I wrote this book of seven and a half short, informal essays to intrigue and entertain you. It’s not a full tutorial on brains. Each essay presents a few compelling scientific nuggets about your brain and considers what they might reveal about human nature. The essays are best read in order, but you can also read them out of sequence. […] The essays don’t tell you what to think about human nature, but they do invite you to think about the kind of human you are or want to be. – Lisa Feldman Barrett

The half-lesson: your brain’s most important job is not thinking. It’s running a little worm body that has become very, very complicated.

ONCE UPON A TIME, the Earth was ruled by creatures without brains. This is not a political statement, just a biological one.

One of these creatures was the amphioxus. If you ever glimpsed one, you’d probably mistake it for a little worm until you noticed the gill-like slits on either side of its body. Amphioxi populated the oceans about 550 million years ago, and they lived simple lives. An amphioxus could propel itself through the water, thanks to a very basic system for movement. It also had an exceedingly simple way of eating: it planted itself in the seafloor, like a blade of grass, and consumed any minuscule creatures that happened to drift into its mouth. Taste and smell were of no concern because an amphioxus didn’t have senses like yours. It had no eyes, just a few cells to detect changes in light, and it could not hear. Its meager nervous system included a teeny clump of cells that was not quite a brain. An amphioxus, you could say, was a stomach on a stick.

Amphioxi are your distant cousins, and they’re still around today. When you look at a modern amphioxus, you behold a creature very similar to your own ancient, tiny ancestor who roamed the same seas.

Can you picture a little wormy creature, two inches long, swaying in the current of a prehistoric ocean, and glimpse humanity’s evolutionary journey? It’s difficult. You have so much that the ancient amphioxus did not: a few hundred bones, an abundance of internal organs, some limbs, a nose, a charming smile, and, most important, a brain. The amphioxus didn’t need a brain. Its cells for sensing were connected to its cells for moving, so it reacted to its watery world without much processing. You, however, have an intricate, powerful brain that gives rise to mental events as diverse as thoughts, emotions, memories, and dreams—an internal life that shapes so much of what is distinctive and meaningful about your existence.

Why did a brain like yours evolve? The obvious answer is to think. It’s common to assume that brains evolved in some kind of upward progression —say, from lower animals to higher animals, with the most sophisticated, thinking brain of all, the human brain, at the top. After all, thinking is the human superpower, right?

Well, the obvious answer turns out to be wrong. In fact, the idea that our brains evolved for thinking has been the source of many profound misconceptions about human nature. Once you give up that cherished belief, you will have taken the first step toward understanding how your brain actually works and what its most important job is—and, ultimately, what kind of creature you really are.

Amphioxi were not our direct ancestors, but we had a common ancestor that was very likely similar to a modern-day amphioxus.

Five hundred million years ago, as little amphioxi and other simple creatures continued to dine serenely on the ocean floor, the Earth entered what scientists call the Cambrian period. During this time, something new and significant appeared on the evolutionary scene: hunting. Somewhere, somehow, one creature became able to sense the presence of another creature and deliberately ate it. Animals had gobbled one another before, but now the eating was more purposeful. Hunting didn’t require a brain, but it was a big step toward developing one.

The emergence of predators during the Cambrian period transformed the planet into a more competitive and dangerous place. Both predators and prey evolved to sense more of the world around them. They began to develop more sophisticated sensory systems. Amphioxi could distinguish light from dark, but newer creatures could actually see. Amphioxi had simple skin sensation, but newer creatures evolved a fuller sense of their body movements in the water and a greater sense of touch that allowed them to detect objects by vibration. Sharks today still use this kind of touch sense to locate prey.

With the arrival of greater senses, the most critical question in existence became Is that blob in the distance good to eat, or will it eat me? Creatures who could better sense their surroundings were more likely to survive and thrive. The amphioxus may have been a master of its environment, but it couldn’t sense that it had an environment. These new animals could.

The hunters and the hunted also received a boost from another new ability: more sophisticated kinds of movement. For the amphioxus, whose nerves for sensing and moving were woven together, movement was extremely basic. Whenever its stream of food became a trickle, it wriggled in a random direction to plant itself in another spot. Any looming shadow prompted its body to dart away. In the new world of hunting, however, predators and prey alike began to evolve more capable systems for movement, or motor systems, to navigate with greater speed and dexterity. These newer animals could dart, turn, and dive deliberately toward things like food and away from things like threats in ways that suited their environment.

Once creatures could sense at a distance and make more sophisticated movements, evolution favored those who performed these tasks efficiently. If they chased a meal but moved too slowly, something else caught the meal and ate it first. If they burned up energy fleeing from a potential threat that never arrived, they wasted resources that they might have needed later. Energy efficiency was a key to survival.

You can think about energy efficiency like a budget. A financial budget tracks money as it’s earned and spent. A budget for your body similarly tracks resources like water, salt, and glucose as you gain and lose them. Each action that spends resources, such as swimming or running, is like a withdrawal from your account. Actions that replenish your resources, such as eating and sleeping, are like deposits. This is a simplified explanation, but it captures the key idea that running a body requires biological resources. Every action you take (or don’t take) is an economic choice— your brain is guessing when to spend resources and when to save them.

The best way to keep to a financial budget, as you may know from personal experience, is to avoid surprises—to anticipate your financial needs before they arise and make sure you have the resources to meet them. The same is true of a body budget. Little Cambrian creatures needed an energy-efficient way to survive when a hungry predator was nearby. Should they wait around until the ravenous beast made its move and then react by freezing or hiding? Or should they anticipate the lunge and prepare their bodies in advance to escape?

When it came to body budgeting, prediction beat reaction. A creature that prepared its movement before the predator struck was more likely to be around tomorrow than a creature that awaited a predator’s pounce. Creatures that predicted correctly most of the time, or made nonfatal mistakes and learned from them, did well. Those that frequently predicted poorly, missed threats, or false-alarmed about threats that never materialized didn’t do so well. They explored their environment less, foraged less, and were less likely to reproduce.

Your brain runs a budget for your body that regulates water, salt, glucose, and many other biological resources inside you. Scientists call the budgeting process allostasis.

The scientific name for body budgeting is allostasis. It means automatically predicting and preparing to meet the body’s needs before they arise. As Cambrian creatures acquired and spent resources throughout the day by sensing and moving, allostasis kept the systems of their bodies in balance most of the time. Withdrawals were fine, as long as they renewed the spent resources in a timely manner.

How can animals predict their bodies’ future needs? The best source of information comes from their past—the actions they’ve taken at other times in similar circumstances. If a past action brought benefits, such as a successful escape or a tasty meal, they’re likely to repeat that action. All sorts of animals, including humans, somehow conjure up past experiences to prepare their bodies for action. Prediction is such a useful capability that even single-celled creatures plan their actions predictively. Scientists are still puzzling out how they do it.

So imagine a tiny Cambrian creature drifting in the current. Up ahead, it senses an object that might be tasty to eat. What now? It can move, but should it? After all, moving takes energy from the budget. The movement should be worth the effort, economically speaking. That is a prediction, based on past experience, to prepare a body for action. To be clear, I don’t mean a conscious, thoughtful decision, weighing the pros and cons. I’m saying that something must occur inside a creature to predict and launch one set of movements rather than another. That something reflects a determination of worth. The value of any movement is intimately bound up with body budgeting by allostasis.

Meanwhile, ancient animals continued to evolve larger, more complex bodies. That meant the insides of bodies became more sophisticated. The amphioxus, the little stomach on a stick, had almost no bodily systems to regulate. A handful of cells were enough to keep its body upright in the water and digest food within its primitive gut. Newer animals, however, developed intricate internal systems, like a cardiovascular system with a heart that pumps blood, a respiratory system that takes in oxygen and eliminates carbon dioxide, and an adaptable immune system that fights infection. Systems like these made body budgeting much more challenging, less like a single bank account and more like the accounting department of a sizable company. These complex bodies needed something more than a handful of cells to ensure that water and blood and salt and oxygen and glucose and cortisol and sex hormones and dozens of other resources were all regulated well to keep a body running efficiently. They needed a command center. A brain.

And so, as animals gradually evolved bigger bodies with more systems to maintain, their handful of body-budgeting cells also evolved to become brains of greater and greater complexity. Fast-forward a few hundred million years, and the Earth is now littered with complicated brains of all kinds, including yours—a brain that efficiently supervises over six hundred muscles in motion, balances dozens of different hormones, pumps blood at a rate of two thousand gallons per day, regulates the energy of billions of brain cells, digests food, excretes waste, and fights illness, all of it nonstop for seventy-two years, give or take. Your body budget is like thousands of financial accounts in a giant, multinational corporation, and you have a brain that’s up to the task. And all your body budgeting takes place in a massively complicated world made even more challenging by the other brains-in-bodies that you share it with.

So, returning to our original question: Why did a brain like yours evolve? That question is not answerable because evolution does not act with purpose—there is no “why.” But we can say what is your brain’s most important job. It’s not rationality. Not emotion. Not imagination, or creativity, or empathy. Your brain’s most important job is to control your body—to manage allostasis—by predicting energy needs before they arise so you can efficiently make worthwhile movements and survive. Your brain continually invests your energy in the hopes of earning a good return, such as food, shelter, affection, or physical protection, so you can perform nature’s most vital task: passing your genes to the next generation.

In short, your brain’s most important job is not thinking. It’s running a little worm body that has become very, very complicated.

Of course, your brain does think and feel and imagine and create hundreds of other experiences, such as letting you read and understand this book. But all of these mental capacities are consequences of a central mission to keep you alive and well by managing your body budget. Everything your brain creates, from memories to hallucinations, from ecstasy to shame, is part of this mission. Sometimes your brain budgets for the short term, like when you drink coffee to stay up late and finish a project, knowing that you are borrowing energy that you’ll pay for tomorrow. Other times, your brain budgets for the long term, like when you spend years to learn a difficult skill, such as math or carpentry, that requires a sustained investment but ultimately helps you survive and prosper.

You and I do not experience our every thought, every feeling of happiness or anger or awe, every hug we give or receive, every kindness we extend, and every insult we bear as a deposit or withdrawal in our metabolic budgets, but under the hood, that is what’s happening. This idea is key to understanding how your brain works and, in turn, how to stay healthy and live a longer and more meaningful life.

This little evolutionary story is the beginning of a longer tale about your brain and the other brains around you. In the next seven short lessons, we’ll take a tour of remarkable scientific findings in neuroscience, psychology, and anthropology that have revolutionized our understanding of what happens inside your skull. You’ll learn what makes the human brain distinctive in an animal kingdom full of astonishing brains. You’ll explore how infant brains gradually transform into adult brains. And you’ll discover how different kinds of human minds can arise from a single human brain structure. We’ll even tackle the question of reality: What gives us the power to invent customs, rules, and civilizations? Along the way, we’ll revisit body budgeting and prediction and their central roles in creating your actions and your experiences. We’ll also uncover the powerful connections between your brain, your body, and other human brains-in-bodies. By the end of this book, I hope you will delight, as I do, in knowing that your thinking cap is for much more than thinking.

  • Your Brain Is Not for Thinking
  • You Have One Brain (Not Three)
  • Your Brain Is a Network
  • Little Brains Wire Themselves to Their World
  • Your Brain Predicts (Almost) Everything You Do
  • Your Brain Secretly Works with Other Brains
  • Brains Make More than One Kind of Mind
  • Our Brains Can Create Reality