Chapter 1
The Invisible Hand

The lions woke me up around two in the morning. The sound wasn't loud so much as big-like the moaning hydraulics of a garbage truck interrupted by the coughs and grunts of an idling Harley-Davidson. My first hazy, sleepy reaction was a kind of grateful joy. Ah, the sounds of wild Africa! I stared up through the gossamer mesh roof of my tent at the stars overhead, felt the night breeze pushing through the dry grass and thorny acacia trees and up against the tent's thin nylon walls, carrying the lions' chorus. I felt fortunate to be there, camped in my little tent in the middle of the vast East African savanna, a place so remote and untrammeled that there were lions just a few hundred yards off. How lucky was I?

Then a pang of adrenaline and fear. This wasn't a zoo or some tourist safari. Those lions weren't pretty pictures in a National Geographic magazine or a PBS nature show. This was real life. A gang of heavily muscled 300-pound feline killing machines was a short stroll away, and they sounded . . . anxious. Maybe even . . . hungry? Of course they could smell me. After days of camping I could smell myself. What was my plan when they came for my soft American carcass, the warm triple crme brie of human flesh? I wondered how close they'd get before I heard them in the tall grass, or if the end would come unannounced, an explosion of claws and hot angry teeth crashing through walls of the tent.

I kept thinking it through, trying to be rational. Judging by where the sound was coming from, the lions would have to walk past Dave's and Brian's tents first. I was Door Number 3 in this particular game of chance. That meant 1 in 3 odds of being eaten by lions tonight, or, if one was a glass-two-thirds-full kind of person, a 67 percent chance of not being eaten. That was a comforting thought. Plus we were with the Hadza, on the outskirts of their camp, and nobody messes with the Hadza. Sure, hyenas and leopards would occasionally slink past their grass huts at night looking for scraps or unattended babies, but the lions seemed to keep their distance.

The fear began to dissipate. Drowsiness seeped back in. I'd probably be fine. Besides, if one had to be eaten by lions, it seemed preferable to be asleep at the time, at least until the last possible moment. I fluffed up the pile of dirty clothes I was using for a pillow, adjusted my sleeping pad, and went back to sleep.

It was my first summer working with the Hadza, a generous, resourceful, and badass people who live in small camps scattered about the rugged, semiarid savanna around Lake Eyasi in northern Tanzania. Anthropologists and human biologists like me like to work with the Hadza because of how they make their living. The Hadza are hunter-gatherers: they have no agriculture, no domesticated animals, no machines or guns or electricity. Each day they wrest their food from the wild landscape around them, using nothing but their own hard work and guile. Women gather berries or dig wild tubers from the rocky soil with stout pointed sticks, often with a child on their back in a sling. Men hunt zebra, giraffe, antelope, and other animals, with powerful bows and arrows they fashion themselves from branches and sinew, or chop open trees with small axes to extract wild honey from beehives built in the hollows of limbs and trunks. Kids run and play around the grass huts of camp or head out in groups to get firewood and water. Elders either head out foraging with the other adults (they are remarkably spry even into their seventies) or stay back at camp to keep an eye on things.

This way of life was the norm worldwide for over two million years, from the evolutionary dawn of our genus, Homo, through the invention of farming just twelve thousand years ago. As farming spread and brought towns, urbanization, and eventually industrialization in its wake, most cultures traded in their bows and digging sticks for crops and brick houses. Some, like the Hadza, held on proudly to their traditions even as the world around them changed and began to encroach. Today, these few populations are the last living windows into humanity's shared hunter-gatherer past.

Along with my good friends and fellow researchers Dave Raichlen and Brian Wood and our research assistant Fides, I was in Hadzaland (as we casually refer to their homeland) in northern Tanzania to learn how the Hadza lifestyle is reflected in their metabolism-the way their bodies burn energy. It's a simple but incredibly important question. Everything our bodies do-growing, moving, healing, reproducing-requires energy, and so understanding how our energy is spent is the first, foundational step in understanding how our bodies work. We wanted to know how the human body functions in a hunting and gathering society like the Hadza, where people were still an integral part of a functioning ecosystem, with a lifestyle still similar in important ways to that of our deep past. No one had ever measured daily energy expenditure, the total number of calories burned per day, in a hunter-gatherer population. We were eager to be the first.

In the modernized world, far removed from the daily work of acquiring our food with our bare hands, we pay little attention to energy expenditure. If we think about it at all, we think of the latest diet, our workout plan, whether we've earned that donut we crave. Calories are a hobby, a nugget of data on our smartwatches. The Hadza know better. They understand intuitively that food and the energy it holds are the fundamental stuff of life. Each day they confront an ancient and unforgiving arithmetic: acquire more energy than you burn or go hungry.

We woke up with the sun still orange and weak on the eastern horizon, the colors of the trees and grass washed out in the diluted morning light. Brian started a cooking fire in our small, Hadza-style three-stone hearth and set a pot of water on to boil. Dave and I milled around bleary-eyed, needing caffeine. Soon enough we were all drinking hot mugs of Africafe instant coffee and spooning up plastic bowlfuls of instant oatmeal and jelly. We discussed research plans for the day. We had all heard the lions during the night and joked nervously about how close they sounded.

Then, sauntering through the tall dry grass, came four Hadza men. They weren't coming from camp, but from the opposite direction, from the bush. They were each carrying large, misshapen loads over their shoulders, and it took me a moment to recognize what it was: legs, haunches, and other blood-matted parts of a big, freshly killed antelope. The men knew we liked to keep track of the foods they brought back to camp, and they wanted to give us a chance to record this kill before splitting it up among the families in camp.

Brian snaps to it, clears off the weigh scale, and locates the Foraging Returns notebook, striking up a conversation in Swahili, our common language with the Hadza.
"Thanks for bringing these by," says Brian, "but where the hell did you get a huge antelope at six in the morning?"
"It's a kudu," say the Hadza guys, grinning, "and we took it."
"Took it?" asks Brian.
"You guys heard the lions last night, right?" say the Hadza guys. "Well, we figured they were up to something, so we went and checked it out. Turns out they had just killed this kudu . . . so we took it."

And that was it. Another day in Hadzaland-a banner day in fact, starting off with the rare prize of big game in all of its fatty and proteinaceous glory. In camp later that morning, gnawing on roasted strips of kudu, hearing the story of how Dad and his buddies chased off a pride of hungry lions in the dark to bring home food, the Hadza kids would understand an important and timeless lesson. Energy is everything, and it's worth risking everything to get it.

Even if you have to steal breakfast from the lions' jaws.

A Small Matter of Life and Death

Energy is the currency of life; without it, you're dead. Your body is made up of roughly 37 trillion cells, each humming along like microscopic factories, every second of every day. Together, they burn enough energy every twenty-four hours to bring eight gallons (about thirty liters) of ice water to a raging boil. Our cells outshine the stars: each ounce of living human tissue burns ten thousand times more energy each day than an ounce of the Sun. A small portion of this activity is under our conscious control-namely the muscle activity we use to move. Some of it we're dimly aware of, like our heartbeat and breathing. But most of this teeming activity goes on completely beneath the surface, in a vast and unseen ocean of cellular processes that keep us alive. We notice only when things go wrong, which, increasingly, they do. Obesity, type 2 diabetes, heart disease, cancer, and nearly all of the other diseases that plague us in the modernized world are, at their core, rooted in the ways our bodies take in and expend energy.

And yet, despite its importance for life and health, metabolism (the way our bodies burn energy) is badly and almost universally misunderstood. How much energy does an average adult burn each day? Every nutrition label in the supermarket will tell you that the standard American diet is 2,000 calories a day-and every label is wrong. Nine-year-olds burn 2,000 calories; for adults, it's closer to 3,000, depending on how much you weigh and how much fat you carry (and for the record, the correct term when we're talking about our daily energy needs is kilocalories, not calories). How many miles do you have to run to burn the energy stored in a single donut? At least three, but again, it depends on how much you weigh. For that matter, where does the fat go when we "burn it off" with exercise? Think it turns into heat? sweat? muscle? Wrong, wrong, wrong. You breathe most of it out as carbon dioxide, and turn a small fraction of it into water (but not necessarily sweat). If you didn't know that already, you're in good company; most doctors don't, either.

No doubt much of our ignorance on the subject of energetics stems from gaps in our education system and the Teflon-like quality with which the human brain repels unused details. When three out of four Americans can't name the three branches of U.S. federal government-an important bit of information drilled into us annually over twelve years of schooling-there may be little hope of people recalling the finer points of the Krebs cycle from high school biology. But our poor understanding is aided and abetted by a host of charlatans and Internet hucksters promoting wrongheaded ideas, usually for personal gain. With a reliably uninformed audience eager to stay healthy, you can sell almost anything no matter how preposterous. Boost your metabolism! they promise. Burn fat with these simple tricks! Avoid these foods to stay thin! scream the glossy magazines, usually without a shred of real evidence or scientific backing.

But the bigger, structural reason energetics is misunderstood is that we have gotten the science of energy expenditure fundamentally wrong. Since the beginning of modern metabolic research around the turn of the twentieth century, we've been taught to think of our bodies as simple engines: we take in "fuel" in the form of food, and burn it off by revving our engine with exercise. Any extra unburned fuel builds up as fat. People who run their engines hotter, burning more fuel each day, are less likely to get fat from accumulating unburnt fuel. If you've already accumulated some unwanted fat, just exercise more to burn it off.

It's an appealing and simple model, a sort of armchair engineer's view of metabolism. And it gets a couple things right: our bodies need food for fuel, and unburned fuel gets stored as fat. But it gets the rest badly wrong. Our bodies don't work like simple fuel-burning machines because they aren't products of engineering, they're products of evolution.

As science is only beginning to fully appreciate, five hundred million years of evolution have made our metabolic engines incredibly dynamic and adaptable. Our bodies have gotten very crafty, able to respond to changes in exercise and diet in ways that make evolutionary sense even if they frustrate our attempts to stay trim and healthy. Consequently, more exercise doesn't necessarily mean more energy burned per day, and burning more energy doesn't protect against getting fat. And yet public health strategies stubbornly cling to the simplistic armchair engineer's view of metabolism, hurting efforts to combat obesity, diabetes, heart disease, cancer, and the other diseases that are most likely to kill us. Without a better understanding of how our bodies burn energy, we grow understandably frustrated when we see our weight-loss plans failing, the bathroom scale refusing to budge despite our earnest efforts at the gym, the latest overhyped metabolic magic letting us down.

This book explores the new, emerging science of human metabolism. As a human biologist interested in our species' evolutionary past as well as our prospects for the future, I've been working on the front lines of metabolic research in humans and other primates for over a decade. Exciting and surprising revelations over the past few years are changing the way we understand the links between energy expenditure, exercise, diet, and disease. In the following pages, we'll examine these new discoveries and their implications for living long and healthy lives.

Much of this new science has come from work with the Hadza and populations like them: small-scale nonindustrial societies still integrated into their local ecology. These cultures have a lot to teach us in the developed world, but it's not the caricatured version of hunter-gatherer life popularized in much of today's Paleo movement. Here, too, my colleagues and I have learned a great deal in the past few years about how diet and daily physical activity keep these populations free of the "diseases of civilization" that bedevil us in modernized, urbanized, industrialized countries. We will visit these groups to see what daily life (and field research) is like in these communities, and what lessons we can bring home. We'll also travel to zoos, rain forests, and archaeological excavations around the world to see how studies of living apes and fossil humans are contributing to our understanding of metabolic health.

But first, we need to get a sense of the immense reach and scale of metabolism in our lives. To truly appreciate the importance of energy expenditure, we have to look beyond the quotidian concerns of health and disease. Like the Earth's tectonic plates, metabolism is the unseen foundation underlying everything, slowly shifting and shaping our lives. The familiar geography of human existence, from our first nine months in the womb to the eighty or so years we might have on this planet, is formed by the metabolic engines burning away inside us. Our big, clever brains and chubby babies are built and powered by metabolic machinery far different from that of our ape kin. As we've come to understand only recently, our evolved metabolism made us the bizarre and wonderful species we are today.