CHAPTER 4
How Humans Evolved to Be the Nicest, Fittest, and Fattest Apes
Early on a cool, dewy July morning I awoke in my little tent and gingerly worked my way out of my sleeping bag and through the damp zippered door. The confines of the yellow nylon dome opened out to a panoramic view of dark forested hills and pale green pastures. My tent and several others were scattered about the scruffy lawn beside the dig house, a two-story dormitory and kitchen for our crowded team of archaeologists, geologists, and paleoanthropologists. We were midway through our annual field season, excavating stone tools and fossils of Homo erectus.
Somewhere far below me, too distant to hear, the Pinasaouri River rushed past the ruins of old bathhouses that once welcomed travelers and traders along the Ancient Silk Road. Across the valley, the collapsed stone tombs of Mongol invaders dotted the fields on the distant hillside. Above them, perched on a high point along the ridge, sat the remains of a proud medieval city. And in the soil beneath those ruins were the 1.8-million-year-old fossilized bones of ancient near-humans. The entire landscape was a layer-cake monument to our impermanence. Wave upon ceaseless wave of ambition and folly.
Something began to rise within me, a dark and churning tide.
I stumbled toward the edge of the small clearing and puked all over the bushes. The hand of some vengeful god gripped my body and squeezed violently to exorcise the evil within. Hands on knees, eyes watering, I spewed hot, frothy garbage. The initial eruption was followed by diminishing aftershocks of convulsion. Retching heaves wracked my body; it felt like my eyeballs might pop from their orbits and dangle helplessly by the optic nerves. And then finally, mercifully, it was done. Wrung out and crumpled like an old tube of toothpaste, I wiped my mouth with the back of my hand and slowly straightened myself up.
There is a deep and perfect calm that follows such abominations, a brief respite from the headache, nausea, and sense of impending doom of a truly horrific hangover. In those moments of clarity, I considered my position. The unlikely circumstances and accidents of history that had made this magical place. My incredible good fortune in somehow finding myself here. Ah, but I had been ungrateful. Spoiled. It hadn’t been enough just to be here. I had overindulged, letting the first glass of wine at last night’s dinner turn into a drunken party under the stars. I wasn’t alone. I turned to walk toward the long communal table on the dig house porch and saw a few of my fellow revelers, bleary-eyed, begin to bravely face their tea and bread.
As I slowly walked over to join them, I had a vague notion of turning things around, of using this moment to grow and mature and leave such self-destructive foolishness behind for good. With battery acid breath I mouthed the silent prayer of idiots everywhere: Next time will be different. This wasn’t my first rodeo. I knew the long odds against real reform, but as I stood there in the calm eye of the hurricane I was optimistic. We were a clever crew, after all—budding scientists pursuing our PhDs at prestigious academic institutions from around the world. We had the intelligence and temperament to earn competitive spots in top-tier graduate programs and to work our way here, one of the most exciting fossil sites on the planet. Surely we had enough good sense to be careful and moderate, to ensure our own self-preservation. True, none of us had demonstrated any talent for self-restraint, but c’mon. Surely we could enjoy the fruits of our evolved human intelligence and collective effort without letting curiosity and hedonism destroy us . . .
The thought disintegrated like a cloud, unfinished. It was time to try to have some breakfast. There were fossils to be discovered, and they weren’t going to excavate themselves. I slumped down onto the long bench next to the others at the table, grabbed a slice of bread with unsure hands, and slathered on some butter and honey. I took a sip of tea. Already I could sense my hangover returning, like the distant hoofbeats of a Mongol horde.
It was my annual pilgrimage to the Lower Paleolithic site of Dmanisi in the Republic of Georgia. Every summer during grad school I’d take a break from treadmills and metabolism and make the trek out to the tiny farming village of Patara Dmanisi in the foothills of the Caucasus Mountains. Month-long detours from thesis work aren’t exactly recommended for success in grad school, but it was too interesting and too fun to miss. What I didn’t appreciate at the time was how connected the site was to my research in human energetics, and how it crystallized a critical period in our metabolic evolution. Our departure from the world of apes, the first evolutionary steps toward something much more human, are captured here. And the secret sauce that made it all work were changes in the way we acquired food and burned calories—changes we’re still grappling with today.
An Unlikely Place
Dmanisi is an unassuming location for one of the most important sites in human evolution. All of the other hominin fossil sites from this period, around two million years ago, are found in the dry, gravelly badlands of East and Southern Africa, familiar to anyone who’s ever thumbed through a National Geographic: Olduvai Gorge, the Great Rift Valley, the cave sites of South Africa. Dmanisi, in stark contrast, is green and leafy, and Georgia, a gorgeous and proud country with a rich history, is distant and obscure to most people outside the region. Yet its geography is precisely what makes Dmanisi so important.
The human lineage split from that of chimpanzees and bonobos roughly seven million years ago (Figure 4.1). But for the first five million years, our ancestors remained in Africa, restricted to a particular set of habitats where their apelike strategies were effective. Then around two million years ago, we hopped the ecological fence. Hominins (species on the human branch of the ape family tree) became smart and adaptable enough to thrive anywhere. Populations grew and spread across Africa and then into Eurasia, stretching from South Africa to Morocco to Indonesia. This was the radical break from an apelike past and into something much more human. Dmanisi is the earliest grainy snapshot we have from this critical period. At 1.8 million years old, it’s the earliest hominin fossil site outside of Africa. The stones and bones beneath the soil there capture the first evolutionary inklings of what makes us human. And the key evolutionary advantage that made these hominins so successful, fueling their global expansion, was a change in the way they acquired and burned energy.
My first trip to Dmanisi began as a conversation with Ofer Bar-Yosef, a gray-haired and inscrutable Harvard professor of Paleolithic archaeology, most famous for excavating Neanderthal burials in the Mideast. As a fresh-faced and eager first-year PhD student, I was told I had to track him down if I wanted to do archaeological fieldwork the following summer. I found Ofer one afternoon leaving his office in the Peabody Museum of Archaeology and Ethnology. He invited me to join as he walked to Harvard Square to pick up some photos (these were still the days of print film). “Vee’ll valk and talk. No one vastes any time,” he offered in his Israeli accent. Of course I agreed.
On the walk, he explained he had connections at two sites where I’d be welcome to do fieldwork: a Neanderthal cave site in the South of France, and Dmanisi. The French site was a bigger operation, more organized, and much easier to get to. “And zee food is better in France,” he added. But the work in Georgia sounded more interesting. Two fossil skulls had been discovered there only a year before, and the site was shaking things up in the field of human evolution. After a bit of discussion, Ofer agreed to make the necessary arrangements. I asked if there was anything in particular I should plan to bring, anything I’d need for Georgia that wouldn’t be on the typical packing list for a summer of fieldwork. Ofer stopped, turning to face me and sizing me up through thick glasses.
“Maybe an extra liver.”
Strangers in a Strange Land
Precisely 1.85 million years ago, a massive volcano erupted in a cataclysm that would have shaken the ground and blackened the skies many miles away, in the low hills that would someday hold the village of Patara Dmanisi. Lava flowed for miles down the nearby Mashavera valley, filling it and obliterating the Mashavera River. As it flowed past the Pinasaouri, a small tributary, it backed up into the side valley and blocked that river, too. The lava cooled into a black basalt, a hundred feet thick in parts. A lake formed as the Pinasaouri River backed up behind its new basalt dam.
As years stretched into millennia, at least two other eruptions filled the skies with ash. The animals that roamed the landscape—a menagerie that included now-extinct species of ostriches, giraffids, horses, gazelles, saber-toothed cats, wolves, bears, and rhinoceroses—must have choked on the ash and wondered, to the extent that they could, what the hell was going on. These ashfalls blanketed the landscape, including the narrow promontory squeezed between the basalt-filled Mashavera River valley and the Pinasaouri lake. This ash became the soil.
Through it all, plucky bands of Homo erectus, early members of the human genus, lived out their lives in the rolling woodlands around the Pinasaouri lake. They were an invasive species, the bleeding edge of an expanding population that had been bubbling out of Africa and spilling out into the rest of the Old World for thousands of years. But none of them would have had the slightest notion of their African origin, or indeed of an origin from anywhere other than right there. With a brain only half the size of ours, they probably thought little about such academic matters at all.
At five feet tall and 110 pounds, the Dmanisi hominins would have been tempting targets for the hyenas, wolves, and saber-toothed cats that prowled the forests. They held their own, though, relying on their wits and simple stone tools. And they were more often predators than prey. The bones of other animals at the site bear the unmistakable gouges and scrapes of stone tool butchery. The Dmanisi hominins and their ilk were not the apelike vegetarians that came before, confined to the African woodlands. They were hunter-gatherers.
They would have been lucky to live into their thirties or forties. Most surely died much younger. Occasionally, the rain would wash their bodies into a nearby gulley, alongside the chewed and scattered remnants of other dead animals. Eventually the gulley would fill with sediment, encasing their remains a few feet beneath the surface.
As eons wore on, the Pinasaouri and Mashavera Rivers cut their way through the thick basalt, reclaiming their valleys and leaving a thin spit of ridgeline once again perched between them. The Dmanisi hominins were long gone at this point, replaced by waves of later hominins. It’s likely that later, larger-bodied populations of Homo erectus inhabited the area, though we’ve yet to find their bones. Stone tools tell us that Neanderthals set up camp a few miles down the valley around forty thousand years ago. Modern humans swept in some time after that. In the early centuries of the Christian Era, a stone church was built atop the promontory. A stone-walled medieval city grew up around it, and the people thrived. Then came the hordes of invaders. Starting around 1080 a.d., they overran the city every couple hundred years like some brutal Mongol time-share. By the fifteenth century the once-proud city was abandoned, and the area was left to the peasant farmers in the valley below.
The initial discovery of the fossil site of Dmanisi was a happy accident. In 1983, archaeologists excavating the medieval city dug into the surrounding soil and pulled out a fossilized rhinoceros molar. Realizing they had stumbled upon the remains of a lost ancient world, they alerted their colleagues at the national museum in Tbilisi. A team of Georgian paleontologists began working the site, focused on the fossils. Stone tools were found a year later, and the first hominin fossil, a jawbone, was excavated in 1991. Paleoanthropologists around the globe were intrigued but skeptical. Then, in 2000, the Georgians reported two new skulls along with solid dates for the Mashavera basalt. They had the oldest hominin site outside of Africa, and the site was producing beautiful, complete fossils. Humanity’s first foray across the globe was captured here, at this little site in the foothills of the Caucasus Mountains. Suddenly, Dmanisi was the center of attention in human evolution.
When I showed up mid-season in the summer of 2001, a small crew of Georgian researchers and volunteers, European and American graduate students, and leading international scholars in human evolution, archaeology, and geology were working to dig the site and reconstruct what life was like for the Dmanisi hominins. The main excavation was a rough rectangle, about five hundred square feet of exposed earth, painstakingly carved flat and level with trowels and brushes. Like every dig everywhere, the site was overlaid with a grid of 1-by-1-meter squares. I spent my days in my assigned square, scraping away claylike sediment with a trowel and a brush while keeping a sharp eye out for the first white flash of fossil.
I was hardly a veteran archaeologist when I showed up, but I had done enough to expect days of finding nothing. Even at an exciting site, most dirt is just dirt. But Dmanisi was different, with rich veins of fossils and stone tools. Rhinos, lions, gazelles, horses; complete skulls and other bones, not the scrappy fragments you learn to cherish on most digs. You’d realize that the person digging in the square next door had gotten awfully quiet and look over to find them absorbed in the ticklish work of wresting some behemoth from the ground, its curved and intricate skull emerging from the surface like Excalibur. The fossils were often softer than the surrounding dirt, and extracting them from the sediment without destroying them was a delicate art.
We found another skull—the third from Dmanisi and the most complete cranium of Homo erectus ever recovered anywhere—at the end of my first season. It emerged from the sediment upside down, its palate facing the sky. Most excavation teams never recover a major hominin find; isolated molars and fragments of skull are celebrated like holy relics. The great Louis and Mary Leakey spent nearly thirty years scouring Olduvai Gorge before finding a hominin skull. The Dmanisi crew had uncovered three skulls in three years. This most recent one, likely from a male in his late teens, was so perfectly complete that the paper-thin, crenulated bone of the upper palate and lower orbits was still intact. The entire team walked around for days with smiles that wouldn’t wear off. Traditional Georgian feasts were held on the long dig-house table, late into the night. Men broke into haunting polyphonic Georgian folk songs.
I was hooked. I knew I’d be back every season for as long as I could. And for the five summers of my grad school career I carved out the time to make the trek to Dmanisi. Every year we found hominins and celebrated (not always in that order). Wine, vodka, and chacha, a grape-based Georgian moonshine, were had in terrifying amounts. And every year I’d find myself yakking into the bushes, ringing in the annual cycle like some putrid combination of Old Faithful and Stonehenge, whispering the same empty promises of reform into the morning air. Ofer was right: I could have used an extra liver.
My second summer at Dmanisi, the crew uncovered yet another skull, the fourth from the area. Its most prominent feature was what it lacked. The U-shaped arcade around the perimeter of the palate that should have held teeth was smooth and rounded. All the teeth were gone. And they had been lost while the individual, probably a male in his late thirties or early forties, was still very much alive, the sockets healing over and filling with bone. Whether from disease or advanced age, this poor guy had lost every tooth in his head and somehow managed to survive, choking down food with pulpy, painful gums while he recovered. The resorption of the tooth sockets was so advanced that he must have lived for years without teeth.
The discovery raised an obvious question. How did he manage to survive? Wild plants and game are nearly all hard to chew; you need teeth. Few wild foods are easy to harvest, particularly if you’re frail. How did he persist for so long?
He owed his life, I believe, to the quintessential human adaptation, the trait that most sets us apart from our ape relatives. It’s a behavior so ingrained that we rarely give it a second thought. Yet it revolutionized the human lineage, changing the way we get our food and altering the way our bodies burn energy. The Dmanisi hominins shared.
Selfish, Lazy Vegetarians
Humans are part of the ape family, a subset of the primate order of mammals. The primate bough of the mammalian family tree first emerged as a green twig roughly sixty-five million years ago, in the wake of the asteroid impact and massive extinction that wiped out the dinosaurs. The K-T Mass Extinction event, as it’s known, left vacant landscapes into which primates and other mammalian groups bloomed.
Early primates were scrappy, squirrel-sized animals who made their home in the trees. Like primates today, including us, they 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 (i.e., more energy-rich) 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.
But we owe more than our hands and love of sugary fruits to these distant ancestors. As was discussed in Chapter 1, my colleagues and I discovered that primates burn only half as many calories as other mammals. It’s so widespread throughout primates today that this metabolic shift must have occurred very early, at the base of the primate radiation. These early primates were playing the long game. Reduced daily energy expenditure meant slower growth and reproduction, but also longer lives. Rather than concentrating all their reproductive effort over a few short years (where a bad year could wipe out most of your fragile offspring), 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. As I write this, I’m sitting across the kitchen table from my four-year-old daughter, who is dexterously eating her cereal and apple slices while chatting about preschool and the years of school ahead. Our modern human lives have very deep roots.
The primate metabolic strategy was incredibly successful. 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. The apes, or hominoids, as the group is technically called, grew to be a successful lot. For fifteen million years, they proliferated and expanded across Africa, Europe, and Asia. There were dozens of species.
Then, for reasons that remain obscure, their fortunes changed. 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 (those two species didn’t split until much later; Figure 4.1). The other population was the founders of our lineage, the hominins. There are at least as many ideas about why this split occurred as there are drunk paleoanthropologists, but little consensus. 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. At least three different fossil species are known from this period, all in Africa. Only one, Ardipithecus ramidus from Ethiopia (Ardi to its fans), is well characterized, with dozens of fossils and a nearly complete reconstructed skeleton, from ape-sized head to long, grasping toes. Others are less complete. The oldest, Sahelanthropus tchadensis from Chad, is known only from a skull and some scrappy fragments of the body. Orrorin tugenensis from Kenya has the opposite problem, with only pieces of the limb bones and some loose teeth recovered.
This might lead to you to ask, “How do the scientists know that these fragmentary finds are different species? Or even that they are hominins at all, and not members of some other lineage?” Congratulations, you’ve just invented the field of paleoanthropology. The gory details of paleoanthropological research are the topic of another larger book, but suffice to say it’s hard work, requiring a keen eye and an encyclopedic knowledge of the morphological signatures of different taxonomic groups. Uncertainty is the norm. Paleoanthropologists routinely work themselves into a lather over the anatomical minutiae that distinguishes one fossil species from another, or get into shouting matches at highbrow academic conferences trying to make the case that their fossil species is a direct ancestor to living humans, while someone else’s pet species is just a side-branch dead end (or gasp! not even a hominin at all). If you want to ruin a paleoanthropologist’s day, suggest that the fossil hominin species that he discovered, named, and has dedicated his life’s work to, is in fact just a local variant of some other, previously described species.
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. One strange and specialized side group of species, assigned to the genus Paranthropus, take this dental inflation to the extreme, with molars five times larger than ours and massive cheekbones to anchor equally massive chewing muscles.
There is even some evidence for increased cognitive sophistication. 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). We used to think that hominins from this period weren’t able to make stone tools, but 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 cumbersome 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.
Figure 4.1. The human family tree. Our lineage, the hominins, is one branch of the ape family, and includes over a dozen known fossil species, several of which are shown. The X in the circle indicates the position of the Dmanisi hominins. Hunting and gathering begins with the genus Homo, and brings with it increased brain size and changes in diet and behavior (gray: early debated evidence; black: strong and continued evidence). Note the change in timescale at five million years. (Adapted from H. Pontzer [2017]. “Economy and endurance in human evolution.” Curr. Biol. 27: R613–21.)
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. Sure, they may have hunted small game occasionally or looted termite mounds for some yummy protein, 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 much larger game—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. But the crucial cognitive leap was not the hunting or the tools—after all, chimpanzees and bonobos hunt and make tools, and it hasn’t led to any radical departure from their ape-ish ways. 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
The first Hadza words I remember learning are amayega and mtana, the back and forth of the basic Hadza greeting. The third word I learned was za.
I’m not sure when I first noticed it. My first trip to the Hadza was a flood of new sights and sounds, and the early days are a bit blurred in my memory. If you’ve ever spent time in a foreign city where you didn’t speak the language, at a café or park, you know how the voices around you can form a sort of abstract sonic tapestry, rich in feeling but devoid of meaning. But at some point, my mind caught on to a repeated, simple command, “za.” Soon I was noticing it everywhere. Two kids hanging out eating a snack, “za.” Grandma feeding berries to her grandson, “za.” A man bumming honey from a friend, “za.”
I asked Brian what it meant, though it should have been obvious. Za means “give.”
What I couldn’t understand was why there was no counterpoint. Nothing was ever said in return, the item in question was simply handed over. Where were all the extras I grew up with, the magic words: “please,” “thank you,” “you’re welcome”? To my utter disbelief, I learned that they didn’t really have them. They have the concepts, of course; there are Hadza words for requesting help and for expressing gratitude. But the “please” and “thank you” drilled into kids across the Western world is absent from the small-scale exchanges that dominate the day. What kind of language doesn’t have the magic words?
The more I saw, the more I understood. Giving—sharing—isn’t a nicety among the Hadza. It’s the rule. Just like you don’t go around saying “Thank you for not spitting in my face” to everyone who doesn’t spit in your face, the Hadza don’t bother saying “please” and “thank you” for sharing. It would imply that the person is doing something beyond simply living up to the societal contract. You need the magic words only if there’s a chance the other person might reasonably refuse, but that’s not how it works with the Hadza.
To be Hadza is to give. Everyone shares with everyone, all the time. That’s the rule. All you have to say is “za.”
In the 1950s and ’60s, researchers in human evolution (nearly all men, it’s worth noting) began synthesizing the available data from the hominin fossil record, field studies of living primates, and ethnographic research in living hunter-gatherer populations. It was an exciting time to be asking, What makes us human? These fields were still young, but enough work had been done, enough fossils found, to move beyond the mere speculation of previous generations and begin synthesizing holistic, evidence-based reconstructions of our evolutionary past.
The movement was codified in the landmark 1966 “Man the Hunter” conference, which generated a book of the same title. The casual chauvinism of the era is obvious from the name. I don’t think it was intended, though that hardly matters. Researchers (again, nearly all men) were struck by what they saw as the major differences between humans and other apes: our proficiency in and dependence on hunting and tools. They saw all the major traits that make humans unique as downstream evolutionary consequences of these key innovations. It was a very influential perspective, even if it wasn’t entirely new. Darwin himself had speculated humans owed our “pre-eminent success in the battle of life” to hunting, arguing it would “have been advantageous to the progenitors of man . . . to defend themselves with stones or clubs, to attack their prey, or otherwise to obtain food.”
Figure 4.2. Hunting and gathering means sharing. A Hadza grandmother shares berries with her grandson after returning to camp with the day’s harvest.
The feminist movement of the 1960s and ’70s and the glaring omission of women in the “Man the Hunter” paradigm led to a predictable and much needed correction. In 1981, anthropologist Frances Dahlberg edited a collection of essays titled Woman the Gatherer, highlighting women’s essential contributions in hunter-gatherer populations. In addition to their irreplaceable roles as mothers and grandmothers, women in foraging cultures invariably provide food and goods that are needed for the community to succeed. In many cultures, women’s foraging provides well over half of the calories. Moreover, by the late 1960s it was clear that chimpanzees occasionally hunted and used tools. If hunting and using tools weren’t uniquely human behaviors, it was hard to argue that hunting and tool use propelled our unique evolutionary trajectory.
Frankly, I think that focusing solely on men’s or women’s contributions misses the crucial point. 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. More than just man the hunter or woman the gatherer, we are human the sharer.
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—not exactly “mother of the year” behavior, by human standards. 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. At the site of Wamba in Congo, Japanese researcher Shinya Yamamoto found that adult bonobos (mostly females) share a particular fruit, the large and fleshy junglesop, about 14 percent of the time.
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 that 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. It’s telling that the most commonly shared foods for chimpanzees and bonobos are monkeys and duikers (a kind of small antelope) they hunt or the large junglesop fruits at Wamba. These items aren’t huge, but they’re more than a mouthful. The lucky hunter typically keeps as much as he can manage, “sharing” scraps to quiet the begging and pestering horde. Even bonobos share junglesop only if a friend begs.
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.
Sharing is the glue that binds hunter-gatherer communities together and provides the fuel that makes 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 (Figure 4.3). As my colleagues and I discovered in our doubly labeled measurements of apes and humans (Chapter 1), we burn about 20 percent more energy every day than chimpanzees and bonobos. Our energetic advantage over gorillas and orangutans is even greater. Those extra calories fuel our big brains, active lifestyles, and big families—the traits that set us apart from the other apes and define our lives. And it began with hunting and gathering, with early members of our genus, Homo, foraging for more than they needed for themselves and giving the surplus away. That extra energy propelled hominins armed with primitive stone tools and ape-sized brains across the globe, from Durban to Dmanisi and beyond.
The Metabolic Revolution
We often discuss evolution in terms of physical traits, the appearance of new anatomical features or changes in their shape and size. After all, it’s the physical traits that are usually preserved in the fossil record. But behavioral changes are often the true instigators. New behaviors arise and the body adapts. Fish started feeding in the muddy shallows of the water’s edge, and those with the strongest fins and best primitive lungs to navigate those puddles had the best reproductive success; the evolutionary transition to land and legs followed. Horse ancestors with unremarkable teeth switched from eating soft leaves to more abrasive grasses. Those with taller teeth survived longer, as it took longer for the teeth to wear out. After millions of years, long teeth became the norm for horses. (Which is why you can tell the age of a horse by looking in its mouth, to see how worn down its teeth are—a savvy move if you’re buying the horse, but rude if it’s a gift.) Polar bears started swimming and diving to hunt, then evolved webbed feet. Behavior leads, form follows.
It would have taken a very particular set of circumstances for sharing to prevail in the hominin lineage: the costs of acquiring more food than you could eat had to be lower than the benefits of giving it away. Foraging for extra food means less energy for yourself, more for someone else—not the sort of thing that natural selection, Darwin’s amoral accountant, usually favors. 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.
Despite the long odds against it, in a population of ape-brained hominins some two and a half million years ago, somewhere in eastern Africa, the right combination of conditions, diet, and behavior aligned. Sharing became the norm. Sadly, the details of its origins may be too fine-grained to be caught in the rough sieve of the fossil record. (Though if you buy a round at the next human evolution conference, you’ll be regaled with any number of nuanced and complex scenarios.) 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, the distant cousins of the potatoes and yams in our supermarkets today, were an important shared food early on. Tubers are a dietary staple for the Hadza and other hunter-gatherer populations around the world. And they are calorific starch bombs, hard for small kids to dig from the ground but easy enough to harvest in surplus for adults. Just as orangutan mothers tend to share foods that are hard for young offspring to get, hominin mothers (or fathers) could have made a habit of feeding tubers to their kids. Perhaps older females, past their childbearing years, began to channel their maternal efforts into sharing food with their daughters and grandchildren.
Whether it was meat, plant foods, or some combination, 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.
We are the descendants of these early, sharing hominins. Over time, hominin physiology responded to this new behavior, ramping up metabolic rates to take advantage of the extra calories. This was the Metabolic Revolution (Figure 4.3), and it has shaped the evolution of our genus, Homo, ever since.
Positive Feedback and Virtuous Cycles
Just as your metabolism reflects the orchestrated activities of all of your body’s systems working together, the Metabolic Revolution changed every aspect of our physiology. Since calories don’t fossilize, it’s difficult to dissect which changes came first. The first sign of metabolic acceleration that we see in the fossil record is an increase in brain size. Brains are metabolically expensive organs, as we discussed in the last chapter. By two million years ago, not long after the earliest cut-marked bones, we find fossil hominins with brains nearly 20 percent larger—consuming 20 percent more calories—than their Australopithecus predecessors.
Figure 4.3. The Metabolic Revolution. Like all primates, apes use their metabolic energy for life’s essential tasks, including growth or reproduction, maintenance (e.g., immune function, tissue repair), and physical activity. They are smart, social animals that invest in brains for navigating their complex social worlds and finding food, but they feed only themselves. Humans combine social and foraging efforts, sharing surplus food energy with other members of their group. Sharing increases the energy available for all tasks including reproduction and maintenance, leading to longer lives, larger families, larger brains, and increased activity. Humans burn more energy each day than other apes to fuel these traits. Greater energy expenditure also favors directing extra calories to fat (far more than in other apes) to survive periods of energy shortage.
The fact that evolution favored channeling those extra calories into expensive brains says a lot about the metabolic strategy of our genus. Normally, we’d expect evolution to favor spending those calories directly on survival and reproduction. After all, reproductive success—the number of surviving offspring produced—is the only measure that natural selection pays attention to. There’s no evolutionary benefit to investing resources in brains or any other feature unless it pays off in more babies. 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. Compared to plant foods, game animals are spread thin on the landscape and are much harder to hunt down. Modern carnivores on the African savanna typically cover four times more ground each day than the herbivores they’re after. The transition to hunting early in our genus would have required a similar increase in the daily distance traveled. And it might have meant more than just a lot of walking. Dan Lieberman (my PhD advisor at Harvard) and Dennis Bramble have made a compelling case that early members of the genus Homo were adapted for endurance running, wearing down their prey under the hot African sun until they collapsed. 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.
The strategy worked. 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. Hunting and gathering had hit its stride. Incredibly, unfathomable eons hence, their descendants would pluck the remains of a handful of these hardy pioneers from the soil of Dmanisi.
Sharing, smarts, and stamina, the key ingredients of hominin cooperative foraging, were a potent combination. 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, like the Big Lebowski’s rug, 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.
It was such a winning strategy that the only way to beat it was to do it better. Each generation, there would be variation among individuals in their cognitive ability, social sophistication, and physical endurance. 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—species more and more like us.
The increase in intelligence is the easiest to track in the fossil and archaeological record. Fossilized skulls like those at Dmanisi enable us to track the growth in brain size—a rough, though reasonable, measure of intelligence when we compare across species. In less than two million years, brain size triples in the genus Homo, steadily expanding like muffins in the oven (Figure 4.1). Stone tool sophistication increases in parallel. The early tools, from places like Dmanisi, are simple, broken cobbles, with all the beauty and sophistication of a first grader’s clay flower vase for mom. By one and a half million years, hominins were making symmetrical, tear-shaped “hand axes,” which are tricky to make (I can’t do it, but my buddies who are into stone tools can). By 400,000 years ago, hominins were using complex, multistep “Levallois” processes to make long, thin blades and other incredible tools—stuff so complicated that you have to be a Level 7 archaeology nerd with years of experience to make it today. Tools get only more intricate from there, their sophistication growing in an unbroken chain of innovation from Paleolithic obsidian blades, to the bow and arrow, to the smartphone in your pocket.
It’s not only the tools, of course. By 500,000 years ago, hominins are controlling fire. (This breakthrough may have come a good deal earlier. Debate on the subjects is . . . heated.) Language abilities must have been developing throughout this period as well, though it is fiendishly difficult to track its evolution. By the time our species, 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.
Reconstructing hominin cognitive evolution is relatively easy because brain size and the tools, art, and the other stuff we make leaves a trail of bread crumbs in the fossil and archaeological records. It’s more challenging to track the pace of evolutionary change in fitness and friendliness, because neither leave much definitive hard evidence behind. What we can say with certainty is that humans today are far and away the best endurance athletes among all of the living apes. Our VO2 max, a common measure of peak aerobic power (see Chapter 8), 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.
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.
It’s a similar story for sharing. Hard evidence of butchered zebra and other big game at sites like Dmanisi tells us that sharing was established early in our genus. In fact, as I’ve argued above, sharing is probably the key behavioral innovation that sparks the evolution of our genus, Homo. But it’s difficult to track the degree or amount of sharing, as it changes over time from then until now. Still, there are some suggestive clues. By at least 400,000 years ago, tool technology and hunting techniques were quite sophisticated. In addition to deadly stone tools, they were making well-balanced spears with fire-hardened tips, and regularly taking down wild horses and other big game. Such dedication to crafting tools and developing hunting strategies indicates, perhaps, that some community members specialized in hunting while others focused on gathering, similar to most hunter-gatherer populations today. Division of labor like that needs a strong commitment to sharing to make it work.
Brain size and behavioral complexity offer another clue about sharing. Enormous brains and a behavioral strategy dependent on learning means that we come into the world at birth as helpless, useless, damp bundles of pudge. We can’t walk, talk, feed ourselves, or keep out of danger for years after we’re born. Instead, we are completely reliant on others—on sharing—for the food, attention, and safety we desperately need. We spend the first decade or two of our lives soaking up the shared resources of generous community members, learning (one hopes) 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—acquiring enough food to feed themselves—until late in their teenage years.
The payoff for all this waiting and learning is incredibly high adult productivity. 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 (see Chapter 9). 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. (Average interbirth intervals for chimpanzees, gorillas, and orangutans are five years or longer.) 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. But even if the essential human metabolic strategy was in place before we got here, it might be our unique twist on sharing that kept us from going extinct.
As our Homo sapiens ancestors expanded throughout Africa and across the globe, they found they weren’t alone. The world was already full of strange and wonderful humanlike species, their evolutionary cousins: 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. The modern science fiction fantasy of meeting some almost-human in a distant realm, communicating, living with them, played out again and again in Paleolithic wilderness.
Some of these species, like erectus and naledi, are helpful reminders that evolution has no momentum. These species evolved slightly larger brains than Australopithecus and were among the earliest hunter-gatherers. But at some point early on, natural selection stopped pushing them toward ever larger brains and more complex foraging. The vagaries of their particular habitats and ecologies didn’t favor it. The costs of bigger brains and the risks of increased generosity were greater than the benefits. So with no pressure to change, they kept their modest brain sizes and early Homo habits for hundreds of thousands of years, even as hominin populations in other parts of the world kept changing. Evolution isn’t trying to get anywhere. Just because brain size increased steadily for a million years doesn’t mean it will continue. We were not inevitable.
Other species, like the Denisovans and Neanderthals, tell us we were not particularly special. These species were smart, adaptable, resourceful, just like us. In fact they were so much like us that we interbred, raising hybrid families and wondering, no doubt, why the in-laws always seemed a bit peculiar. We find bits of their DNA in our chromosomes today, a few scattered bricks of a lost civilization recycled into modern construction.
Why they went extinct and we persisted—why we’re the only hominins left on the planet today—remains one of the great mysteries. 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 it was just dumb luck, a cosmic roll of the dice where chance happened to favor us. 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, my 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 (or in Wrangham’s telling even 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.
Whether humans were unique among hominins in our propensity to work together, it’s clear that our extreme sociality, huge brains, and capacity for physical activity are the key traits that set our species so fundamentally apart from the other apes. And we owe it all to our two-million-year legacy of hunting and gathering, stretching from Dmanisi to today. Our complex social worlds and empathy, our ability to explore the galaxy and split the atom, our ability to endure, our willingness to share lunch—all of it is literally part of our DNA. And all of it is fueled by our high-energy metabolic strategy. Our metabolism—the way we get our energy and the way we spend it—was essential to our radical evolution.
Did I mention there’s a downside?
The Downside
I grew up in the rolling, remote hills of the Appalachian Mountains of northwest Pennsylvania, in a little town called Kersey. Like yours, my childhood was filled with daily lessons and constant reminders of my social identity. I was a Pontzer, a Catholic, a public school student, a Kersey kid, a Steelers fan (even if I rarely watched a game). Each of those layers meant something. It defined who my friends were and who was inherently suspect (private school students, kids from St. Mary’s). None of these identities was stronger than being a Penn State fan.
My parents, older sisters, and many aunts, uncles, and cousins had all gone to Penn State. We watched few televised sports in my house growing up, and neither my mom nor dad cared much about sports, but if we were home on a Saturday in autumn, Penn State football was on the TV. My senior year of high school, I applied to exactly one college: dear old Penn State. I honestly couldn’t imagine going anywhere else. Penn State was my tribe.
The ultimate tribal ritual—the ecstatic rite of passage of my freshman year—was attending a Penn State football game. For a true believer, it’s a religious experience. Perched on the steep aluminum bleachers with 115,000 rapt fans, all adorned with the colors and other signifiers of the tribe, we cheered on the gladiators below. It didn’t matter that we didn’t know one another. Anyone in the stadium (aside from the small, brave-faced contingent in the visitors’ area) was an instant friend. And in full voice we’d shout the defining Penn State cheer, a deafening call-and-response across the stadium. WE ARE . . . PENN STATE! It was nearly as intoxicating as the sleep deprivation, freedom, and alcohol that defined my freshman year.
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. Come to think of it, they seem to have an awful lot of resources, things our group could really use. Look at them! Just sitting there with all their stuff. The smug jerks. I mean, it’s practically criminal of them to keep it all to themselves. I say we go over there and strongly recommend that they give us what is rightfully ours. After all, We are Penn State . . . and they are not.
You see how this sort of thing can get out of hand.
Sharing made us incredibly generous to our fellow group members, but also gave us the capacity to be frighteningly callous and evil to those who are not. It’s part of what Brian Hare and Vanessa Woods describe in their book Survival of the Friendliest. Hundreds of thousands of years of evolution for sharing and friendliness within our tribe has made daily life a miracle of peace, harmony, and collaboration for most of us. We volunteer, donate our time and money, coach kids’ soccer or organize the school bake sale. We can watch a tense movie in a crowded theater with hundreds of strangers and no one bats an eye. A theater full of unfamiliar chimps would be a bloodbath before the opening credits. But the flip side is that we are generally indifferent and even hostile to anyone we deem an outsider. 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 . . . ad infinitum. 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.
So many of the atrocities that scar our history and shake our faith in humanity today—genocides, slavery, human trafficking—are born from our evolved ability to view outsiders as less than human. In the past, this horrific behavior was often sanctioned, even required, by religion or the state. Biology and evolutionary science were coopted in this effort in the 1800s and 1900s, with appalling and wrongheaded “science” used to justify racist policies and behavior. The slimy tendrils remain today in the “intellectual” arguments for racism (in fact, there is no evidence whatsoever that the minute genetic differences among ethnic groups today affect behavior, intelligence, or anything else that we value in our fellow humans). It’s chilling to see these themes rising again in our increasingly tribal politics, in countries that are supposed to be civilized enough to know better, dehumanizing people we disagree with and anyone seen as an “other.”
The crucial argument of our time is, Who is part of our group? Who counts as one of us, and who doesn’t? Of course, the only morally acceptable answer to that question is everyone. Everyone counts. We are all people. We are all part of the same human tribe.
To win this argument, which we must, we will need to overcome our suspicion of outsiders, the evolutionary price we pay for our incredible willingness to share.
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 Hadza don’t get them. They are what people in public health call “diseases of civilization,” the unintended consequences of development. And they have come to the forefront even as, by some accounts, human societies globally have become less violent. We have graduated as a species from brutally killing each other to mindlessly killing ourselves.
The problem isn’t simply our built environments. It’s deeper. 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.
When Steve Ross, Mary Brown, and I conducted doubly labeled water measurements in dozens of apes living in zoos around the United States and compared them to humans, we found differences in more than just energy expenditure. We also found that apes are incredibly lean. Chimpanzees, bonobos, gorillas, and orangutans idling away in zoos and sanctuaries don’t get fat, at least by human standards. Chimpanzees and bonobos put on less than 10 percent body fat in captivity, on par with elite human athletes in training. Even active hunter-gatherers like the Hadza put on more fat than that. And for sedentary people in modern cities (the equivalent of zoo-living apes), the sky is the limit. Men can easily carry 25 or 30 percent body fat, and women more than 40 percent.
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. In our modern-day built environments, those rainy days never come. 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. The World Health Organization puts the worldwide number of deaths each year from inactivity at 1.6 million. The number of healthy years lost as people struggle with heart disease, diabetes, and other consequences of a sedentary lifestyle is far greater. And it is a uniquely human problem. Apes in zoos, with modest amounts of exercise each day, don’t develop high blood pressure, diabetes, humanlike heart disease, or the host of other maladies that plague the developed world.
Modernization has brought an incredible amount of good into the world, from modern medicine and global connectivity to warm houses and sanitary indoor plumbing. But its unintended consequences have grown increasingly scary (and we haven’t even touched on climate change, habitat loss, the threat of nuclear annihilation . . .). Our species is only 300,000 years old. If we’re going to survive another 300,000, or even enjoy the next three hundred, we’ve got to start building better human zoos.
Our one hope is our enormous, intelligent, creative brain. A long evolutionary history as hunter-gatherers has given us the cognitive capacity to shape our world. We have been smart enough to tame fire, build incredible machines and send them to distant planets, create new species, and piece together our own evolutionary history. Are we smart enough to take control of our future? Or are we destined to stumble, to give into temptation and fall short, puking into the bushes yet again and suffering needlessly from self-inflicted misery? Will our distant ancestors pluck our fossils from the soil and marvel at our genius or shake their heads at our inability to avoid disaster?
To figure out how to set things right, we need to figure out what’s gone wrong. How did we get so off track, and how do we get back? It’s time to head back to Hadzaland and see what they can teach us about living well and staying healthy.