cortical areas that are critical for language, Broca’s area which is responsible for speech production, and Wernicke’s area which is responsible for language understanding. Interestingly, these two areas seem to have evolved from the motor cortex and auditory cortex, respectively. Most sounds made by other animals, from grunts to calls and birdsongs are produced in the midbrain, by areas closely connected to those controlling emotional responses and general arousal levels. Some human sounds, such as crying and laughing, are also produced by midbrain areas, but speech is controlled from the cortex. In most people both of the main language areas are in the left hemisphere, so that the two halves of our brains are not the same. Most of us are right–handed, meaning that our left hemisphere is dominant. Although some apes show handedness most do not and there is nothing like our systematic brain asymmetry in other primates. Clearly, our brains have changed in many ways other than just size.

I have described very briefly what needs to be explained – that over a period of about 2.5 million years hominid brains steadily increased in size, an increase that carries obvious costs and must have been driven by a powerful selection pressure. But we do not know what that pressure was.

Theories of the big brain

Theories abound. Most early theories suggested that toolmaking and technological advances drove the need for a larger brain. For theories of this kind the selection pressure came from the physical environment and from other animals. Human brains were needed to outwit their prey. Tools provided obvious advantages and bigger brains could make better tools. Among problems for this kind of theory are that the increase in
brain size seems to be out of all proportion to the scale of the endeavour. Big brains are so expensive that if you could catch your prey with a slightly smaller one you would have an advantage. Many pack animals hunt extremely effectively with brains that are small by human standards. Indeed, as we have seen, it rather looks as though
Homo erectus
began eating more meat to feed the growing brain rather than vice versa. Something else must have been driving brain size.

Early hominids obtained much of their food by foraging. So perhaps a big brain was needed for extracting difficult foods or for the spatial ability and cognitive maps needed to find food in patchy and unpredictable environments. However, animals with very small brains manage to store and find food in vast numbers of separate locations, and many animals, such as squirrels and sewer rats, make cognitive maps of large areas. Species with such good spatial skills do show differences in brain structure but not in overall size. Also, predictions concerning brain size and foraging range have not generally supported this kind of theory (Barton and Dunbar 1997; Harvey and Krebs 1990).

Other theories emphasise the social environment. The Cambridge psychologist Nicholas Humphrey (1986) suggested that early hominids took an important step beyond their ancestors by beginning to look into their own minds as a way of predicting what others would do. For example, if you want to know whether that huge male gorilla is likely to attack you if you try to mate with this attractive female, you should try to imagine what you would do in the same situation. This introspection is the origin of what Humphrey calls
‘Homo psychologicus’,
of humans capable of understanding that others have minds, and ultimately of self–awareness.

Consciousness itself is something we value highly and tend to think of as uniquely human and special, but whether it provides any selective advantage is a fiercely debated issue (e.g. Blakemore and Greenfield 1987; Chalmers 1996; Dennett 1991). Some argue that consciousness could not have evolved unless it had a function, while others maintain that consciousness is not the sort of thing that could have a function. For example, if consciousness is an epiphenomenon of attention or language or intelligence, then the selective advantage would be for those capabilities, not for consciousness itself. More radically, some believe that consciousness is an illusion, or that the whole idea of consciousness will ultimately be dropped, just as the idea of the ‘life force’ was dropped when we began to understand the mechanisms of life. Clearly, consciousness cannot help us explain the big brain; you cannot solve one mystery by invoking another.