The Domesticated Brain (4 page)

The modern adult human brain weighs only
1/50
of the total body weight but uses up to
1/5
of the total energy needs. The brain’s running costs are about eight to ten times as high, per unit mass, as those of the body’s muscles; and around ¾ of that energy is expended on the specialized brain cells that communicate in vast networks to generate our thoughts and behaviours, the neurons that we describe in greater detail in the next chapter.
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An individual neuron sending a signal in the brain uses as much energy as a leg muscle cell running a marathon.
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Of course, we use more energy overall when we are running, but we are not always on the move, whereas our brains never switch off. Even though the brain is metabolically greedy, it still outclasses any desktop computer both in terms of the calculations it can perform and the efficiency at which it does this. We may have built computers
that can beat our top Grand Master chess players, but we are still far away from designing one that is capable of recognizing and picking up one of the chess pieces as easily as a typical three-year-old child can. Some of the skills we take for granted depend on deceptively complex calculations and mechanisms that currently baffle our engineers.

Each animal species on the planet has evolved an energy-efficient brain suited to deal with the demands of the particular niche in the environment that the animal inhabits. We humans developed a particularly large brain relative to our body size but we don’t have the largest brain on the planet. Elephants can make that claim. Nor do we have the largest brain to body ratio. The elephant nose fish (which looks like an aquatic elephant) has a much larger brain to body size ratio than the human. Despite the recent brain shrinkage described earlier, the human brain is still around five to seven times larger than expected for a mammal of our body size.
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Why do humans have such big brains? After all, big brains are not just metabolically expensive to run but they pose a considerable health risk to mothers. You only have to look in a Victorian graveyard to see the number of mothers who died during childbirth as a result of haemorrhaging and infection to understand why giving birth can be such a dangerous event.
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Babies with large brains have large heads, which makes them more difficult to deliver. This became a particular problem during the evolution of our species when we started to navigate the physical world on two legs. When we began to walk upright with our heads held high, this increased the danger of childbirth but, inadvertently, this risk may have been responsible for
a significant change in the way we looked after each other. It could have contributed to the beginning of our domestic life as a species.

Although most mammals are up and running about pretty soon after birth, human babies require constant care and attention from adults for at least the first couple of years. The newborn brain also has to undergo considerable growth. At birth, it is nearly twice as large as that of a chimpanzee when you take into consideration the size of the mother, but still only about 25–30 per cent the size of the adult human brain; a difference that is mostly made up within the first year.
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Both our large growing brains and immaturity have led some anthropologists to claim that humans are born too early.
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It has been estimated that instead of the standard nine months, humans would required a longer gestation period of eighteen to twenty-one months to be born at the same stage of brain and behavioural maturity equivalent to a chimpanzee newborn.
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Why do humans leave the womb so early?

We do not have records of the brains of our ancient ancestors because the soft tissue deteriorates in the ground whereas bony skulls fossilize, and we can use these to estimate how big the brain they housed must have been. One of our first ancestors in the hominid tree of evolution appeared on the planet around 4 million years ago.
Australopithecus
or
southern ape
was very different from all the other ape species because it was able to walk upright on two legs. We know this because of the bone structures of their fossilized skeletons and the analysis of footprints that were preserved in the mud. The most famous fossil of
australopithecus
is called
‘Lucy’ after the Beatles song ‘Lucy in the Sky with Diamonds’ that was playing on the radio when she was unearthed in Ethiopia in 1974. Although Lucy was a young woman when she died, she was only about the height of a modern three-to four-year-old child and had a brain the size of a human newborn. She had long arms and curved fingers, so she was probably making the transition from living in trees to living on the land. One reason that Lucy may have come down from the trees was that the climate in Africa changed so that there was less jungle and more grassland savannahs. On a savannah, you are more vulnerable to attack from predators and so moving across flat land is much easier and faster on two legs than scrambling around on all fours like other apes.

Most of us take walking for granted, but moving on two legs is remarkably difficult. Just speak to any engineer who has tried to build a walking robot. We are familiar with science-fiction robots walking on two legs, but the reality is that this is extremely complex and requires sophisticated programming as well as a very level surface. This is because two legs provide only two points of contact with the ground, which is very unstable. Just try getting two pencils to balance against each other and you get the idea. Even big feet don’t make it much easier. Add to that the problem of coordinating the shift in weight to lift one foot off the ground and then transfer that weight to the other foot as you stride. No wonder walking is considered to be a form of controlled, continuous falling forwards.

Walking and running were both adaptations to the changing environment of the flat grasslands but they came at a cost. First, even a nimble early hominid was not going to be
able to out-run sabre-toothed cats or bears, so they had to be able to out-smart animals that were physically much larger, stronger and faster. Hominids had to evolve a brain not only capable of bipedal locomotion but one that was strategic enough to avoid capture. Second, when our female ancestors began to stand upright, this changed the anatomy of their bodies. For efficient movement on two legs, the hips have to be within a certain size, otherwise we would end up waddling like a duck – which is not the ideal way to run to catch prey or avoid being eaten. So there was adaptive pressure to keep the hips from becoming too wide, which, in turn, meant that the pelvic cavity, which is the space in between the hips, could not become any larger. The pelvic cavity determines the size of the birth canal, which effectively determines the size of the baby’s head that a mother can deliver.

Up until 2 million years ago, the relative brain size of our hominid ancestors was the same as that of the great apes today. However, something happened in our evolution to change the course of our brain development, which grew significantly larger. Human brain-size increased to be 3–4 times larger than the brain of our ancestral apes.
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As our head started to increase in size to accommodate our expanding brains, this put pressure on hominid mothers to deliver their babies before their heads got too big. However, this is not a problem for our nearest non-human cousins, the chimpanzee. In terms of movement, chimps do not naturally walk upright and so did not develop a narrow pelvis. Their birth canals are large enough to give a relatively easier birth to their babies, which is why chimpanzees waddle when they do try to walk upright. They usually deliver by themselves in less
than 30 minutes, whereas human delivery takes considerably longer and is most often assisted by other adults.

This problem of birthing big-brained babies in slim-hipped mothers is known as the ‘obstetrical dilemma’ and until recently was the accepted account of why human infants are born so early relative to other primates. However, anthropologist Holly Dunsworth at the University of Rhode Island has argued that another reason why our infants are born so early is that mothers would starve if the gestation period was any longer.
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Pregnancy is incredibly demanding on the mother in terms of the energy required to support both herself and the rapidly growing foetus. In primates and across other mammals, there is a reliable relationship between the relative size of the newborn compared to the mother that indicates that each species’s delivery date represents the point where the energy demands of the foetus begin to exceed what the mother can safely provide.
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Bigger foetuses require more energy. Dunsworth argues that pelvic size is not the only problem, but rather feeding babies without starving the mother is why humans are born prematurely.

What is undeniable is that human childbirth is not easy. One of the more intriguing ideas about the evolution of humans and their growing brains is that the difficulty and dangers posed by childbirth could have led to the development of assisted deliveries and ultimately contributed to the evolution of human domestication.
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Humans needed help in order to give birth, which means that the onset of midwifery may have contributed to the social development of our species. No other animal has assisted childbirths and this unique feature which appeared early in our history may have been
significant in shifting our species towards greater prosocial interactions. Other primates give birth relatively quickly in trees or bushes by themselves. It is possible for humans to give birth alone, and many do, but it is not the norm and especially not for first-time mothers, who typically experience longer, more painful labours. Assisted childbirth is part of our domestication. Having other members of the group present would have helped to protect against predators and reduce the stress of the situation by offering reassurance as well as provide physical assistance in actually delivering the baby.

Assisted childbirth could have been an early behaviour that fostered the right conditions for compassion, altruism, trust and other social exchanges that would become the behavioural foundations of our cultural domestication. Even if helping a mother to deliver entailed nothing more than being present to obscure, distract or confuse a potential opportunistic predator, these behaviours could have been the basis for reciprocal relationships with others in the group. Moreover, the stress and relief associated with a potentially dangerous birth could have triggered emotions that foster motivations to shape behaviours. Those who sought and offered assistance could have passed on such traits to their own offspring, thus increasing the likelihood of this cooperative behaviour becoming an established social pattern in the species.

In the same way that domesticated dogs seek assistance, when faced with a problem, our earliest ancestors began to look to others for help. Childbirth as shared emotional experience in the evolution of social behaviour may be highly
speculative, but for anyone who has witnessed a birth for the first time, the extent of the experience is unexpected, surprisingly emotional and often beyond reason and control, suggesting that it triggers behaviours that lie deep in the history of our species to help others.

Brain size and behaviour

Considering all the problems that giving birth to big brains seems to entail, we are still left with the question, ‘Why did our ancestors evolve much larger brains about 2 million years ago?’ One possibility that is consistent with the argument we began with is that a larger brain enabled animals to move around and keep track of where they have been.
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If you look at the animal kingdom, different patterns of feeding are related to different brain sizes. Primates who eat mostly fruits and nuts have larger brains than those primates who eat only leaves. Leaves are readily available in predictable locations and so require less foraging. Primates who live mostly on leaves have to consume much larger volumes of these low nutritional foods that then have to be broken down by enzymes in the stomach. This is why leaf-eating primates have much larger guts for fermenting the material. It also explains why they have to spend most of their day sitting around and simply eating and digesting.

In contrast, fruits and nuts are more nutritious but they are also sparse, more seasonal and more unpredictable. Coming down from the trees and learning to walk upright meant that foraging over greater distances by our ancestors would become the typical behavioural pattern. Bigger
brains would have been necessary to find higher value nutritional foods that would have been necessary to maintain a bigger brain.

This is why fruit-eating primates have to travel much further to satisfy their dietary needs. They also have much smaller guts and proportionally larger brains. Their habitats are more extensive and require greater navigational skills so they are generally more active. Take spider monkeys and howler monkeys, two closely related species that live in the tropical rainforests of South America. The diet of the spider monkey is 90 per cent fruit and nuts, whereas howler monkeys live mostly on the rainforest’s canopy leaves. This difference in diet and the need to forage could explain why the spider monkey’s brain is proportionally twice the size of the howler monkey’s, with a corresponding greater level of problem-solving abilities.

But our early ancestors were not simply foraging for nuts and berries – they were beginning to process food and carcasses with rudimentary stone tools. Animals with large brains are better tool users and humans are experts who far exceed any of the tool-making skills of other animals. Even making the earliest simple stone tools required special skills that are uniquely human. The anatomy of the human hand and the brain mechanisms that coordinate dexterity enabled our ancestors to hold a flint in one hand and knap it into the right shape with the other – a skill so far not observed in non-human primates.
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Animals also tend to fashion tools from what is immediately available and abandon them soon after, whereas our ancestors hung on to their manufactured tools, carrying them around for future use. That requires
a level of knowledge, expertise and intelligent planning to develop technology unprecedented in the animal kingdom – one notable exception being the sea otter that is said to carry a stone in its pouch that it uses for cracking seashells!