I'm going to be writing a series of posts based on two books published in the last couple of years that have come into my hands: Burn by Herman Pontzer of Duke University (2021), and Exercised by Daniel E. Lieberman of Harvard University (2020).
Both these books have similar themes: diet and exercise in an evolutionary context. Both the authors are evolutionary anthropologists. Both study different aspects of human evolution.
Lieberman is mainly interested in the anatomical and physiological changes that have taken place in the human body over millions of years of evolution. He is best known for describing the changes that allowed humans to become the animal kingdom's best endurance runners and projectile throwers. His work was featured prominently in Christopher McDougall's best-selling book, Born to Run.
Pontzer is interested in how the body spends energy. As he puts it, evolution basically boils down to a game of converting energy into offspring. To this end, he studies how animals burn energy, and what makes humans’ use of energy special compared to other animals. There's another connection—Lieberman was Pontzer's thesis advisor in graduate school at Harvard.
And, importantly, both authors work with the Hadza hunter-gatherer people of Tanzania. What they've found in the course of their research is essential in understanding the kind of animals we are, how we can stay healthy in our human zoos, and what's going wrong in our modern world. We won't find all the answers to those questions, but hopefully anthropological science can help clear up a number of misconceptions and mistakes on the topic.
Lieberman's book is mostly about exercise, and Pontzer's book is mostly about metabolism. Put them together and you arrive at an interesting perspective on how humans evolved and what steps we might take for better health. In this installment, we'll begin by focusing primarily on Pontzer's work. I will be taking excerpts from both books over the course of these posts, including some others, notably The Obesity Code (2016) by Dr. Jason Fung, a Toronto nephrologist. Fung's book takes on many misconceptions about obesity and its relationship to diet and exercise and cites much of the same research included in the other two books. The letters in the citation will identify which book I am referring to. As always, emphasis is mine unless noted otherwise.
So without further ado, let's dive in.
The Cost of Living
First, let's start with the basics. What exactly is metabolism?
Metabolism is all of the energy our 37 trillion or so cells collectively burn to keep us alive. That energy is taken in through the food we procure from our natural environment—the same as for every living organism.
You may have heard of mitochondria, the "powerhouse" of the cell. Mitochondria are symbiotic bacteria within eukaryotic cells. They take a chemical called ATP and transform it into the energy required to power the cell. This cellular respiration process requires oxygen and produces carbon dioxide, both of which are acquired and expelled by our lungs and transported by our bloodstream. Fats, proteins and carbohydrates are extracted from the food we eat by our digestive system and used as the source for ATP. Pontzer goes into much more detail about exactly how metabolism works, but I will skip over it here. His explanation is far more pleasant and readable than the freshman introductory course in biochemistry that makes people drop out of medical school, so check out the book for more detail.
The amount of energy we burn in a twenty-four hour period is referred to as our daily energy expenditure, or DEE.
The amount of energy is measured using calories. A calorie is officially defined as the amount of energy it takes to raise one gram of water one degree Celsius at sea level (now usually defined as 4.1868 joules). One thousand calories is a kilocalorie, or kcal.
Nutritional science, rather confusingly, decided to capitalize the word 'calorie' and use it as a substitute for kilocalorie, akin to capitalizing meters to represent kilometers. Thus the Calorie in nutrition is actually a kilocalorie from a scientific standpoint, and both terms are used more-or-less interchangeably. When you read the Calorie amounts on food labels, they are technically referring to kilocalories, or one thousand calories.
Because both people and animals come in all different shapes and sizes—not to mention ages and genders—scientists have developed a number of concepts which allow apples-to-apples comparisons to be made regarding energy expenditure.
One is resting metabolic rate, or RMR. This is basically the amount of energy you burn while doing basically nothing but sitting around on the couch watching episodes of Schitt's Creek.
Of course, when you're sitting around you aren't really doing nothing. Your body is holding you up and keeping you awake if nothing else. You might be digesting the slice of pizza you ate while sitting in front of the TV.
So to really get a handle on the minimum amount of energy your metabolism burns there’s a concept called the basal metabolic rate, or BMR. BMR is the amount of energy expended keeping you alive in a near comatose state—in other words, the minimum viable amount of energy you need to stay alive, which is about 10 percent lower than your resting metabolic rate.
It turns out that even in a vegetative state your body still requires a fair bit of energy. It's engaged in wide array of metabolic housekeeping tasks including breathing, maintaining your body temperature, keeping your heart pumping, maintaining your vital organs, brain function, lever function, and kidney function; healing from injuries, fighting off infections, and so on. The vast majority of calories you take in (95 percent) are used for basal metabolism. (OC: 52)
If you are an average adult American male who weighs 180 pounds (82 kilograms), your rate of energy expenditure while resting quietly in a chair is approximately seventy calories per hour. This is your resting metabolic rate (RMR), so named because your resting metabolism comprises all the chemical reactions going on in your body while you are not being physically active.
Based on your RMR, we can calculate that if you do nothing but sit in a chair for the next twenty-four hours, your body will expend about seventeen hundred calories.
Seventeen hundred calories is a lot. Even when you are sitting you are not entirely at rest. Some of that energy is being spend digesting the last meal you ate, regulating your body temperature, and preventing your body from slumping to the floor.
To correct for these expenses, we could measure your energy expenditure in bed just after you woke up from an eight-hour sleep in a dark 70°F room following a twelve-hour fast. That measurement, your basal metabolic rate (BMR), would be roughly 10 percent lower than your your RMR (in our example, 1,530 calories). Your BMR is the energy you use to maintain the most basic processes of your body necessary to stay alive in a nearly coma-like state. (E: 30-31)
Your biggest energy hogs are the brain and the liver. They both use about 20 percent of your metabolism, despite the liver weighing only three pounds and the brain slightly more. Digestion consumes about 10 percent of your calories, or 250 to 300 kcal per day for a typical adult. Skin and bone require relatively small amounts of energy compared to their weight, and fat even less.
By measuring how much blood and oxygen go in and out of organs, physiologists can approximate how much energy different body parts consume. Such measurements indicate that nearly two-thirds of a person's resting metabolism is spent on just three very expensive tissues: brain, liver, and muscle. Your brain and liver each consume about 20 percent of your resting metabolism, and if you are a typically strong human, your muscles expend 16 to 22 percent of your resting metabolism. The remaining 40 percent accounts for everything else including your heart, kidneys, guts, skin and immune system.
If you are sitting while reading this, for every five breaths you take, one pays for your brain, another for your liver, a third for your muscles, and the last two pay for the rest of your body. (E: 36)...Of the more than 20 trillion calories consumed today by human beings on Earth, the majority are devoted to paying for the most basic needs of their bodies at rest (E: 32)
Our digestive apparatus is undersized compared to our great ape relatives, and so is a lot less metabolically expensive. It's hypothesized that the ability to cook food using fire greatly diminished the need for the large guts of chimps and gorillas who digest raw, fibrous foods all day long. Leaf-eating primates have larger guts and smaller brains than fruit-eating primates, for example. The calories not spent on maintaining a large digestive system were instead channeled into brain growth and reproduction:
The inclusion of meat in the diet had big effects throughout the body. Eating animals means more energy—particularly fat—in each bit of food, which meant less food was needed to meet daily energy demands. The need for big molars and other digestive machinery was reduced. Natural selection favored smaller teeth and guts, freeing up energy for other tasks. Today, our digestive tracts are 40 percent smaller, and our livers 10 percent smaller, than they would be if our digestive systems were proportional like our vegetarian great ape brethren. These reductions freed up about 240 kcal per day, which we spend on bigger brains and other energetically expensive adaptations. (B: 196)
Of course, no animal can stay alive permanently in a comatose state; it needs to be physically active in order to capture the energy it needs from its environment to survive and reproduce. Your physical activity level, or PAL, is a multiple of your basal metabolic rate (the energy it takes to keep you alive). This ratio allows comparisons to be made across species as well as between people of different sizes.
So an animal with a PAL ratio of 2.0 expends twice its basal metabolic rate every day. An animal with a ratio of 3.0 spends three times its basal metabolic rate, and so on. It follows that the higher the PAL ratio of an animal, the more active it is.
Your PAL is calculated as the ratio of how much energy you spend in a twenty-four hour period divided by the amount of energy you would use to sustain your body if you had never left your bed. The ratio has the advantage of being unbiased by differences in body size. Theoretically, a big person who is very physically active will have the same PAL as a small person who does the same activities. (E: 19)
Incidentally, most wild mammals have PAL ratios between 2.0 and 4.0, with humans near the bottom of that range. Hadza have PALs of 2.3 for men and 1.8 for women; sedentary Westerners and most other ape species between 1.5 and 1.6: "Stated differently, apes and sedentary industrialized people are unusually inactive compared with most mammals, and hunter-gatherers are in-between." (E: 44)
The amount of energy burned by an animal also depends on its size, as well as its body composition. A big animal like an elephant will obviously burn more calories per day than a dog or cat. Lean muscle tissue burns more calories than fat which is more metabolically inert; thus, the amount of lean muscle tissue relative to fat will also affect the basal metabolic rate. Age is also a factor—growing animals will burn more calories than mature ones. And pregnancy and nursing is the biggest energy hog of all. The total energy cost of a healthy nine-month human pregnancy is 80,000 kcal, and nursing requires up to 500 kcal per day—over the course of a year roughly the same amount of calories as hiking the Appalachian Trail (B: 93)
Activities we voluntarily engage in have MET (metabolic equivalent) values assigned to them, which is a multiple of the amount of energy you spend just hanging out, or 1 MET. So an activity with a 3.5 MET rating like walking uses three and a half times the energy of relaxing; 2.0 twice the amount of energy, and so on. The physical activity ratio (PAR) is a similar concept.
Often energy expenditures are reported in metabolic equivalents, or METs. One MET is defined as 1 kilocalorie per kilogram of body mass per hour, roughly the energy cost of resting.
The Compendium of Physical Activity...is the definitive resource for anyone wondering about the caloric cost of a particular activity. It has MET values for over eight hundred activities, from the everyday ("typing: electric, manual, or computer," 1.3 METs), to the unexpected ("fishing with a spear, standing," 2.3 METs), and from the curiously vague ("sexual activity, general, moderate effort," 1.8 METs) to the disconcertingly specific ("walking backwards, 3.5 mph, uphill, 5 percent grade," 6.0 METs). (B: 69)
In case you were wondering, the exercise activities with the highest MET values are climbing and swimming.
It used to be thought that all one had to do to determine a person's daily energy expenditure was take an average of the PAR values (essentially the same as METs) of their daily activities and multiply it by their basal metabolic rate. More active people would burn more calories, and less active people would burn less calories. This is known as the factorial method:
For example, if a person spent twelve hours sleeping (1.0 PAR) and twelve hours doing laundry and other light housework (2.0 PAR), their average twenty-four hour expenditure would be 1.5 PAR, or one and a half times their basal metabolic rate. Multiply 1.5 by their estimated BMR and you've got their estimated daily energy expenditure.
This approach...assumes that daily energy expenditure is simply BMR plus the costs of physical activity and digestion...[but] it's not that simple—not by a long shot. The bottom line is that your daily activity level has almost no bearing on the number of calories you burn each day. (B: 101-103)
The Doubly Labeled Water Revolution
Fortunately there are better and more accurate ways of measuring energy expenditure than the factorial method.
Since burning energy generates heat, we could measure the heat given off by an animal to get an estimate of its metabolism (called direct calorimetry). Pontzer informs us of an experiment carried out by two of the fathers of energetics—Antoine Lavoisier and Pierre-Simon Laplace. They placed a guinea pig in a container and immersed it in a bucket filled with ice. By measuring the amount of melted water drained from the bucket they could approximate the amount of heat given off by the animal. Lavoisier and Laplace compared this to the amount of CO₂ produced by the guinea pig and found that ratio to be exactly the same as the ratio of heat and CO₂ generated by burning a candle. This led them to conclude that, "la respiracion est donc une combustion": that breathing is combustion. That's why when we talk about calories burned, that's literally what is happening (hence the title of Pontzer's book).
Pontzer informs us that, "[b]urning carbs, fats and protein consumes oxygen and produces CO₂" (B: 68) Therefore, you can also measure the amount of carbon dioxide produced by an animal's respiration to determine the amount of energy burned by its metabolism. Since carbon dioxide is much easier to measure accurately than heat, this is the technique that's normally used today (called indirect calorimetry).
Over the years scientists strapped oxygen masks onto dutiful volunteers to determine the amount of energy they expended doing various tasks. That's how they came up with the MET values for the activities listed above. Of course, that raises the question of the exactly how they determined the MET values of people having sex while wearing oxygen masks, and what exactly "moderate effort" means.
This method still suffered from a number of problems, however. It was easy to measure metabolism in a laboratory where people could be strapped into a mask and put on a treadmill, but not when they were going about their daily activities, especially in remote areas. And it was difficult to determine the metabolism of non-human animals—especially wild animals—since they do not like to go around wearing oxygen masks as a general rule.
Then came a revolution.
A scientist by the name of Nathan Lifson devised another method for measuring the amount of CO₂ produced without using a mask or equipment.
The body is about 65 percent water. Water is comprised of hydrogen and oxygen atoms. Water enters the body through food and drink and leaves it via urine, feces, the vapor in our breath, sweat, and tears (for me, mostly tears). Lifson's critical insight was that hydrogen leaves the body only as water, whereas oxygen leaves the body as both water and carbon dioxide.
This meant that you could measure the amount of CO₂ produced by tracking the relative ratios of hydrogen and oxygen atoms leaving the body. To do this, you needed to give your subject something called doubly labeled water. Doubly labeled water contains a known quantity of (rare) heavy hydrogen and oxygen atoms, which allows the ratio of hydrogen and oxygen atoms to be measured as the water leaves the body. Carbon dioxide, as we saw before, is a by-product of cellular respiration. By measuring CO₂ production using this method you could determine daily energy expenditure. All you needed was to collect blood or urine samples and then process them in a lab.
This sounds like a creepy magic trick, but by measuring the rate at which these heavy atoms become less abundant in urine, we can calculate the rates at which both the hydrogen and the oxygen atoms leave the body from sweating, urinating, and breathing. Because hydrogen exits the body only in water but oxygen leaves in both water and carbon dioxide, the difference in the concentration of these two atoms in urine allows us to compute exactly how much carbon dioxide someone produced from breathing, hence how much energy he or she used.
If you weigh 180 pounds, your DEE is probably about twenty-seven hundred calories per day. Because we already learned your RMR is about seventeen hundred calories a day, that means nearly two-thirds (63 percent) of the energy you expend each day is spent just on your resting metabolism. Who would have thought that being a couch potato is so expensive? (E: 31-32)
With this technique you could accurately measure the metabolisms of various animals. You could also get accurate measurements for people in remote and inaccessible areas without using special equipment. All you have to do is give them doubly labeled water, collect some urine samples, and analyze them in a lab. This was a major breakthrough for the study of metabolism.
Despite its advantages, for a long time this technique wasn't used very much because it turns out that doubly-labeled water is very expensive to produce. Plus the bigger the animal, the more doubly labelled water is needed to measure its metabolism, so it was usually confined to small animals like mice or rabbits, limiting its applicability.
But over time doubly labelled water became much cheaper to produce, which meant that it became economical to use this method on larger animals. It was also now economical to do studies on humans, including humans living in remote areas like hunter-gatherers. You could now determine with a high degree of accuracy exactly how much energy people and animals were spending each and every day—no more guesswork.
We used to think we understood why people differed in their daily energy expenditure. Surely, we thought, one could simply add up the kilocalories spent on physical activity, organ function, growth, thermoregulation, digestion, and the rest, and calculate a person's daily energy expenditure. And it's true, of course, that daily expenditure must include all those costs. But the doubly labeled water revolution has provided a wake-up call, a surprising reality check.
Rather than simply adding up like the grocery bill at checkout, all the pieces of daily energy expenditure—activity immune function, growth, and the rest—interact and affect each other in dynamic and complex ways. Daily energy expenditure isn't just the sum of its parts. (B: 112)
This revelation forms the core of Pontzer's research described in the book.
Life in the Fast Lane
Since Dr. Pontzer's field is evolutionary anthropology, it made sense for him to study our closest animal relatives—apes and other primate species—to find out how they were burning energy.
It used to be thought that metabolism was basically the same across all warm-blooded species, besides some oddballs like marsupials. After all, since metabolism is a cellular process, the amount of energy burned per kilogram of lean body mass should be same no matter the animal. This meant that that the amount of energy an animal burned was simply a function of its BMR plus its activity level—more active animals burned more calories; less active animals burned less calories.
There were several signs that this wasn't the whole story. Some animals mature early, leave behind a lot of offspring, and don't live very long past their reproductive years. Other animals mature very slowly, produce fewer offspring, and live for quite a long time. In other words, animals have either "fast" or "slow" life histories. Perhaps this was related to how fast their metabolisms were burning energy?
The differences between how fast or slow an animal's life history is and their metabolism is still not completely understood. There's more at work in growth and reproduction rates than simply metabolism. Still, it's generally true that animals with slower metabolisms tend to live longer and devote less energy to reproduction. It's also known that the cells of smaller animals burn energy faster than the cells of larger animals for some reason, and they tend to reproduce faster. And every organism expends about a billion heartbeats regardless of its size. (B: 97)
Pontzer started by looking at orangutans. Orangutans are among the slowest-maturing and longest-lived mammals. They don't reproduce until they are at least fifteen years old, and they have babies only once every seven to nine years—the longest interval of any animal.
It turned out that orangutans do indeed have a very slow metabolism: they were burning only around a third of the energy that would be expected for an animal their size. Ponzer tells us that a 250 pound fully-grown adult male orangutan burns about as much energy as a nine-year-old, 65 pound human boy. A 120 pound female burns even less energy—about 30 percent fewer calories than a human of similar size. The only species with lower expenditures for their body size were three-toed sloths and pandas. He posits that this was an adaptation to periodic food shortages in their Indonesian tropical rain forest habitat.
But it's not just orangutans. Pontzer soon discovered that all primates have far slower metabolisms than other mammals! Primates were typically burning calories at half the rate that would be expected for other placental mammals their size. This accounts for the relatively long lifespans that primates—including humans—enjoy. And these slow metabolisms are hardwired into primates across the spectrum, from tiny lemurs to massive mountain gorillas. Compared to primates, most other animals live fast and die young.
Primates burn only half as many calories as other placental mammals. To put that in human terms, consider that the normal daily energy expenditure for human adults is between 2,500 and 3,000 kilocalories per day...Our analyses showed that a typical placental mammal our size burns well over 5,000 kilocalories per day. That's the daily average energy expenditure of Olympic athletes at the peak of training!
But it's not that these other mammals are incredibly active; they walk a couple of miles per day at most and spend much of their time eating and resting, Their bodies simply burn energy faster, much faster, than our diminished primate metabolism can sustain. (B: 21)
The fact that this trait is shared collectively by all primate species means that it must have evolved somewhere around the time when the primate branch first emerged on the tree of life sixty-five million years ago. Nobody is quite sure why primates evolved such slow metabolisms, but Pontzer speculates that, since prey animals tend to have faster metabolisms, early primates might have had few predators since they spent most of their time in the tree canopy. Also, food sources like leaves and fruits were fairly abundant and easy to find in the canopy, so early primates didn't have to burn a whole lot of energy acquiring food.
And while our remote ancestors—known as australopithecenes (“eastern ape”)—started out similarly to other primates as arboreal vegetarians, our species took a different path. We came down from the canopy and branched off into a new species known as Homo (Latin for “man”). The Homo genus embarked on a high-energy, high metabolism, high risk strategy compared to the other apes due to the shrinking of forested areas and the expansion of grasslands during the Pleistocene era starting around 2.5 million years ago. In the process, human anatomy and physiology were radically transformed.
But all of it was only possible thanks to another innovation in the Homo genus: hunting and gathering.
Sharing is Caring
Chimpanzees—and other primates—simply gather. Their slower metabolisms and general lethargy are enabled by the fact that plants—their main food source—don't run away. Since chimpanzees and gorillas live in what's been described as a "giant salad bowl" they don't have to expend a whole lot of energy to survive: "Chimpanzees typically devote about half their waking hours to filling their stomachs with highly fibrous food, and for much of the rest of the day they rest, digest, groom each other, and take long naps. On an average day, they climb only about a hundred meters and walk just two or three miles." (E: 28)
Chimpanzees do hunt on occasion, but they hunt opportunistically. But hunting is too risky as a primary strategy for getting calories, since animals—unlike plants—can run away. That means that they would go hungry if they failed. Since getting enough calories is the name of the game in natural selection, this strategy would result in extinction. Consequently, meat forms only a small and relatively insignificant portion of their diet—dessert, not the main course.
Chimpanzees also, as a rule, don't share their food. Sometimes they will give food to someone who begs hard enough for it, especially close relations; but not voluntarily, and begrudgingly. Some bonobos have been observed sharing fruit in the wild, but this is still atypical behavior and not standard practice. Chimps and bonobos typically forage alone and generally eat only what they themselves can procure from the forest, despite being highly social animals. They're more likely to share if there's a surplus, but even then they will try to keep as much as possible for themselves:
Apes, despite their intricate, lifelong social relationships, live lives of dietary solitude. When it comes to food, they are on their own. Consequently, they are compelled to go for the sure thing, to make certain they get enough food each day to keep from starving. And there's little upside to pursuing big game or gathering more than they need; anything they can't shove in their mouths right now will go to waste or be pilfered by beggars, who are unlikely to ever return the favor. (B: 132)
Starting with Homo erectus around two million years ago, we start to see large animals being regularly butchered for meat with stone tools. Killing large animals like zebras must have required smaller, weaker humans to band together in order to bring them down. Hunting—especially big game—is a high-risk strategy. Often times you might come home empty handed, which would mean starvation if that was your only food source. And, importantly, time spent hunting is time you can't spend gathering.
However, human ancestors came up with a new strategy: hunting and gathering. Hunting and gathering is a uniquely human behavior, because half of the population (usually men) goes after one type of food, while the other half of the population (usually women and children) goes after another type. But hunting and gathering as a means of procuring food is only possible thanks to another hominid innovation: sharing.
Sharing is absolutely essential for hunting and gathering to be viable strategy. That's why Pontzer argues that hunting and gathering should really be referred to as "hunting-gathering-sharing". Sharing is means of pooling risk. If some hunting parties come back empty handed, other hunting parties might be successful. And if no hunters manage to bag anything that day, the band still has plenty of plant foods to eat so that they don't starve. But whether you've gotten your hands on meat or plants, you come back and share it with the entire group whether they are related to you or not! Sharing is therefore absolutely essential to the evolution of hominids.
Humans are social foragers. We routinely bring home more than we need, with the intention of giving it away to our community. That means we have one another as a safety net; if someone comes home empty-handed, they won't go hungry. This allows us to diversity and take risks, to develop complementary foraging strategies—hunting and gathering—that maximize the potential for big gains while limiting the consequences of failure.
Some group members hunt, and will occasionally bring home a big game bounty of fat and protein. Others gather, providing a stable, dependable source of food to get through the days when the hunters are unlucky. It's an incredibly flexible, adaptable, and successful strategy. And the foundation of it all is the inviolable, ironclad, unspoken understanding that we will share.
Sharing is the glue that binds hunter-gatherer communities together and provides the fuel that males them run. It radically changed the hominin metabolic strategy. Sharing meant more food, more calories, more energy for growth, reproduction, brains, activity...all of it...Despite the long odds against it...this strange act of foraging for others had profound consequences for hominin evolution. Sharing meant more energy for life's essential tasks. Survival and reproduction, the currencies of natural selection, improved. The sharing hominins and their kin outcompeted their less generous neighbors. (B: 132-135)
Pontzer points out that what makes humans special in the animal kingdom has often been described as hunting or tool use. But both of these behaviors have been observed even in chimpanzees. What really makes humans unique is the fact that we freely share our food. Sharing is what makes us human. Sharing and cooking food is a still a cornerstone of human sociability to this day (along with dancing and storytelling). Pontzer informs us that the Hadza share absolutely everything without hesitation; all you have to say is "za"—give—no "please" or "thank-you" required.
Sharing behavior also led to the evolution of tribes—the social groups humans from organically to ensure their survival. Tribalism facilitated sharing, but it also has a downside—it means we have a tendency to "otherize" people who fall outside our tribe, that is, to see them as less than human. This is an ugly tendency for which we need to be constantly vigilant. Sebastian Junger, in his book Tribe, notes that food sharing is essential to belonging to a tribe:
Two of the behaviors that set early humans apart were the systemic sharing of food and altruistic group defense. Other primates did very little of either but, increasingly, hominids did, and those behaviors helped set them on an evolutionary path that produced the modern world. The earliest and most basic definition of community—of tribe—would be the group of people that you would both help feed and help defend. A society that doesn't offer its members the chance to act selflessly in these ways isn't a society in any tribal sense of the word. It's just a political entity that, lacking enemies, will probably fall apart on its own.
Sebastian Junger, Tribe; pp. 109-110
Hunting and gathering and sharing—and the extra calories that resulted—shifted us into a permanent high-energy lifestyle. All humans, even sedentary Westerners, are "gas guzzlers" compared with other primates. Humans burn 20 percent more energy on average every day than chimps and bonobos, 40 percent more than gorillas, and 60 percent more than orangutans. (B: 25) The Hadza burn twice as much energy per day per pound of fat-free body mass as chimpanzees, and Hadza women will spend 115,000 more calories on average over the course of a year than a similarly sized chimpanzee female—enough energy to run from New York to Miami. (E: 43-44)
Our physique changed, as did our physiology. We run and walk for our dinner instead of climb for it. Our distinctive upright gait is more energetically efficient than other primates—chimps spend twice the amount of energy as we do walking the same distance, and we instinctively switch from walking to running at almost the exact moment when it becomes more energy efficient to do so. (E: 44) This allowed us to cover far more distance per day than other animals in the food quest.
In fact, Daniel Lieberman has argued that many of these anatomical changes emerged to allow us to run down prey animals in the hot, dry conditions of the African savanna—a technique known as persistence hunting. We became hot, sweaty long-distance runners, with long legs and ramped-up metabolisms to match.
…humans today are far the best endurance athletes among all of the living apes. Our VO2 max, a common measure of peak aerobic power, is at least four times that of chimpanzees. We carry more muscle in our legs (though less in our arms) than other apes, and we have a much greater proportion of fatigue-resistant "slow twitch" muscles. Our blood holds more hemoglobin to ferry oxygen to working muscle. And our naked, sweaty skin (by far the sweatiest on the planet) keeps us cool, protecting us from overheating even when exercising in hot conditions. (B: 139)
All of this allows us to go farther and faster than any of the other apes. Chimpanzees travel less than two miles per day, on average. Other apes are even lazier. Humans, particularly hunter-gatherers like the Hadza, walk five times farther each day. People run marathons for fun. We are built for intense, all-day activity.
Many of the anatomical traits that help make us such prodigious walkers and runners, like our long legs, the springy arches in our feet, and our short toes, are present in early Homo, suggesting our endurance abilities were present fairly early in our genus and have been honed by evolution as part of the hunting and gathering strategy over the past two million years. (B: 139-140)
Perhaps the biggest physiological change was our ballooning brain. Meat and starchy tubers helped to fuel explosive brain growth. Hominid brain size tripled in the last two million years. By around 700,000 years ago, the human brain reached the low end of its modern range in a species called Homo heidelbergensis.
As we saw earlier, brain tissue is very metabolically expensive. For this reason, it's highly unusual for natural selection to channel extra calories into larger brains unless it confers immediate reproductive benefits. Yet this exactly what happened in our lineage. Increased brain power, in turn, made us more effective foragers, allowing a single individual to bring home thousands more calories per day than he or she needs. The slow life history inherited from our primate ancestors meant that children could mature slowly and learn as they grow, helped along by their parents and even by their grandparents who live far beyond their reproductive years. Raising children, like gathering food, was also a collective endeavor.
The key thing is, the behavior had to emerge first, then the anatomical changes. Evolution can't "look ahead" and see what will potentially work; you have to continually survive and reproduce along the way for an evolutionary strategy to succeed. Hunting and gathering and sharing was the "killer app" which propelled our lineage to become human. We swapped strength for endurance, brawn for brains, and fighting for sharing.
No one knows for sure why we are the only hominid species that survived. One hypothesis is that we became more social than other hominids through a gradual process of self-domestication. Another is that the leaner, more muscular bodies of other species like Neanderthals were too metabolically expensive to maintain in a world where the climate was rapidly changing and large prey animals were quickly disappearing. Getting sufficient calories proved too difficult a challenge in this new environment, it’s thought, and Homo sapiens' scrawnier, fatter bodies and omnivorous diets were better suited to the new reality. Perhaps we reproduced faster or more successfully. Or perhaps it was simply random chance where something like a virus wiped out everyone else. Whatever the reason, today the only living remnants of those other hominid species exist in our genome through interbreeding.
We Evolved to be Fat and Lazy
Our active, high-energy lifestyles also led to our distinction as the fattest ape species. Because our metabolism burns so much faster, and requires so much more energy, our bodies evolved the capacity to store larger amounts of reserve energy for times of scarcity in the form of fat. Humans carry about twice as much body fat (about 23 to 41 percent—more if you live in Wisconsin) than other apes (about 9 to 23 percent). Orangutans are the chubbiest ape species; chimps and bonobos the leanest. (B: 25). They simply don't need to store as much fat because their metabolisms don't use as much energy: "With a faster metabolism demanding a continuous supply of calories, selection to buffer us against energy shortages led to a second, complimentary solution: more fat." (B: 147)
That's why apes in zoos get big, but they don't get fat. They invest extra calories in more muscle tissue, but they stay relatively lean, even though they sit around scratching themselves and munching on shoots and leaves for over ten hours a day in both zoos and in the wild. Humans, on the other hand, tend to put away excess calories as additional fat, with all of the negative health consequences that entails.
Our legacy as hunters and gatherers means that we evolved to require large amounts of physical activity on a daily basis, which was absolutely essential for our survival. It also means that we are vulnerable to a whole raft of metabolic diseases if we don't get enough exercise. Obesity, type-2 diabetes, heart disease, depression, and a whole host of other diseases we're only now beginning to identify (like dementia), happen to us which don't happen to other primates who are inactive. Unlike chimps and gorillas, we cannot simply lounge around in the forest all day stuffing our faces and still remain lean and healthy.
For us, a life of apelike idleness is a recipe for disaster. Sedentary humans are far more likely to develop cardiometabolic disease, including heart disease and diabetes. Yet, despite their laziness, apes don't get sick. Diabetes is exceptionally rare among apes, even in zoos. They have naturally high cholesterol levels, but their arteries don't clog.
The primary case of death in captive apes is cardiomyopathy, a pathology of the heart muscle, the causes of which aren't entirely known. But they seem to by immune to the kind of heart disease that fells humans. Apes don't develop hardened vessels or have heart attacks from blocked coronary arteries. They stay lean too...chimpanzees and bonobos in zoos carry less than 10 percent body fat… (B: 235-236)
Though hunter-gatherers like the Hadza don't work incredibly hard and the spend many hours a day being physically inactive, apes make them seem like workaholics. And because we evolved from apelike ancestors who largely resembled chimpanzees and gorillas, that suggests it is evolutionarily normal humans who are unusual in terms of how much they work and rest. (E: 29)
The bottom line is this: humans evolved to absolutely require large amounts of physical activity every day. Without it we get sick. Paradoxically, we are the animals least suited to living in a zoo, especially the depressing, unnatural, maladapted zoos we've managed to create for ourselves (and which for some reason we seem to be unable to change).
What we didn't evolve to do, however, is exercise.
That's because by definition, exercise is a voluntary expenditure of energy without any sort of end goal or purpose. None of our hunter-gatherer or farming ancestors ever ran or walked several miles a day just for health reasons. There's no reason for evolution to build in an urge to expend energy over and above what is required for survival. Hunter-gatherers were active because they had to be.
That’s the main point of Daniel Lieberman's book, Exercised. He argues that it's a myth that we should naturally want to work out. In fact, such behavior is profoundly unnatural. As he told an interviewer, “When people don’t exercise, we label them as lazy, but they are actually doing what we evolved to do - which is to avoid unnecessary physical activity.” 1
In fact, no animal evolved a tendency to voluntarily expend calories over and above what is strictly necessary to survive. Spending hard-to-get calories on frivolous activities not tied to increased reproductive success would be an evolutionary dead-end, and humans are no exception. That's why you always see animals lounging around in zoos and nature documentaries whenever they're not doing something absolutely necessary for their survival (except, perhaps, playing). Just like every other animal, we evolved to be lazy. It's just that regular physical exertion was a baseline requirement for survival for almost everyone on the planet until the last hundred years or so. Today, in our modern industrialized world, we need hardly lift a finger like those chair people in WALL-E.
In fact, Lieberman informs us that the very word exercise didn't originally refer to something desirable at all—rather it was a state of being anxious or agitated about something. That's why he chose it for the title of his book: "We are exercised about exercise."
Stated simply, we evolved to be as inactive as possible. Or to more precise, our bodies were selected to spend enough but not too much energy on nonreproductive functions including physical activity...Decades of research show that hunter-gatherers manage to avoid starvation and maintain about the same weight throughout the year. That doesn't mean hunter-gatherers don't face tough times. They do. In fact, they frequently complain of being hungry. But one of the ways hunter-gatherers survive is by not foolishly squandering scarce calories on unnecessary activity. (E: 40-41, italics in original)
The bottom line is that humans evolved to acquire and expend much more energy than chimpanzees...by walking long distances, digging, sometimes running, processing our food, and sharing, we spend a lot more energy being active every day than chimpanzees, but that effort yields more calories that enable us not only to be more physically active but also to reproduce at about twice the rate. The extra energy also allows us to have bigger brains, store more fat on our bodies, and do other good things. But there is a cost. The more calories we need, the more we are vulnerable to not having enough. Although the hunter-gatherer strategy is a boon to our reproductive success, it selects against wasting calories on discretionary physical activity.
Of course, this logic applies to all animals. Whether you are a human, ape, dog, or jellyfish, natural selection will select against activities that waste energy at a cost to reproductive success. In this regard, all animals should be as lazy as possible. However, the evidence suggests that humans are more averse to needless physical activity than many other species because we evolved an unusually expensive way of increasing our reproductive success from an unusually low-energy-budget ancestor. When your expenses are high, every penny saved is valuable. (E: 45-46)
This puts those of us in the modern industrialized world in a quandary. We have designed a world in which physical activity is by-and-large unnecessary or voluntary. Yet our evolved physiology absolutely requires us to engage in large amounts of physical activity every single day in order to remain healthy. At the same time, we have evolved behavioral tendencies that make us reluctant to engage in unnecessary physical activity to an even greater degree than most other animals. On top of it all, our bodies evolved the tendency to store a higher percentage of body fat than other primates. It's like some sort of cruel cosmic joke.
Energy Mechanics
It used to be thought that the huge amounts of exercise hunter-gatherers engaged in every day kept them lean. By staying so lean they avoided the metabolic diseases that plague the modern world. Their highly active lifestyles meant that they were burning a massive amount of calories every single day, we presumed, and even getting enough calories to fuel such an athletic lifestyle must have been a challenge.
In this view, the human body was effectively seen as a steam engine: calories in equals calories out. Because "calories out" was so high for most of our existence, the story went, we never had to worry about an obesity problem. But in our modern industrialized countries where most people have desk jobs, drive cars, and sit around watching Ozark, our lack of physical activity is the main reason why we become obese, and obesity is the root cause of all these chronic metabolic conditions that are affecting us in greater and greater numbers at younger and younger ages. The solution to this is, we are told, is "willpower." All we have to do is to simply, "eat less and move more."
But Pontzer's research completely upended this overly-simplistic narrative. It turns out exercise may have been what was keeping hunter-gatherers healthy, but it wasn't what was keeping them thin.
That's what we'll be talking about in our next installment.