Toumaï opens her eyes. Rays of sunlight break through the forest canopy. Lush green leaves cocoon the branches of her nest. She sees her family by her side and takes comfort in their well-being. Toumaï thinks and feels in whispers, for her mind is simple and ancient. She knows not what she is – but senses, perhaps, that she and her offspring are part of something much bigger than themselves. And indeed they are. For in the forests and woodlands of the Sahara, before natural climate change turned it into a desert, lived the earliest known human ancestor: Sahelanthropus tchadensis – otherwise known as Toumaï, an ancestor who lived 7 million years ago, 230,000 generations before you.
Toumaï had a tiny brain (350 cm3), about the size of a child’s fist. It’s hard to say exactly what it looked like. A fossilised cranium, called an endocast, leaves only an impression of the brain on the skull; and comparing modern human brains to those of our closest ape relatives takes us only so far. Nonetheless, it was a brain that had to deal with many challenges. Sabre-toothed cats prowled the land; crocodiles patrolled the waters. Being in the middle of the food chain meant that Toumaï was always searching for food – and nearly always relying on scraps left by other, more lethal predators. With distinctive features including thick fur, strong arms, a sloping face and a prominent brow bone, Toumaï looked more ape than human. Whether she walked upright on two legs, a hallmark of human evolution, is unknown.
We first learned of Toumaï’s existence on the morning of 23 March 2001, in the Djurab desert of northern Chad, west of the Great Rift Valley. Ahounta Djimdoumalbaye, a Chadian student working with a group of French scientists, unearthed what we would later learn was a 7-million-year-old early-human skull. He named it Toumaï, which means ‘hope of life’, the name given to babies born in the Djurab before the dry season.1 Paleoanthro-pologists had not witnessed a breakthrough of this magnitude since 1925, when the Australian anthropologist Raymond Dart uncovered a 3-million-year-old child’s skull in Tuang, South Africa.2 Toumaï, however, was even more impressive because she was the last common ancestor we shared with chimpanzees – the first chapter in the story of human evolution.
Humans first evolved in central Africa about 7 million years ago. After the dinosaurs perished, and mammals thrived and diversified, primates flourished in the treetops, where advanced social behaviours led to an increasing demand for greater cognitive power. Over time, different human species including Ardipithecus, Australopithecus, Homo habilis, Homo heidelbergenis, Homo neanderthalensis, Homo naledi and Homo floresiensis (to name a few) branched off from other apes and evolved with brains with unique characteristics. Some of these human species lived contemporaneously with one another; we Homo sapiens, for example, lived for a time alongside at least one other member of our genus, Homo neanderthalensis. Although you might feel superior to these now-extinct humans, it is important to remember that evolution has no aim; there is no inevitable march of progress. The famous cartoon of ‘monkey to man’ is the most misleading drawing in the history of science. It flies in the face of how evolution actually works – random mutations leading to non-random change – and is almost religious in its sentiment, imbuing us with what C. S. Lewis aptly called the ‘snobbery of chronology’.
As a neuroscientist, I am familiar with human brains – which are a bit like large, squishy grapefruits – and I’ll never forget the first time I held one.
It was September 2009, the start of term, and I was standing in a laboratory at University College London for an intimate lesson on brain anatomy. I looked at the brain in my hands. The light cast shadows over its wrinkled surface, and a pungent scent of preservative drifted up into my nostrils. It was heavier than expected, like a paperweight; it was beige with a pinkish tinge, like clay; it was soft but unyielding, like tofu. I slowly turned it over, eager to inspect it from every angle, before carefully passing it back to my professor. She pointed out the various lobes, cavities and ventricles, trailing her finger across all the regions we partly understand, plus all the ones we don’t. This particular brain had belonged to a healthy elderly female – donated to help scientists understand how it works. Amid the bewitching folds of grey and white matter was a spellbinding tapestry of neurons and synapses, a cellular and molecular universe. Even today, the memory evokes a visceral sense of reverence and awe.
Alas I cannot – can never – hold a preserved version of Toumaï’s brain. Neuroscientists are not detectives, but if we were, and if early human brains were the equivalent of missing persons, Toumaï’s would be the ultimate cold case: a mystery so distant it lingers at the edge of comprehension, long unresolved yet tantalisingly open to new evidence. And such evidence is now emerging. Today paleoanthropologists have advanced software to create virtual imprints of ancient brains, which they use to show how and when particular brain shapes evolved. In January 2018 anthropologist Simon Neubauer and his colleagues showed that the brain started its journey as an elongated sphere (a bit like a small rugby ball), which gradually began bulging out into the globular shape of modern human brains.3 This is a fascinating discovery. It suggests that the human brain existed as a kind of seedling version of itself, like a deflated beach ball waiting to be pumped with air. Eventually this expansion would give rise to our brain’s four lobes: frontal, parietal, occipital and temporal, each housing different circuits for tasks such as thinking, speaking, seeing and feeling.
About 3.5 million years after Toumaï, another early human – Australopithecus, the ‘Southern Ape’ – lived in savannahs across Africa. Here, lions, leopards and hyenas posed the greatest threat. To survive, this ancestor needed to walk upright on two legs, freeing her hands to use primitive tools in the same way that chimpanzees use rocks to crack nuts and twigs to breach termite mounds. In 1974 scientists discovered a collection of fossilised bones in the Afar Triangle in Ethiopia belonging to a female member of this species, called Australopithecus afarensis.4 The scientists liked listening to the Beatles’ song ‘Lucy in the Sky with Diamonds’ while on the expedition, so they named her ‘Lucy’.
If we met Lucy today, she would look more ape than human. She’d spend much of her time frolicking in the trees, enjoying a tree-dwelling lifestyle as well as a bipedal one. Crucial for this were her strong upper-limb bones, inherited leftovers from a close relative called Australopithecus anamensis, who was probably only a few hundred thousand years older. With a large body and extraordinarily long arms, Lucy would also have walked differently to modern humans, though what this movement looked like remains a mystery.
Although Lucy’s brain was small (600 cm3, about the size of a chimp’s), it was starting to show subtle changes in shape and structure. By boosting the number of brain cells in a region called the neocortex, evolution gave Lucy her first glimpse of higher-order thinking, involving spatial reasoning, abstract thought and planning. The neocortex (Latin for ‘new bark’, because in the evolutionary tree of life 3 million years is practically brand new) is the folded outer layer of the brain, responsible for nearly all our higher faculties. It is unique to mammals, and is so important the astronomer Carl Sagan called it the place ‘where matter is transformed into consciousness’. We can only guess what this was like for Lucy. I like to think of her thought processes as being similar to a young child’s, not fully developed but impressive nonetheless.
What generated this increased brain size? As with so many evolutionary questions, genetics is at the root of the answer. In February 2015 a group of geneticists at the Max Planck Institute in Germany identified a stretch of DNA that appears to have triggered the boost in neocortex size.5 The gene (rather unfortunately named ARHGAP11B) is highly active in human cortical stem cells (the progenitors of neocortex neurons), and, crucially, is not present in chimpanzees. It is uniquely human. Moreover, when the team inserted the gene into developing mice it made the animals’ neocortex 12 per cent larger than usual. Their brains even started to display the characteristic folding pattern unique to the human neocortex. Toumaï and Lucy’s brains might therefore have had similar folding patterns to our own.
Where this gene came from, we don’t know. Evidence suggests that it appeared when a different gene partially copied itself – a process geneticists call gene duplication – after humans split from chimpanzees 7 million years ago. Of course, we still need to explain exactly what ARHGAP11B is doing to produce higher cognition in early humans. For now, that piece of the brain’s evolutionary puzzle is conspicuously missing.
Then came our genus (that is, a group incorporating multiple species), Homo, which emerged in East Africa about 2.5 million years ago – 1 million years after Lucy. For the brain, this represented a spectacular leap forward. Humans with brains of 900 cm3 started to appear, followed by humans with a capacity of 1,000 cm3. Soon afterwards, about 500,000 years ago, brain size ballooned in humans to a staggering 1,500 cm3 – the size of a cantaloupe. Gone were the lifesaving attributes of other primates: thick fur, large muscles, a strong bite. Instead, evolution prioritised the brain. The neocortex expanded to occupy 80 per cent of the brain’s mass, and new regions for intelligence, language, memory, creativity, self-awareness and conscious thought flourished. In an evolutionary heartbeat, a minuscule 0.014 per cent of the 3.5 billion years of life on Earth, the brain went from consuming 8 per cent of the body’s energy to a massive 20 per cent, despite being a mere 2 per cent of total body weight. Though elephants and whales have bigger brains than humans, our brain is actually three times larger than what would be expected for an ape of our size. Relative to body mass, we have the largest brain of any living creature. If the human brain ever had a Big Bang, this was it.
You might wonder why this leap is so impressive, and not just another stage of development in the history of human brains. Though it may seem strange, our brain exhibited an unusual rate of change in evolutionary terms. It is not, as was once believed, just a linearly scaled-up primate brain. Our neurons are unique in many ways. They possess a unique genetic code, with at least thirty-two distinctly human genetic signatures shared across 132 brain regions.6 They have unique membrane and synaptic properties, allowing them to boost their connectivity and computational power. Most important, they are more flexible and more malleable than the brains of our closest ancestors, giving them a competitive edge in cognition and learning ability. Acknowledging these facts is not a prescription for human exceptionalism; it is simply the recognition that with Homo sapiens, the brain changed dramatically.
Throughout Homo evolution, many types of human brain evolved simultaneously. One belonged to Homo habilis, which means ‘handy man’, in deference to their tool-making abilities. These humans were nomadic hunter-gatherers, living in small bands in the grassy plains of northern Tanzania. Another type belonged to Homo erectus, ‘upright man’, who thrived in Africa and Eastern Asia for so long (2 million years), they are what biologists call a chronospecies: a species that changes and improves without ever becoming a new species altogether – a biological time traveller. Quite how this human changed yet did not change enough to become a new species is an on-going riddle in anthropology, and so some scientists distinguish the African variant (Homo ergaster) from the Asian variant (Homo erectus sensu stricto). Several other brains also flourished at this point, including that of Homo heidelbergenis, Homo neanderthalensis, Homo naledi and Homo floresiensis, though we know very little about these species, what their brains looked like or how they lived.
Such variety arose because the brain, like any organ, is subject to the pressures of natural selection. The planet has withstood immense changes over the past 2.5 million years. The grasslands of Africa and Arabia, once shaded by thick woods and watered by torrential monsoon rains, morphed into a fierce desert. The Earth’s unsteady orbit, shifting every 20,000 years, triggered an ice age every 100,000 years. Over a small period of geological time, the planet’s surface warmed and cooled, warmed and cooled. And each period brought special problems that only specialised parts of specialised brains could solve. Homo habilis and Homo erectus, for example, probably possessed neural circuitry for advanced social cognition, allowing them to persist through the ice age by hunting in groups. But as the ice melted and groups swelled, species such as Homo naledi evolved circuitry for goal memory – the power to remember certain objectives and targets of action; in the ruthless environment of primate society, remembering who your allies were was critical. And on it went. Time waxed and epochs waned; random mutations fuelled non-random advantages (or disadvantages); and natural selection filtered out what worked for the brain and what didn’t.
The human genome has experienced massive selection in the past few millennia – thought to be as significant as the artificial selection seen in domestic dogs, all of which trace their roots to a single group of grey wolves. Perhaps the most distinctive evolutionary change for our species was the loss of human body hair, with one theory suggesting that we went through a semi-aquatic phase (hence our slightly webbed hands), and another that our ancestors needed to keep cool when they migrated across the hot African savannah. But body hair is one thing, the human mind quite another. So how was the change achieved? Recent studies suggest that at least a dozen new genes contributed to humans’ advanced cognitive capacity, each one coding for a slightly different molecular spring, cog, gear and dial in the ever-expanding clockwork of the mind.7 While this is clear evidence for the role of genetics in supersizing the human brain, the real causes can be found in the interplay between DNA and the environment. Two diverse theories, as interesting as they are surprising, go some way towards explaining what really happened.
In 2004, Hansell Stedman, a molecular geneticist at the University of Pennsylvania in Philadelphia, studied the genomes of people from across the world – including natives of Africa, Europe, Russia, Iceland, South America and Japan – and compared them to several non-human primates alive today, including gorillas and chimpanzees. While the results remain the subject of debate, it appears that a rare gene mutation severely shrunk the jaw and weakened the bite of our early human ancestors.8 Biting and chewing are controlled by powerful muscles in the jaws of most primates, and genetic research shows that a gene called myosin heavy chain 16 (MYH16) plays a crucial role in this kind of muscle contraction.9 When chimpanzees bite, MYH16 switches on, and the muscles apply a strong force over the skull, restricting its growth. When mutated, however, MYH16 causes the jaw muscles to be eight times smaller than those of other apes. And this is now thought to be one of the reasons our species, Homo sapiens, has the largest brain of any primate. By eliminating the restraints of a bulky jaw, the human skull expanded, freeing the brain to grow to its modern size – three times the size of the average chimpanzee’s.
‘The first thing to note about the human brain is its size,’ Stedman told me during our long conversation. ‘I’m not suggesting that this mutation alone buys you a human brain, but it certainly could have started the brain’s evolutionary process.’ Because the cranium and jaw are both made of spongy bone, Stedman thinks the process was inevitable. ‘Muscle sculpts bone,’ he said. ‘And the human skull has always been modified by the forces acting on it, like a river slowly changing the landscape over time.’ He added that powerful jaws cannot coexist with powerful brains, due to the anatomy of cranial structures in primates. Consider the chimpanzee: its skull has no forehead; it has a large brow bone and a distinctly projecting face. This means its brain is forced into a small case like a tiger in a cage.
Stedman emailed me an image of a human and chimpanzee skull. The contrast between skull size and jaw size is stunning. It looks to be by design, but of course it isn’t. When it comes to evolution, bone is just as haphazardly constructed as everything else. A good example is the spine. It evolved to be stiff for climbing and moving in trees. Then we walked upright and it curved inward to deal with the weight of the head. But all that extra pressure causes back problems and vertebral fractures and a whole host of other issues. Then there’s the foot. No one would design it to have twenty-six bones. It’s built that way because our ancestors needed flexible feet to grasp branches – but again, the pressure of walking upright causes problems such as ankle sprains, shin splints and Achilles tendonitis. The MYH16 mutation in the jaw seems to be one of the few evolutionary instances where we got lucky. Really lucky. ‘All the other primates’ brains were constrained by their jaw muscles,’ says Stedman. ‘But Homo sapiens had nothing holding them back. They got smarter and smarter and smarter.’
It’s easy to assume that something within the brain must have triggered its evolutionary ascendency. The idea that it all started with a genetic accident in the jaw is almost too surprising. But the truth – as we will discover throughout this book – is that evolution is a game of chance, not strategy. DNA mutates by various means – from sloppy cell division to climate change to interstellar cosmic rays. That’s not to say that evolution is a bad engineer. Quite the opposite. The blind nature of the process, unshackled by the imaginative limits of human engineers, permits untold brilliance – the kind that allowed the pectoral fins of fishes to become the forelimbs of horses, the flippers of whales, the wings of birds and the arms of humans.10 And a single mutation is all it takes: if a mutant organism produces just 1 per cent more offspring than its non-mutant rivals, it leaps from representing 0.1 per cent of the population to 99.9 per cent in just 4,000 generations, a mere 100,000 years. (To put that in context, if the height of the Empire State Building represented the history of the planet, 100,000 years would be a postage stamp at the top.) We don’t know what caused the MYH16 mutation – and may never know – but what’s clear is that the revolving door between fate and happenstance is far more significant than has been previously recognised.
Another possibility is the invention of cooking, a unique human skill that saves energy by allowing us to expend less on chewing and digesting food. Cooking breaks down the connective tissue in animal flesh and dismantles the carbohydrates in plants, both of which improve absorption in the gut. Around 2.7 million years ago, before microwave meals and the dawn of the Big Mac, humans probably only cooked things such as wheat, root vegetables, fibrous fruit and, most importantly, meat: a complete protein containing all twenty amino acids and packed with energy-rich fat. In 2007 the American physiologist Stephen Secor showed that Burmese pythons fed a meal of cooked versus raw beef expend 23.4 per cent less energy digesting the food if the beef is cooked.11 Others have found similar results across the animal kingdom.12 Meat is also a good source of niacin (vitamin B3), a nutrient known to enhance brain development, and cooking increases niacin extraction rates from meat by 50 per cent.13
As humans ate more and more meat, they consumed enough calories to allow their gastrointestinal organs to shrink, which diverted even more energy to the brain. Essentially, humans swapped guts for brains. This theory, called the Expensive Tissue Hypothesis, is compelling. After all, the human gastrointestinal tract is only 60 per cent of the size it would be for an ape of our stature, and tiny guts have been observed in six other species of brainy primates, including capuchin and howler monkeys. One might think the gut was losing out in the deal, but as primatologist Richard Wrangham points out, cooking allowed humans to build an external stomach in which fire, rather than digestive enzymes, was used to break down food before it was processed by the body. ‘It makes sense that we like foods that have been softened by cooking,’ Wrangham writes,
just as we like them chopped up in a blender, ground in a mill, or pounded in a mortar. The unnaturally, atypically soft foods that compose the human diet have given our species an energetic edge, sparing us much of the hard work of digestion.14
Evolutionary trade-offs are not unique to humans. Male howler monkeys, for example, have traded a loud roar for bigger testicles: the louder the roar in these primates, the smaller the testicles. Believe it or not, this is thought to help howlers mate. A tiny-balled male might struggle to attract a female, but having a ferocious roar will certainly help scare away any male competition. On the other hand, a big-balled male is so desirable he can afford to live in a group with other big-balled males, keep quiet and simply wait for the females to mate with him and his friends – which they do. Research tells us that trading guts for brains works because humans embraced a particular life history. Unlike other animals, humans extended their juvenile years, delaying reproduction until much later in their lifecycle. In consequence, humans had more time to shrink their guts and grow larger, more sophisticated brains before adulthood. The postponement of development – called neoteny in evolutionary biology – is especially active in the human brain. A recent genetic analysis found that 40 per cent of genes linked to the development of the prefrontal cortex only become active well into adolescence.15
If cooking and our resulting micro guts really did provide the energy needed for our macro brains, we would expect to find evidence dating back at least 300,000 years, when Homo sapiens emerged, of the most vital ingredient – fire. The evolutionary advantage fire gave humans was immense. In addition to providing heat to cook food, fire brought light, warmth, protection from deadly predators and a meeting point for groups to socialise, share stories and form meaningful bonds. There is evidence that humans controlled fire (scorched earth, charred bones, charcoal, ash, stone hearths) from 800,000 years ago, in Israel – so well before the brain’s Big Bang 500,000 years ago. Some scholars point out that seared clay and burned stone tools have been found at campsites dating back 1.5 million years, at Olduvai Gorge in Tanzania and Koobi Fora in Kenya. Others posit that 1.4-million-year-old burned clay has been found in the Baringo Basin of Kenya, and that 1-million-year-old charred animal bones have emerged at Swartkrans, South Africa. But all such evidence comes with a big caveat: were these fires human-made or natural? After all, lightning and volcanoes are just as able to ignite a fire. But even if fire was tamed millions of years ago, we are still left with the mystery as to when humans prepared the first cooked meal.
One thing is certain: between 500,000 and 2 million years ago something extraordinary happened. The brain of Homo erectus alone was suddenly a colossal 25 per cent bigger than that of its predecessor, Homo habilis. On the scale of things, this is an even greater evolutionary change than the whale graduating from walking on land to a life in the water in only 10 million years.
The MYH16 mutation and the advent of cooking are only two in what will probably turn out to be a long list of theories about why the human brain is so big. Indeed, as I write, a gene called ZEB2 has been found to be an important molecular switch in brain development, nearly doubling the number of neurons in the human brain compared with that of other great apes.16 In the years ahead, scientists will increasingly rely on such evidence to get a clearer picture of what occurred. Discovering how the brain evolved is the most ambitious excavation of its kind, because the mind, as George Elliot wrote, ‘is not cut in marble – it is not something solid and unalterable. It is something living and changing.’17
But what if it didn’t change? What if fate stepped in and stopped the brain ascending the evolutionary ladder? I’ve contemplated this sort of question before. When I was a research scientist at the University of Washington, Seattle, my colleagues and I would attempt to understand something about the brain by imagining its absence. What if, for example, the fatty myelin sheaths that wrap around neurons disappeared? (Answer: the brain’s electrical impulses slow down and the symptoms of disorders such as multiple sclerosis appear.) What if the protein molecule ‘tau’ suddenly vanished? (Answer: the neuron’s internal skeleton collapses and the symptoms of Alzheimer’s disease appear.) It became something of a game, with brownie points reserved for the scientist who posed the most intriguing question.
To understand how miraculous the invention of the human brain was, we can ask what might have happened if, 2.5 million years ago, there had been no MYH16 mutation in the genus Homo, or that some variety of natural disaster – an earthquake, an asteroid, a plague – had wiped out every variety of Homo on the African subcontinent. No mutant, no massive brain, no momentous increase in cognitive power and cultural sophistication. The human race as we know it ceases to exist.
A good bet for a brainy primate that might have taken our place is the Indonesian species Homo floresiensis, a miniature human-like ape that rarely grew over three feet and had huge, fuzzy feet. In 2003, when scientists found remains of the tiny humanoid in a cave on the island of Flores, one of them exclaimed, ‘Holy shit, hobbits!’, which they have been nicknamed ever since.18 It’s thought we Homo sapiens wiped out the hobbits when we invaded their island some 50,000 years ago. No surprises there.
But now, with Homo sapiens gone, hobbits reign supreme. Their brains are small (about the size of Lucy’s, Australopithecus), but they can still build stone tools, hunt elephants, use fire and fend off giant komodo dragons. So they’re certainly intelligent enough to migrate out of Indonesia. Facing no real competition, they make their way by raft to Australia, Malaysia, Vietnam and the Indian Subcontinent, colonising as they go, seeding new hobbit societies and turning them into warring tribes that compete for land and resources. Within a few million years, during a period of intense migration and plummeting temperatures, hobbit colonies in the northern hemisphere spawn a new generation of weather-hardened ‘woolly’ hobbits, who huddle around fires on the ice sheets of Eurasia, surviving on seal meat and the occasional mammoth, pondering where next to call home.
But that’s it. With no mutations boosting rapid brain evolution, the hobbits’ brains remain unchanged. And they continue to live as nothing more than clever apes – wandering nomads, waiting to inherit a genetic recipe that never comes, oblivious to the scientific and civilisational marvels that nature had planned for their long-lost Homo sapiens cousins. No doubt this alternative world would have been an infinitely more primitive place, but it is one that could have easily existed.
When viewed in the light of this evolutionary game of chance, it is even more extraordinary that it is our species, Homo sapiens, ‘Wise Man’, whose brain triumphed above the rest, after it entered into existence some 400,000 years ago. The era into which Homo sapiens emerged was one of extreme ecological instability. African megadroughts depleted the land’s fresh water; vanishing grasslands diminished the number of animals available to hunt. To survive and flourish, Homo sapiens spread across the world, encountering and interbreeding with other species of Homo along the way. One was the Neanderthals, early humans who split from modern humans about 500,000 years ago and went extinct around 40,000 years ago. In recent years, we have learnt that modern humans living outside Africa carry around 2 per cent Neanderthal-derived DNA, the result of Neanderthal–human interbreeding 40,000 to 60,000 years ago. Another was the Denisovans, cousins of the Neanderthals who thrived across Asia between 500,000 to 30,000 years ago. Humans living today in Oceania, particularly Papua New Guinea and Australia, possess up to 6 per cent Denisovan DNA from ancient interbreeding. It’s thought that Southeast Asians once interbred with Homo erectus and possibly Homo floresiensis, the ‘hobbits’. Species such as Homo naledi and Homo rhodesiensis also probably coexisted with what became modern humans (though whether they interbred is unknown). When it comes to the brain, this genetic mixing between different humans may have contributed to its astonishing size and complexity.
Today our advanced social behaviours have led to an increasing demand for even greater cognitive power, sending the modern human brain into hyperdrive. While the underlying changes remain hidden from our conscious minds, we now have the tools – advanced microscopy and molecular genetics – to spot them in unprecedented detail. Just as we use the rings in a stump to learn the age of the tree (the more rings it has, the older it is), so too can we use the mosaic of neurons, the constellation of synapses and the tributaries of molecules to learn the age of the brain and the transformations it has seen. As we will see, understanding these changes is the key to improving our behaviour, our health, our environment, our education and our morality.
Homo sapiens’ brain anatomy – our brain anatomy – is breathtaking in its complexity. Though it may look like a homogenous ball of grey and white matter, the brain has three basic parts. The largest part, the cerebrum (Latin for ‘brain’), sits at the top. Whenever you see a picture of the brain, with its characteristic folds and wrinkles that make it look a bit like a walnut, you are looking at the cerebrum. It controls all our higher intellectual functions and helps knit together all the sensory information flooding into our minds from the outside world. It divides into two hemispheres, the left and the right, which for unknown reasons control opposite sides of the body. These hemispheres communicate with each other through a thick bundle of nerves called the corpus callosum (Latin for ‘tough body’), which is larger in musicians, ambidextrous people and homosexual men.
The cerebrum can be divided again into four lobes: frontal, temporal, parietal and occipital. The frontal lobe is essentially the control panel of our personality: important for thinking, feeling, speaking, judging, planning, socialising and controlling our sexual behaviour. The temporal lobe is a different beast altogether; it’s a kind of translator for auditory information, turning every signal received from the ear into a message that our brains can understand. The parietal lobe is all about sensation: our sense of touch, temperature and the location of our body in space. Damage to the parietal lobe can cause depersonalisation disorder (DPD), a condition where people feel completely detached from their own bodies, as though they are living their lives on autopilot. The occipital lobe is the seat of vision: it’s here that everything we see – all the shapes, colours and movements – are analysed and interpreted to produce a seamless cinematic-like projection of the world. If even one small part of the occipital lobe is lost – to a stroke, say – a person can experience symptoms ranging from the inability to recognise faces to seeing the world in snapshots.
At the back of the head, where the spine meets the brain, is the cerebellum (Latin for ‘little brain’), which houses more than half of the brain’s neurons. For a long time it was thought that the cerebellum’s only role was to control voluntary movement, the kind you need to pass a breathalyser test. But with so many neurons – the result of the cerebellum expanding over evolutionary time – it came as little surprise to learn that it also does a great deal of thinking. We just don’t know what sort of thinking. Some believe it checks and corrects thoughts in the same way it checks and corrects movements;19 others believe it acts as a kind of editor not only for thoughts but for emotion, language and memory.20 As Nico Dosenbach, a neurologist at Washington University, puts it: ‘We have an explanation for all the bad ideas people have when they’re drunk. They’re lacking cerebellar editing of their thoughts.’
Beneath the cerebrum and in front of the cerebellum is the brainstem, the gateway between the brain and the body. It oversees all the bodily functions that we’re not normally aware of: sleeping, breathing, swallowing and controlling the heartbeat. It is the most ancient part of the brain, evolving more than 500 million years ago, and is crucial for keeping us alive. When snipers need an instant kill, they aim for the brainstem.
Dozens of smaller structures are spread throughout the brain, with names as exotic as the amygdala, hypothalamus, hippocampus, telencephalon and habenular commissure, to name a few. Each is specialised for a particular aspect of brain function and each, as we will see, has its own evolutionary history. Fanning out across all this are the brain’s blood vessels. Though tiny in size they are amazingly long: if they were laid out in a line they would measure more than 60,000 miles, enough to circle the globe.
For all its staggering sophistication, the brain is a remarkably vulnerable organ. The skull fails to protect it against serious falls or collisions. A tiny clot, or stroke, can wipe out whole chunks of brain tissue. Viruses can infect it and trigger an immune response, both of which can be fatal. Tumours can burrow in and hollow it out. Psychiatric afflictions such as schizophrenia, depression and bipolar disorder can traumatise it. And neurodegenerative diseases such as Alzheimer’s, Huntington’s, Parkinson’s and multiple sclerosis can bore holes in it until it resembles Swiss cheese. According to the World Health Organization (WHO), one in four people will be affected by a neurological illness at some point in their lives.
You might be wondering why the brain, unlike other tissues in the body, doesn’t seem to heal itself. Santiago Ramon y Cajal (1852–1934), a Spanish physician and one of the founding fathers of neuroscience, was convinced that new neurons are only added to the brain before birth. Scientists have been investigating this and recent research shows that he was wrong. At least two brain regions can produce new neurons: a part of the hippocampus known as the subgranular zone (SGZ) and an area flanking the brain’s many cavities, or ventricles, known as the subventricular zone (SVZ).21 The problem is that the birth of new neurons in the mature brain, so-called neurogenesis, replaces very few of the neurons lost to common brain diseases such as Alzheimer’s and stroke. For this reason, scientists are zealously pursuing a way to boost the brain’s healing powers artificially.
Nevertheless, the brain has evolved with some remarkable tricks up its sleeve. In 2007 The Lancet reported the case of a forty-four-year-old French man who had been living normally despite the fact that he was missing 90 per cent of his brain.22 A disease called hydrocephalus had been silently destroying his brain since childhood, leaving only a thin outer layer of brain tissue and a gaping hole in his head. A civil servant with a wife and two children, the man was leading a healthy life and had only gone to his doctor complaining of mild weakness in his left leg. His brain had literally reorganised itself, radically changing to compensate for what would otherwise have killed him or left him in a vegetative state. In recent years, scientists have given a name to this phenomenon: neuroplasticity.
This is the brain’s ability to rewire its synaptic connections, reorganising itself in response to new circumstances or changes in the environment. Discovered by Michael Merzenich in the early 1970s (when he was in fact trying to prove the exact opposite: that the brain is fixed and, thus, unchangeable), neuroplasticity is thought to exist in some form in all primates, and is the driving force behind the brain’s evolution – a turbo-charge for its ability to change. This remarkable capacity for adaptation is the reason we can learn a new language, play a new instrument and navigate a new world. It’s the reason stroke victims can recover. It’s our brain’s perpetual second chance.
Today, scientists are desperately trying to unlock the secrets of neuroplasticity to treat all manner of brain diseases. For a long time, I was one of them. Back in the early days of my research, my interests lay in understanding the more fundamental properties of nervous systems, including their regenerative abilities. So every week, my lab mates and I would band together to produce a culture of neurons (taken from a rat) and watch how they behaved under different conditions. We had no grand hypothesis. We were just doing what all neuroscientists do at that stage of training – frolicking in the lab, learning by failing, trying not to break anything. It was a thrilling time and a brilliant opportunity to ask basic research questions. Something I wanted to know was: how did the neurons stand up against a small dose of the bacterial toxin lipopolysaccharide (LPS)? LPS decorates the surface of many bacteria, such as Salmonella and E. coli, and evolved to shield bacteria from harm as well as help them escape the host’s immune system. (We had a lot of LPS in our lab because other scientists were using it to study the neuroinflammatory disorder multiple sclerosis – MS.) My idea was a simple experiment, and since relatively little is known about how neurons respond to LPS, I figured it would be a nice way to observe the resilience of brain cells in real time. I treated my cells, returned them to the incubator, then checked on them a day later.
The LPS had had a devastating effect. More than half the neurons were dead, their microscopic carcasses reduced to flotsam and jetsam in the culture medium, shrivelled husks of their former selves. Those that survived looked healthy enough, though I couldn’t assess their internal state with just a microscope, and I assumed they’d be dead soon too. I remember feeling saddened by the result; clearly neurons weren’t that resilient. How many of my own had I killed after a big night out, I wondered. How many was I harming right now just by worrying about their terrifying fragility?
The next day, I checked on my remaining cells and couldn’t believe what I saw. Not only were they alive – they were thriving. Each of the surviving neurons was sprouting new projections, forming new synapses and making new connections. Like the forty-four-year-old French man missing 90 per cent of his brain, they had reorganised themselves in response to the injury, enhancing and seemingly ramping up their function to offset the loss of their neighbouring cells. It was a breathtaking moment, something I’d only ever read about until that day. From then on, neuroplasticity was an endless source of fascination to me. More than anything, the phenomenon is a striking reminder of how malleable our brains are, and how evolution has made even the most remarkable brain changes possible.
Such marvels of anatomy are not unique to Homo sapiens. Other species have brain anatomies that are beautiful, vulnerable and often surprising. The brain of a bird is completely smooth and uniform, with no wrinkles or lobes to speak of. The brain of a spider is so large that it fills their body and spills into their legs. Teleost fish, the most diverse of all fish, can continuously regenerate their brains in adulthood. Once a sea squirt finds a nice spot to call home, it eats its own brain. Such novelty exists for the same reason we see novelty across species: diverse environments promote a diversity of brains adapted to them.
Among the brain’s characteristics that might be deemed uniquely human is its ability to accumulate culture. We call this phenomenon cultural evolution, and it’s a big deal. Unlike purely genetic evolution, in which traits are passed directly between parents and their offspring, cultural evolution allows any member of a population to inherit traits from any other member. It is still considered a form of Darwinian evolution; the key difference is that traits are acquired not by DNA, but by the relationships between human beings. We see cultural evolution happening all around us, all the time. When a teacher explains the fundamentals of physics to her students, she is passing on information that will help them flourish later in life. When society demands the equal treatment of women, a new selection pressure to copy and imitate such behaviour is transmitted culturally. And when a politician or activist calls for action on climate change, those who listen are inheriting knowledge crucial for the planet’s – and consequently their own – survival.
But steady on. Isn’t this all just dependent on the norms and social conventions of the day? What does the way society treats women or the climate possibly have to do with the cognitive advancement of our species? The answer is: everything. In fact, cultural evolution may be more important than biological evolution. To understand why, let’s imagine two groups of people: the ‘Savs’ and the ‘Dims’. Both are defined not by their DNA but by their respective cultures. The Savs promote an inclusive culture in which every member of their society is treated fairly and human rights are upheld. The Dims, on the other hand, enforce a culture of gross inequality, heavily curtailed freedoms and the persecution of minorities. Needless to say, the offspring of each group will inherit their group’s unique cultural traits by way of teaching, imitation and other forms of cultural transmission. Thus, the Savs will continue to be cultural Savs, and the Dims cultural Dims.
Yet what if there is something about the Savs’ pro-equality stance that makes them better at performing some task – creating a strong economy, say – than the Dims? We know that people born in wealthier countries live longer than those born in poor countries (a cursory glance at the Preston curve demonstrates this).23 We know too that gender equality boosts economic growth and shrinks mortality rates.24 In consequence, the Savs produce healthier offspring who are more likely to reproduce. They are the fitter group, and the Dims know it. Try as they might to convince their citizens otherwise, the Dims will lose many members of their society over time. Like the Afghans fleeing the Taliban and the Venezuelans fleeing the Maduro regime, they will vote with their feet and migrate to the Savs’ more enlightened world.
This is how the brain empowers evolution by culture. By simply running the cognitive software of good ideas – in our example the idea of freedom over tyranny – the human mind became capable of improving the biological fitness of entire groups. DNA replication became almost an afterthought. With the mind of Homo sapiens, an entirely new selective pressure was born.
There is a sense of wonder and bewilderment when we consider the brain that evolution has built for us. You only have to look at the other great apes – the orangutans, gorillas and chimpanzees – to see the vast difference between their way of life and our own. Of all the primate brains to evolve, ours was not just the best innovator, the best problem solver, the best society-builder; it was also the fastest and the most energy efficient. In its exclusive identity and marvellous eccentricity, the human brain became our crowning achievement. We really do have a brain like no other. And so let us begin by understanding one of the oldest parts of the brain: the emotional brain.