The Road to Sapiens

Burn Notes

Selfish, Lazy Vegetarians

Early primates had dexterous, grasping hands tipped with fingernails instead of claws. One persuasive theory of primate origins is that early primates coevolved with flowering plants, which also got their evolutionary start after the dinosaur extinction. In this scenario, primates adapted to eating fruits of these plants, unintentionally providing them with a means of dispersing their seeds throughout the forest in their poo. Plants with more attractive fruits were dispersed more effectively and had better reproductive success. An evolutionary partnership was formed, with plants selected to produce fleshy, sugary fruits and primates adapted to seek them out and eat them.

Rather than concentrating all their reproductive effort over a few short years, primates had longer reproductive careers that lowered the consequences of encountering a poor season or two. Slower growth also meant more time for learning during development, with more opportunities for innovation and creativity.

Over millions of years, they expanded into a diverse group with two main branches: the lemurs and lorises on one side, and monkeys on the other. Around twenty-one million years ago, a new shoot sprouted from the monkey branch: the apes. For fifteen million years, they proliferated and expanded across Africa, Europe, and Asia. There were dozens of species. Then, for reasons that remain obscure, the bushy ape bough was pruned to just a few branches. By six million years ago we lose nearly all trace of apes in the fossil record. Only a handful of hominoid species persist today: chimpanzees, bonobos, and gorillas in equatorial Africa; orangutans, and several species of gibbons (“lesser apes” in the casual condescension of primate taxonomy) in the rain forests of Southeast Asia. The only other ape lineage to survive was ours, the hominins.

Around seven million years ago in Africa, a population of apes gradually split in two. One of the resulting populations would become the founding stock of the chimpanzee and bonobo lineage. The other population was the founders of the hominins. From the fossil record, we know that the earliest hominins walked on two legs and had stubby, less lethal canine teeth. Otherwise, they were very apelike: chimpanzee-sized bodies and brains; long arms, long fingers, and grasping feet for scrambling high up in the trees. This first chapter of hominin evolution lasted from seven to four million years ago.

The second chapter of the hominin lineage, from about four to two million years ago, is known from a much more complete fossil record. This is the era of the genus Australopithecus, including the famous Lucy and her kin, Australopithecus afarensis. Several species come and go in the fossil record throughout this period, each with their own anatomical distinctions. Still, there are common trends. The grasping foot of earlier hominins like Ardi is gone, morphed into a foot much more like ours with the big toe in line with the others. This, along with changes in the pelvis, suggests these species were more proficient on the ground, burning fewer calories to walk and perhaps venturing a bit farther each day than either living apes or the earliest hominins. Teeth get larger, the enamel much thicker.

• Brain size ticks up a bit in Australopithecus, from just under half a quart to just above (but still only about a third the size of ours).
• In 2015 researchers reported large, rudimentary stone tools from a 3.3-million-year-old site in northern Kenya. We don’t know what these tools were used for, or whether they represent a widespread phenomenon or just a short-lived, early experiment. Regardless, they suggest that at least some Australopithecus species were a bit more clever and resourceful than living apes, who use rudimentary tools to fish for termites or crack nuts but aren’t known to manufacture stone tools.
• And yet, for all the anatomical diversity and hints of creativity, these hominins were most likely apelike in terms of their metabolism. We can be confident in that assessment because, like the living apes today, species from the first two chapters of hominin evolution were essentially vegetarians. They may have hunted small game occasionally or looted termite mounds, as chimpanzees and bonobos do. But a look at their teeth and their tree-climbing adaptations tells us Ardi, Lucy, and the others were getting the vast majority of their calories from plant foods. An apelike, plant-based diet, in turn, tells us these species didn’t need to walk very much to find food. It’s a general rule of ecology that plant-eaters don’t travel very far each day, because plants are plentiful and don’t run away. Living apes rarely cover more than a mile or two in a given day.

But around 2.5 million years ago, hominins started behaving in strange, un-apelike ways. Rather than hunting the occasional monkey or small antelope, they began targeting zebras and other big animals. Stone tools begin to show up all across East Africa in large numbers, and animal fossils from sites in Kenya and Ethiopia show signs of butchery. Meat was no longer a rare delicacy; it was a regular part of the menu. This was the dawn of hunting and gathering, the start of the third and latest chapter of hominin evolution. It marks the early emergence of our genus, Homo. The big dietary innovation that would change our metabolism and our evolutionary destinies wasn’t the food these hominins ate; it was the food they gave away.

Human the Sharer

Men and women both make essential contributions in hunting and gathering societies, but neither is enough on their own. What makes hunting and gathering so successful isn’t the hunting or the gathering, it’s the and. 

In stark contrast, the living apes hardly ever share. Sure, mothers of all ape species will occasionally share some food with their infants or young children. Orangutan mothers in the wild share food with their young kids about one out of every ten meals, usually foods that are difficult to obtain. Sharing among adult apes is even less common.

• Gorillas have never been observed sharing food among adults in the wild. Adult chimpanzees in the Sonso community in the Budongo Forest of Uganda share food about once every two months, and much of what passes for “sharing” is more like tolerated theft. Bonobos share the most, but even they fall well short of the human norm.
• Apes, despite their intricate, lifelong social relationships, live lives of dietary solitude. 

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 diversify 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.

The Metabolic Revolution

To the extent that the recipient is related to you and shares the same genes, their reproductive success is partly yours. But the discounting is steep: even your child shares only half your genes. The costs of acquiring extra food would need to be low, and the payoff to the receiver really high, for sharing to be worth it. It’s easy to understand why no other apes—in fact, hardly any other species at all—have hit upon sharing as a successful strategy.

The earliest hard evidence for sharing comes from cut-marked bones on large animals like zebra. No hominin could eat a zebra by himself, no matter how hungry. And targeting a zebra, dead or alive, would require teamwork, either to hunt it or to push other hungry carnivores off the corpse. Teamwork pays only if there’s an agreement to share the spoils. Perhaps hominin sharing grew from apelike hunting, with some individuals giving more than the limited, grudging scraps we see with chimpanzees.

Or perhaps hominin sharing grew from the sort of fruit-sharing behavior we see among female bonobos at Wamba. A strong case can be made that wild tubers were an important shared food early on.

Positive Feedback and Virtuous Cycles:

• The caloric investment in brains tells us that cognitive sophistication was so critically important for those hominins that it was worth spending precious calories on more brain power. Physical activity must have increased substantially as well. Relying on meat for a good portion of the diet requires a lot of work each day to get food.
• Regardless of how they hunted, hominins had embarked on a high-energy strategy, hunting and gathering, spending lots of calories on intellect and effort with the expectation of even greater, shared returns. Populations grew and ranges expanded. Homo erectus, the first hominin species to go global, appears in East Africa nearly two million years ago and quickly spreads out across the Old World. Within 100,000 years, its range extended from southern Africa, through central Eurasia, and all the way to East Asia, with stone tools recovered in China and fossils as far away as Indonesia.
• Greater brainpower improved our ancestors’ capacity to locate and procure the best fruits, tubers, and game, while simultaneously improving their abilities to plan and scheme together. Greater endurance allowed them to cast a wider net, hunting down prey and exploiting the bounties of a much larger home range. And sharing, tied it all together. With the newfound capacity to acquire more food than they needed, and the social contract to share the surplus, hominins found themselves awash in energy.
• In each generation, the smartest, fittest, and friendliest individuals were the ones who tended to survive the best and reproduce the most. An arms race developed within the hominin lineage, the early, incipient changes snowballing into an ever more grotesque species, with big bulbous heads, delicate faces, and hairless, sweaty bodies.
• In less than two million years, brain size triples in the genus Homo. Stone tool sophistication increases in parallel. By one and a half million years, hominins were making symmetrical, tear-shaped “hand axes.”
• By 500,000 years ago, hominins are controlling fire (debated). Language abilities must have been developing throughout this period as well, though it is fiendishly difficult to track its evolution. By the time Homo sapiens emerges in Africa around 300,000 years ago, trade networks for highly prized raw materials stretch for miles, and natural red pigments are being used for decoration and perhaps symbolic art. By 130,000 years ago, if not earlier, humans along the coast of southern Africa were harvesting shellfish on an annual schedule, paying attention to the seasons and the tides to get the best catch. Our species expands out of Africa and into Eurasia around 120,000 years ago, echoing the earlier waves of Homo erectus, bringing art and innovation wherever we go. By 40,000 years ago, we’re painting lurid murals on cave walls from Bordeaux to Borneo.
• 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 keeps us cool, protecting us from overheating even when exercising in hot conditions. All of this allows us to go farther and faster than any of the other apes.
• Division of labor like that needs a strong commitment to sharing to make it work. We spend the first decade or two of our lives soaking up the shared resources of generous community members, learning to be a functioning, productive adult. Our brains burn so much energy on learning, building, and pruning neural connections as information floods in that our body’s growth slows down during the early elementary school years. In hunting and gathering societies like the Hadza, people don’t become self-sufficient until late in their teenage years.
• Adult hunter-gatherers, both men and women, can easily bring home thousands of extra food kilocalories per day, far more than they need for themselves. This is the extra energy that fuels our faster metabolic engines and greater daily energy expenditures. The extra energy is shared with children, as well as with their moms and other caregivers. In fact, because the energetic burden of reproduction is shared, with moms getting lots of help, mothers in hunter-gatherer societies typically have a kid about every three years, a much faster pace than that of ape mothers who do all the work themselves. It’s the human life history paradox: each kid takes longer to grow up, but we still manage to reproduce faster than our ape relatives. And it’s our commitment to sharing and unique metabolic strategy that make it work.
• Brain size creeps up into the low end of the modern human range by about 700,000 years ago, in a species called Homo heidelbergensis that is found throughout Africa and Eurasia. Their big brains and technological sophistication suggest that long childhoods and super-productive adult foraging was established well before our particular species, Homo sapiens, evolved in Africa. Likewise, their big, expensive brains and hunter-gatherer lifestyles tell us they likely had the same accelerated metabolic rates that we see in humans today, burning more energy than their Australopithecus forebears.
• As our Homo sapiens ancestors expanded throughout Africa and across the globe, they found they weren’t alone. The world was already full of humanlike species: Neanderthals in Europe, Denisovans in central Asia, relict populations of Homo erectus in Asia, an erectus-like species in southern Africa called Homo naledi, and a miniaturized species called Homo floresiensis, nicknamed the Hobbit by paleoanthropologists, in the islands of Indonesia.
• It’s often been argued that we were simply smarter or more creative, but it’s not at all clear that was the case. Neanderthals had brains a bit larger than ours and were making cave art, playing music, and burying their dead long before we showed up. Perhaps we brought new diseases into Eurasia with us as we expanded across the globe, wiping out the Neanderthal and Denisovan populations in the same way that European diseases devastated Native American populations after contact.
• One compelling explanation is that humans persisted because we were friendlier. Richard Wrangham at Harvard University, as well as Brian Hare and Vanessa Woods, his colleagues at Duke, have argued that Homo sapiens became hyper-social through a long process of self-domestication. In this scenario, individuals (particularly men) who tried to get their way through violence and intimidation were ostracized/executed by members of their group. Over time, friendliness and the gene variants that promoted it were favored; mean people didn’t have as many kids. Humans took the sharing behavior of earlier Homo species to the next level. Our communities began to function as hyper-cooperative superorganisms, like beehives or ant colonies. In this scenario, our greater social cohesion was our key advantage over Neanderthals and Denisovans as we spread into Eurasia. When we found ourselves on the same landscapes as Neanderthals and other hominins, our hyper-cooperative strategy won out.

The Downside

An integral part of being hyper-social, sharing apes is our insatiable need to belong to a group. From childhood we are keenly aware of who our tribe is. We pick up the language, the appearance, the signifiers of our group, and we adopt them. We want to belong. This makes a good deal of sense when we consider the evolutionary importance of sharing. Without our group, we’re dead. And we need to know who to be nice to. The social contract demands that we are generous with those in our community.

Just as important is understanding who is not in our group. Sharing with outsiders is an enormous risk. If they aren’t part of our tribe, they might not reciprocate. Even worse, they might be hostile. 

We divide our world into an in-group and an out-group. Penn State and Pitt, Steelers and Patriots, Republicans and Democrats, citizens and immigrants, my race and yours, Tutsi and Hutu, Muslims and Christians… It matters very little whether the groups are defined by something meaningful or completely arbitrary. Members of our group are family for life. Outsiders might even not rate as human.

The other downside of our evolved metabolic strategy is our evolved propensity for metabolic disease. Obesity, type 2 diabetes, and heart disease don’t evoke the same moral horror as genocide, but they kill more people globally each year than violence. These diseases aren’t inevitable.

The faster metabolism and greater daily energy expenditures of the hominin metabolic revolution put our hunter-gatherer ancestors at an increased risk of starvation. Greater daily energy needs mean sharper consequences when food is in short supply. Of course, sharing helps mitigate most of this risk. But there are many potential threats to our energy supply, from prolonged illness wiping out our appetite to unpredictable weather wiping out local plants or game. With a faster metabolism demanding a continuous supply of calories, selection to buffer us against energy shortages led to a second, complementary solution: more fat.

Raise an ape in a zoo, with lots of food and limited exercise, and they get big but they don’t get fat. Their bodies use the extra calories to build more lean tissue, bigger muscles, and other organs. As a result, zoo apes weigh considerably more than they do in the wild, but they stay lean. In contrast, hominins like us evolved to store a lot of those extra calories away as fat, a rainy day fund to survive future food shortages, prolonged illnesses, or other disruptions in our energy supply. Too many of us end up with far more fat than our bodies need, and the negative health consequences that come with it.

Our hominin bodies are also evolved to support, and in fact depend on, the high levels of daily physical activity that were the norm throughout the past two million years of hunting and gathering. We have evolved to require daily exercise. Without it we get sick.

More Data, Less Shouting

In 1999, David Dunning and Justin Kruger, psychologists at Cornell University, had a brilliant insight that seemed to explain why incompetent people are so annoying: their very incompetence blinds them to how incompetent they are. To test this hypothesis, they had dozens of Cornell undergrads take tests in logic, grammar, and the ability to identify humor. Then they asked the students to rate themselves on how well they thought they did. To no one’s surprise the worst performers—those least knowledgeable—routinely rated themselves as experts at what they were doing.

Paleo diet evangelists have distinguished themselves by projecting a hardnosed, steely-eyed view of human nature and evolution. Humans, they assure us, have evolved to eat meat. They push high-fat, low-carb diets that send the body into ketogenesis, arguing that our ancestral diet was all bison and no berries. Paleo proponents, particularly the self-styled carnivores, reject the notion that vegetarian or vegan diets are healthy or natural, dismissing plant-based recommendations or cautions about fat as politically correct pandering or corporate propaganda. In their view, no self-respecting hunter-gatherer would eat a starchy, carb-rich diet, and they sure as hell wouldn’t eat any sugar. Vegans can be just as bad. 

There are three lines of solid evidence that tell us something about the diets our ancestors ate: the archaeological and fossil record, ethnographies of living hunter-gatherers, and functional analyses of the human genome. The details differ and it’s easy to get lost in the weeds, but the overarching message from each is clear: we evolved as opportunistic omnivores. Humans eat whatever’s available, which is almost always a mix of plants and animals (and honey).

Archaeology and the Fossil Record:

• For the first four to five million years of hominin evolution, the different species we see in the fossil record (including the famous Lucy skeleton and her Australopithecus kin) had molars with rounded cusps for eating plant foods. They had long arms and slightly curved fingers as well, which tells us they were climbing into trees often, presumably for fruit and other plant foods. Sure, they probably hunted monkeys or other small game occasionally like chimpanzees and bonobos do today. Insects might have been a regular part of the menu, too, much the same way that chimpanzees target honey and eat ants and termites. But all the evidence from the long early period of hominin evolution points toward a heavily plant-based diet.
• One innovation during this period might have been the exploitation of tubers. Australopithecus species, which are found in the fossil record roughly four to two million years ago have really large molars with thick enamel. Their teeth also preserve scratches that suggest sediment in their food, and the isotopic signature of the enamel is similar to that of wild tubers.
• At around 2.5 million years ago, we see a momentous dietary shift with the origins of hunting and gathering.
  • As the genus Homo began hunting and scavenging more, meat became an ever-larger part of the diet. We see cut marks from stone tools on animal bones starting about 2.5 million years ago, and that continues right up to the present day.
  • At 1.8 million years ago, the Homo erectus population we were excavating at Dmanisi was eating antelope and other animals.
  • By 400,000 years ago, Homo heidelbergensis was regularly taking down wild horses and other big game.
  • By 100,000 years ago, Neanderthals were regularly eating reindeer and mammoth. The cave floors of Neanderthal sites are often thick with the butchered remnants of their meals, and their position as meat-eaters in the food web is evident from the telltale isotopic signatures of their bones (animals that eat other animals have elevated levels of the isotope nitrogen 15, which gets concentrated as you move up the food chain).
  • Our own species was equally adept at hunting, with charred bones from a staggering number of species found in ancient hearths.
• Eating animals means more energy—particularly fat—in each bite 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 proportioned like our vegetarian great ape brethren. These reductions free up about 240 kcal per day, which we spend on bigger brains and other energetically expensive adaptations.
• Some of the newest and most exciting research on hominin diets comes from analyses of food particles trapped in the plaque stuck to the teeth of fossil hominins.
• Neanderthals were the quintessential big game hunters, but they balanced all that meat with carb-rich grains, starchy tubers, sweet fruits, and nuts. Amanda Henry has found similar evidence in the fossilized teeth of members of our own species from this period.
• Archaeological excavations in Jordan have recently uncovered an ancient oven and charred bread remnants dated to over 14,000 years ago, thousands of years before the emergence of agriculture. The bread flour was made from wild cereals.

Ethnography:

• When we plot the proportion of calories from plants and from meat against latitude, two things are immediately obvious. First, there is a lot of variation. Within 50° latitude of the equator (that is, south of Winnipeg, Canada, and north of the Falkland Islands), you can find meat-heavy diets, plant-heavy diets, and everything in between. People eat whatever is available. And this brings us to the second point. In really cold climates, more than 50° from the equator, populations eat a lot of meat. (It’s worth noting, though, that Arctic populations worked to get plant foods wherever they could, even pillaging rodent burrows to steal their stores of wild tubers.) Why do Arctic groups eat a lot of meat? Because plants don’t grow there, at least not very well. We eat what’s around.
• Honey (including the stuff the Hadza get) is just sugar and water, with nearly the same proportions of fructose and glucose as high-fructose corn syrup. In fact, our blood sugar and fat metabolism respond identically to honey, high-fructose corn syrup, and table sugar (sucrose, which is formed from fructose and glucose). If carbs—especially sugar—were particularly bad for you, these high-carb cultures should all have diabetes and heart disease. Instead, they have exceptionally healthy hearts and virtually no cardiometabolic disease. Diets among populations like the Hadza, Tsimane, and Shuar are also low in fat, which makes up less than 20 percent of their calories each day (the typical American diet is 40 percent fat). In fact, outside of the far north, there aren’t any well documented hunter-gatherer groups (like the Hadza) or horticulturalists (like the Tsimane and Shuar) with diets that are high in fat.
• Humans can be healthy eating a broad range of diets, and have done so in the past. There is no single Paleo diet.

Genetics:

• Pastoralists like the Maasai offer a great example of a local adaptation to diet. Milk is a large part of the diet in pastoralist cultures, and much of the energy in milk is provided by lactose, a disaccharide sugar made of glucose and galactose. Like all mammals, we need the enzyme lactase to break lactose into its glucose and galactose during digestion.
  • Infants make lactase in abundance to digest their mother’s milk, but in most people, and in all human populations prior to ten thousand years ago, the gene that makes lactase usually shuts off after childhood. That’s a problem for lactose intolerant adults who eat dairy, causing all sorts of digestive distress as lactose sugars pass unscathed into the large intestine, where they’re digested by gas-producing bacteria. In pastoralist populations, a mutation in the lactase gene arose about seven thousand years ago that causes it to stay active into adulthood.
  • In a pastoralist society, this mutation conferred a big advantage to those who carried it. Those unbloated, dairy eaters had more calories at their disposal. They survived better and had more kids.
  • Remarkably, this happened twice, independently, among early pastoralist groups in East Africa and northern Europe. Today, the descendants of these early pastoralists carry the lactase persistent version of the gene that doesn’t switch off.
• All humans have more copies of the gene that makes salivary amylase (an enzyme in your spit that digests starch) than other apes, resulting in twice the amount of amylase in our spit and reflecting the importance of starchy foods in the hominin diet.
  • But while all humans living today carry plenty of salivary amylase genes to digest starch, populations vary in the number of gene copies. Cultures with deep traditions of eating more carbohydrate tend to carry even more copies of the salivary amylase gene, increasing their levels of salivary amylase even further and improving their ability to digest starch.
• A variant of the NAT2 gene, which produces an enzyme involved in several metabolic pathways, is thought to have become more common in farming cultures in response to decreasing levels of dietary folate. Farming in African and Eurasian cultures, and the resulting shift in the types of fatty acids in the diet, appears to have driven changes in the fatty acid desaturase genes (FADS1 and 2), which are important in lipid metabolism.
• Indigenous groups living in the Atacama Desert of Chile have adapted to the naturally high levels of arsenic in their groundwater, with natural selection favoring a variant of the gene that speeds up clearance of arsenic from the body. 
• Arctic populations have also adapted to eating a lot of meat, but not in a way that most Paleo diet proponents would predict. Work with Inuit populations in Greenland and Canada has shown that the FADS genes have changed in these groups as well, presumably in response to the high fat content (particularly omega-3 fats) in their diet, which has traditionally included a lot of seal and whale blubber.
  • But remarkably, most people in these groups can’t go into ketosis. Instead, they carry a mutant variant of the gene CPT1A that essentially prevents the production of ketones (the “normal” variant of the gene regulates the ketone production in the mitochondria). The non-ketogenic variant was so advantageous among the Inuit and other Arctic cultures that it is ubiquitous among these populations today.

Magical Ingredients: Sugar, Fat, and Testicles

When it comes to your metabolism, there are very few foods shown to have any measurable impact beyond the normal costs of digestion. “Energy-boosting” drinks and supplements, like Dr. Oz’s detox water, are universally bullshit. “Negative calorie” foods that supposedly take more energy to digest than they contain, like celery and leafy greens, are also a myth, though filling up on low-calorie, high-fiber veggies is a good way to lower your daily calorie intake.

Drinking ice water won’t change the amount of energy you burn each day. Even for foods proven to ramp up metabolic rates, the effects are usually modest. The 100 milligrams of caffeine in a cup of coffee will increase your daily energy expenditure by around 20 kilocalories, the equivalent of five M&M’s.

Fat Versus Sugar

At the core of the anti-sugar argument is a plausible mechanism that really could promote obesity, diabetes, and other metabolic disease. Called the carbohydrate-insulin model, it works as follows: eating carbohydrate-rich foods, particularly those high in easily digested sugars, raises your blood glucose levels (blood sugar). In response, the pancreas produces the hormone insulin. Insulin has wide-ranging effects throughout the body, but one important role is to move glucose out of the blood and into cells to store as glycogen or to make ATP. But there’s a limit on how much glycogen our body can hold, and insulin stimulates the conversion of excess glucose into fat and inhibits the pathways that mobilize and burn fatty acids.

Low-carb proponents often complain that mainstream science has ignored the carbohydrate-insulin model, but in fact a number of scientists over the last decade or so have sought to test its predictions.

• The low-carb ketogenic diet was no different than the high-carb baseline diet in promoting fat loss. Daily energy expenditure was slightly higher (57 kcal/d) on the ketogenic diet, but by much less than the carbohydrate-insulin model predicts.
• Large-scale real-world studies examining the effects of low-fat and low-carb diets on weight loss generally find that they are equally good (or bad). The DIETFITS study, which was funded in part by Taubes and the Nutrition Science Initiative, randomly assigned 609 men and women to either a low-carb or low-fat diet. After twelve months, both groups had lost thirteen pounds and 2 percent body fat, on average. Resting energy expenditure dropped in both groups, as we’d expect for people losing weight, but there were no differences between diets (if anything, average resting energy expenditure trended slightly lower in the low-carb group).
• Low-carb diets were tested in a large real-world sample, and they fared no better (and no worse) than the traditional low-fat approach.
• Cancer deaths in the U.S. peaked around 1990, a decade before the decline in sugar consumption. The amount of sugar consumed (including high-fructose corn syrup) peaked around 2000, but the prevalence of overweight, obesity, and diabetes have continued to climb even as people eat less sugar.
• Sugar certainly isn’t healthy (it holds zero vitamins, fiber, and other nutrients, for a start), and sugary foods are easy to overconsume. But there’s little evidence that calories from sugar (including high-fructose corn syrup) are any worse or better for your weight or metabolic health than the calories from fat.

Why Low-Carb Keto Diets (and Others) Succeed

The reason that low-carb diets work is simple: they reduce energy intake and impart negative energy balance. You burn more calories each day than you eat. Low-carb diets may be particularly effective in the short term because they force the body to burn through your glycogen. On a very low-carb diet (usually 20 grams or less of carbohydrate per day), the carbohydrate metabolic pathway shuts down. As that happens, glycogen stores are depleted—the last passengers to take the carbohydrate line into the mitochondria. Unlike fat, glycogen holds water. Because the body stores glycogen in its hydrated form, with three or four parts water per glycogen, burning it also leads to water loss and a rapid reduction in body weight.

Once glycogen stores are depleted, the body relies on the fat metabolic pathway to provide energy. You’ll start burning your stored fat, but only if your daily energy expenditure exceeds your intake.

It’s possible that low-carb diets are helpful for people with type 2 diabetes, since a large dose of carbohydrates can send blood sugar levels soaring to unhealthy levels in people who lack the usual response to insulin.

And it doesn’t seem to matter much whether you restrict the calories at each meal or skip some meals altogether. Intermittent fasting, in which you abstain from eating for large portions of the day, has been widely touted for weight loss. In randomized control trials similar to the Dansinger study, people assigned to intermittent fasting diets are no more successful at losing weight and keeping it off than those assigned to traditional calorie restriction diets.

Hungry Hungry Hypothalamus

Sensory information from your taste buds and guts, along with nutrient contents and hormones circulating in the bloodstream, provides your hypothalamus with a detailed account of the calories coming in and going out. The hypothalamus reacts accordingly, manipulating your hunger and metabolic rate to keep you in energy balance. Normally, this system does an incredibly good job matching intake and expenditure. When we eat enough to meet our needs, we feel full and stop. When we burn our stores of glycogen and fat, we get hungry and eat. If we happen to overeat or starve, our metabolic rate responds appropriately to correct the imbalance.

But the strange and wonderful universe of foods we’ve developed in the industrialized world have exposed a weakness in the system. For far too many of us, the foods we eat overwhelm the usual checks and balances that moderate intake. In short, our modern diets are too delicious. We like food for the same reason we like everything: it triggers the reward system in our brains. Like all animals, from the simplest worms to the most complex primates, we have brains that are evolved to reward behaviors that improve our chances of survival and reproduction. Sex, sugar, social connection… all the essential, universal cravings are built into us from the beginning. We are prewired with neurons waiting to sense “good” things and release reward molecules like dopamine and endocannabinoids in response, to keep us going back for more. The evolutionary logic is simple: organisms with reward systems that are well tuned to their social and physical environments seek out more food and more sex, and tend to have more offspring that inherit their neural reward systems.

Counteracting our desire to eat palatable foods is a set of signals that reduces the reward they bring and makes us feel full. As food is digested and absorbed into the bloodstream, our pancreas releases insulin and our fat cells release the hormone leptin, both of which act in our brain to muffle the reward response to food. Stretch receptors in the stomach and hormonal and neural signals from the digestive tract communicate to our brain that we’re filling up. Protein intake is monitored as well, making us feel fuller the more we eat (in fact, there’s compelling evidence that we monitor the amount of protein we’re eating and don’t feel satisfied until we’ve had enough). All of these satiety signals essentially turn the volume down on the reward signals that food provides and make us feel full, leading us to stop eating, even if the food is delicious.

Modern diets overwhelm our hypothalamus and its ability to balance intake and expenditure in two ways. First, we’re bombarded with far more variety than our hunter-gatherer ancestors ever encountered. This variety sabotages our ability to judge intake by jumping from one set of reward neurons to another. Our brain shuts down the reward response for flavors it’s experiencing but leaves others exposed, a phenomenon called sensory specific satiety.

The other major problem with modern foods is that they are literally designed to be overeaten. Much of the food we buy at the supermarket, the canned and packaged foods, has been engineered beyond anything our ancestors would have recognized. Fiber, protein, and anything else that will make you feel full is removed. Sugar, fat, salt, and other things to tickle your reward system are added. As a result, added sugars and oils are the two leading sources of calories in the American diet today, accounting for fully one-third of the energy we consume. Our evolved reward systems are unprepared for the intensity and breadth of reward signals that these processed foods provide. Our hypothalamus is too slow to shut down our appetite, and we overconsume.

How Does Anyone Avoid the Obesity Trap?

The revolution in genetic research over the past two decades has uncovered over nine hundred gene variants associated with obesity. Just as we’d suspect, nearly all of these genes are active primarily in the brain, clearly pointing to the brain as the epicenter of dysregulation in obesity. The food reward system is complex and expansive, as are the systems that regulate hunger, satiety, and metabolic rate. The myriad pieces of those systems are built by our genes, and those genes vary a bit from person to person. Some genetic variants make our reward and satiety systems more prone to overeating, others make them more resistant.

One obvious strategy to manage our weight and maintain good metabolic health is to build our diet around foods that are filling and nutrient rich without packing in a lot of calories.

Diets that work, including both low-carb and low-fat varieties, are effective because they cut out low-satiety foods and help us feel full on fewer calories. Vegetables, fruits, meat, and fish can all be part of a healthy diet, as long as we avoid foods that prod us to overconsume. Low-carb enthusiasts rightly point out that sugary foods are too easy to overeat: they jangle our reward systems without making us feel full. Sugar-sweetened beverages (sodas and sports drinks), fruit juices, and processed carb-rich foods are dangerous because they carry lots of reward response without any of the fiber that make whole fruits and vegetables so satiating. But fatty foods, particularly processed foods devoid of protein, can cause the same problem.

Getting calorie-rich processed foods out of your house and off your desk at work, and replacing them with protein- or fiber-rich alternatives (like plain nuts, fruit, or fresh veggies), can help reduce the number of calories you consume each day while still feeling full. Cooking for yourself more often can also help, as most restaurants are in the business of making delicious food that’s easy to overeat. We can also try to lower the stress in our lives. Emotional and psychological stress, as well as physical stress like sleep deprivation, can cause dysregulation in our neural reward systems that can lead to overeating. Our brains can also learn to substitute food reward for the emotional and psychological rewards we crave when we’re feeling isolated, scared, or sad.

The distinction between our internal metabolic engine and the external engines that run our world is largely an invention of language, a verbal sleight of hand we’ve played on ourselves. A calorie is a calorie, whether it’s in the food we eat, the sunlight we trap in a solar panel, or the fossilized plants we burn in our cars. Our two engines, internal and external, are deeply interdependent and intertwined in ways we rarely appreciate. We’ve been burning energy externally, harnessing it for our purposes, ever since our hunter-gatherer ancestors got hold of fire, hundreds of thousands of years ago. As we shaped fire, it shaped us. Just as our metabolism today reflects its evolutionary roots, our modern energy economy, and our dependence on it, is an extension of our hunter-gatherer past.

From Focusing Your Energy to Playing with Fire

Simple tools, from the stone choppers at Olduvai to the knives in your kitchen, are useful because they allow us to concentrate our energy. You have the strength to cut a steak with your bare hands, but only if you can focus your power along the edge of a blade. 

Fire was the great technological leap forward. Stone tools, a bow and arrow, and other simple tools allow you to manipulate the way you store, focus, and release your body’s own energy. With fire, our hominin ancestors had access to a completely new engine. Unlike their internal metabolic engines, our hunter-gatherer ancestors could burn these fires as hot as they liked, for as long as they wanted. Most important, they could harness the power of fire in service of essential evolutionary tasks: growth, maintenance, and reproduction. It was a first in the two-billion-year history of life: external energy expenditure to augment your own metabolism.

It seems fire initially had three uses: cooking food, staying warm, and keeping potential predators away. The use of fire for warmth meant our ancestors didn’t have to shiver through the night. Even mild cold can elevate our metabolic rates by 25 percent, or around 16 kcal per hour. Sleeping cold for eight hours could cost a stone-age hunter-gatherer over 100 kcal. With fire to keep warm, those calories could be spent on other important physiological tasks, like growth, reproduction, and repair. Our ancestors might have also slept more soundly knowing that big cats and other species instinctively shied away from fire.

Cooking completely changed our diets and in turn changed our bodies. Wood fires release about 1,600 kcal per pound of fuel. In a simple campfire, most of that energy is lost to the air. The energy that is captured as heat in the food changes its structure and chemistry. Meat becomes easier to chew. Proteins are denatured, making them easier to digest. Starches that are otherwise indigestible are transformed; their carbohydrates accessible in our guts. The effects are largest with root vegetables, which are full of resistant starches that our guts can’t digest: we get double the calories from a cooked potato as we do if we eat one raw. In short, fire supercharged the hominin diet, increasing the amount of energy per bite and decreasing the energy spent on digestion.

Over time, our hunter-gatherer ancestors evolved to rely on fire to prepare our food. Digestive capabilities were reduced, the energy for a big gut and intensive digestion diverted to other tasks. Some of this extra energy seems to have been allocated to reproduction, just as we’d expect from natural selection. The energy boost from cooking may have also contributed to the evolution of larger, more energetically expensive brains.

Raw Foodists eschew cooking for a variety of philosophical reasons or misguided ideas about the “life force” in food. The largest study of their health and physiology comes from a group of over three hundred men and women following raw food diets in Germany. People eating uncooked diets have a hard time maintaining healthy weight, with many below a BMI of 18.5, the threshold for being considered malnourished. Women on raw food diets often stopped ovulating, and the degree of ovarian disruption was directly correlated with the proportion of uncooked food in the diet. Men’s reproductive function was sometimes compromised as well, with some reporting a loss of libido. Without cooked food, humans’ ability to survive and reproduce—the two nonnegotiable measures of evolutionary fitness—are seriously diminished. Even with access to modern foods and high calorie oils.

Fires could be used to change the landscape, burning swaths of forest or scrub to push game and promote new plant growth. Flame also unlocked a universe of chemistry and new materials.

• Paleolithic hunter-gatherers learned to use fire to harden the tips of wooden spears, a practice Hadza women still use today to prepare their digging sticks.
• Our ancestors discovered that fire-treated rocks often made better stone tools.
• Neanderthals and modern humans learned to use kilns to made bitumen, a strong adhesive derived from birch sap, to glue stone axe heads and other blades onto wooden handles.
• As early as thirty thousand years ago, humans were building fires hot enough to fire pottery.
• Around seven thousand years ago, early farming cultures were figuring out how to smelt ore to make copper and other metals.
• By three thousand years ago, they had figured out how to make iron and glass. One hundred generations later, their descendants would be walking around with smartphones in their pockets and sending rocket-powered robots to distant planets.