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.11 An individual neuron sending a signal in the brain uses as much energy as a leg muscle cell running a marathon.12 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.13 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.14 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.15 Both our large growing brains and immaturity have led some anthropologists to claim that humans are born too early.16 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.17 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.18 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.