HOW EATING MEAT—AND ESPECIALLY FAT—JUMP-STARTED OUR EVOLUTION FROM APELIKE TO HUMAN
The fresh kill left by the saber-toothed cats lay still in the dewy morning grass as the sun rose over the parched savanna. The heat was already beginning to rise as Lucy’s eyes scanned the horizon for any sign of the predators that had brought down the giant, bloodied buck. The saber-toothed cats were busy chasing off hyenas, but they would soon return.
The smell of smoke from distant wildfires was a reminder that edible plant life in the area had been scarce since the long drought had set in. In the vast distance beyond the plain, a cloud of ash billowed high into the hazy sky from a looming volcano. Small particles of ash littered the landscape, smothering the few patches of green grass that were still growing. Only a few of the grazing animals that were still around were finding much to eat.
Lucy rose on her two legs to peer cautiously above the tall, dry grass for any sign of immediate danger and made her way a few feet to the kill. Her mate stood nearby, watching for the return of the cats.
The carcass was just too large and heavy to steal away, but Lucy had planned ahead. Her rough, leathery hand grasped a large, razor-sharp stone for carving flesh. With swift and powerful blows, she cut deeply into the partially eaten hindquarters and managed to disarticulate the large, meaty shank. She pulled and twisted it free from the rest of the carcass and quickly fled with her mate, scrambling for cover in the nearby brush. When they’d found a protected place, they ravenously feasted on that meat until all that was left was a femur and a smattering of smaller bones. Lucy raised her sharp stone tool and struck the bone repeatedly until it cracked open to yield the creamy contents within . . . rich, life-giving fat.
This scene, and others like it, took place about 3.39 million years ago. Not quite an ape and not quite human, Lucy has become something of a celebrity in paleoanthropology circles, the earliest hominin ever discovered. (She’s a member of our hominid species group, but from a separate line called Australopithecus afarensis.) The unearthing of Lucy’s partial skeleton in Ethiopia radically challenged the notion that our very earliest ancestors were herbivores. When it became clear that Lucy’s kind used stone tools to cleave meat and marrow from animal prey, the practice of hominid meat eating became more deeply ingrained in history than previously thought—by a whopping eight hundred thousand years.1
When the global climate began to change and warm radically, less than a million years later, the earliest proto-members of our own human species finally emerged from the dwindling trees, forced by the change in climate to live out and exposed on the grass-filled savanna. The scarcity of edible plants pressured them to make meat and animal fat their primary food, with fruit and leaves becoming side dishes. This dependence on an extremely nutrient-dense food was a turbo-boost to this emerging brand of intelligent primate,2 who learned to stand upright in order to better scan the horizon for predators, grasp and use spears (thanks to their opposable thumbs), and adapt to a hunting and scavenging way of life. Notably, they also learned how to scavenge brain tissue along with fatty bone marrow.
Before long, what anthropologists term “persistent carnivory”—the full-time dependence on meat eating—became our new consistent ancestral norm.3,4 And it became the biological impetus for the development of a body that was very different from that of the apes that preceded us: our digestive tracts changed in order to process meat efficiently, and our stomachs developed the high acidity levels normally associated with scavengers in the wild.5 Stomach acid is necessary to effectively digest animal protein, absorb minerals, and protect us from potentially harmful microbes. Our gallbladders evolved to handle significant amounts of fat and fat-soluble nutrients in the fatty animal foods we consumed. We also evolved the ability to absorb (and developed a distinct preference for) heme sources of iron, which come from blood and are found only in animal-source foods.6 In addition, our heads and teeth evolved to meet our ever-greater carnivorous appetites, with skull, jaw, and teeth changes to accommodate the ripping and shearing of flesh.7 Most dramatically, the hominin brain began to grow in size and sophistication at a rate that is unmatched in natural history. Our behaviors developed, our intelligence grew, and the first Homo sapiens—the humans we are today—began to emerge.
It’s hard to overstate the significance of this dietary development. Though we often identify ourselves as “omnivores,” able to consume a variety of food sources, the truth is that not all foods are created equal in the eyes of our physiological makeup. For most of our evolutionary history we have been mostly carnivores and scavengers—not only actively carnivorous, but actually even more so than the wolves, cats, foxes, and bears we lived among. We were cunning and sophisticated enough to procure more of our caloric intake from meat than they did, especially from the extra-large megafauna that were available to us until the end of the last glacial period. (This has been measured using high-tech devices called isotope ratio mass spectrometers, which can determine aspects of dietary composition from bone collagen samples of prehistoric humans and animals.)8
With apologies to vegetarians and those for whom omnivorous is shorthand for “just eat whatever you want,” the fact is that it was a key dependency on meat and fat eating, and the development of increasingly sophisticated hunting practices, that allowed us to spread out of Africa and to adapt to new environments across the entire world.9
To understand why we became fat burners, you need to factor in the environment we were living in for the majority of our evolution. The past 2.58 million years are known as the Quaternary Ice Age (it hasn’t ended yet, by the way), and for most of it, the global climate has been far less stable and friendly than it appears to us today. It has been marked by extreme bouts of climate change, including major periods of glacial advance alternating with (shorter) periods of glacial retreat. The key word here is extremes—extreme cold in some places and extreme heat in others, with drought, wildfires, and chaotic and quite frequent volcanic activity that destroyed plant-based foods in many regions, among other disruptive things. It was during one of the prolonged, colder, and more inhospitable periods roughly two hundred thousand years ago that our modern human physiology emerged more or less fully complete. We became humans by, quite literally, surviving catastrophes.
This means that the most powerful pressure we faced was the urgent need for nutrient-dense calories, enough to fuel our large, energy-hungry brain and to warm our fur-less bodies. (Though the majority of the world’s humans were likely huddled closer to the equator, even those places experienced extreme, uncomfortable conditions—it can get perishingly cold even in a stormy jungle at night.)
Thus evolved what is perhaps the most critical but little-discussed aspect of our hunting and scavenging past: prehistoric humans didn’t hunt and scavenge in the pursuit of meat per se but were driven by an urgent need for fat, the most calorically dense food. For several million years before the dawn of agriculture, our ancestors relied on abundant animal protein and fats of all kind—saturated fats and the delicate polyunsaturated fats known as omega-3s and arachidonic acid that exist alongside them in meat. We developed a dietary preference for large, fatty, and robust land animals and learned to use every part of them, nose to tail. This preference persisted even when, during the Mesolithic period—the transitional period of the Stone Age between the Paleolithic and the Neolithic periods from about twelve thousand years ago to roughly eight thousand years ago—humans began to eat more fish.10
In fact, this preference persisted even in post–Ice Age primitive, indigenous, and traditional societies wherever possible, even in the absence of the megaherbivores whose fat was far more plentiful. When we scavenged carcasses, we created techniques to harvest the nutrient-dense parts that other predators could not—the juicy marrow inside the bones and the ultra-fat-rich brains. The saturated fats and key polyunsaturated fats we derived from them in turn fed our brains, which are fat-based and require animal-source fats as the critical building blocks. A positive feedback loop was created: the more animal fat we ate, the more our brains grew and intelligence developed, and the better we became at creating tools to procure even more meat and fat. When survival is at stake, you can’t afford to get by on shreds of scavenged meat or wait for fruit trees to come into season. You want the most energy-dense food you can get. And the variety of fats from animal sources—immensely satisfying and nourishing in relatively small amounts—is exactly that.
It wasn’t just any old animal sources of meat we sought. For prehistoric humans, the fatter and sassier the animal, the better the meal, and in Paleolithic terms, this meant big. Anthropologists call the extra-large land mammals of the time “megafauna”—these include woolly mammoths, mastodons, ground sloths, prehistoric bison (much different from today’s hybridized buffalo), elephants, camels, Irish elk, and the giant auroch (the prehistoric ancestor of our cattle today). The larger the animals, the slower they moved, and the more body fat they accumulated. A woolly mammoth, for example, likely consisted of at least 50 percent fat (extrapolating somewhat from today’s elephants), with stores of subcutaneous fat four inches thick—as well as a gigantic brain and megabones rich in marrow. It could weigh up to eleven tons, so taking it down required sophisticated technologies, bravery, and cunning—but the pursuit was worth it. Snag a full-size woolly mammoth, and you’d basically have a family barbecue—or maybe an all-you-can-eat buffet—that lasted for weeks. Early humans would have gorged on this fat-rich meat, seizing the opportunity to fill up their bellies when they got it. (This may also help explain the pronounced levels of hydrochloric acid in the human stomach, as compared with those of our other primate brethren: it takes a lot of hydrochloric acid to break down all that protein.)
Prehistoric people also hunted mountain sheep, beavers, bears, and wild pigs—all animals having high subcutaneous fat content.11 They weren’t after the lean cuts of muscle meat; research has shown that both ancient and Neolithic hunters preferred tallow-rich fat in bison and camel humps and in nutrient-dense brains (which were 60 to 80 percent fat), tongues, kidney fat, and other organ meats.12 Muscle meats—the cuts we tend to eat today—would have been dried and mixed with tallow to make energy-dense pemmican. At times the muscle meat got tossed down the food chain: according to stable isotopic analysis of Ice Age hunting tribes living in what is now the Czech Republic, humans ate the premium fatty mammoth meat, while the lean reindeer meat went to their dogs. Had Hanna-Barbera cartoonists understood all this, they would have depicted Fred Flintstone chowing on brain puree instead of Bronto Burgers at the Bedrock Stone Age drive-in.
Fueled by this food, our predecessors became master survivalists, able to persist despite the exceedingly challenging conditions throughout the Paleolithic era. In a feast-or-famine, climatically extreme world, fat came to mean survival—and survival trumps everything else. Our bodies, psychology, and biochemistry were forged around this basic principle, and we evolved inherent mechanisms for outlasting inevitable periodic famine, such as the fat-sensing hormone leptin (a subject covered at length in Primal Body, Primal Mind).
Today, we understand that fat was not only a preference but an absolute necessity.13 We know that not only are animal-sourced fats the body’s most efficient source of energy, but these fatty meats offer critical nutrients such as vitamin B12, heme iron, zinc, and elongated omega-3 fatty acids that ensure the proper functioning of the brain and the entire nervous system, the health of our very cellular structure, and the strength of our immune system. Fats contain powerful anti-microbial properties to fight infection and fuel our most important system for homeostasis. They facilitate the absorption and utilization of protein, cushion our organs and keep us warm, and support the heart, bones, hormones, and lungs. And that’s just the CliffsNotes version of everything fat does for us.
(An aside for foodies: Our ancestors developed a sophisticated sensory system designed to draw us irresistibly to fatty foods. Humans respond immediately to the smell, taste, and appearance of fat-rich foods because to survive, we had to be able to assess the energy content of foods with remarkable speed and accuracy. Next time someone laughs at your love of heirloom pork belly, tell them it’s your evolutionary imperative at work.)
This veritable meat-fest was occurring during a relative dearth of usable plant foods. Not only would edible plants have been challenging to procure in this extreme climate, but gathering was time- and labor-intensive compared to hunting, with a low cost-to-benefit ratio in terms of calories acquired. It was also challenging due to the high level of toxicity in the very plants that could offer some protein or calories, such as wild legumes and wild potatoes. For survival, man needed nutrient density. The prehistoric mantra: meat, and especially fatty meat, is where it’s at.14
Astonishing as it may sound to modern ears, eating all that fat did not make us fat. In fact, our ancestors were far leaner and more muscular than many professional athletes today. We ate lots of fat in the absence of carbohydrates, and this became the body’s formula for optimal energy burning, sans the love handles, as you will discover. This is still the formula your body understands today.
About 12,800 years ago, things changed—suddenly, violently, and irrevocably.15 A massive global cataclysm abruptly changed both the climate and growing conditions necessary for the survival of at least half the world’s megafauna species, and these plus-sized creatures died out in the greatest mass extinction since the dinosaurs. Now living in a post-glacial recovery epoch—our world today, which anthropologists and geologists call the Holocene era—prehistoric humans were presented with a whole new challenge for procuring their primary food, fatty meat, in such volume. All of a sudden, hunting fare consisted mainly of smaller, leaner animals such as deer, elk, birds, and smaller game, which moved quickly and were harder to catch up to. This didn’t change our preference for animal foods, however. To the contrary, it made hunting for fat an even more important focus, which forced us to further hone our hunting and observation skills.
Holocene humans became expert at selecting the fattest animals of the herd—bulls and females at the peak of maturity—through visual cues, such as the pronounced curves of the body and the sheen of the coat, and through seasonal selection, by tracking the animals’ migration patterns.16 To understand how remarkable this is and how different we are from other predators, consider that wild wolves always target the youngest or most infirm (often the leanest) animals in a pack. I have observed this firsthand many times: when wolves hunt, they aim for what is easiest for them to catch on foot. We, on the other hand, being the slowpoke predators we are, compensated for our physical limitations with lethally effective technology. Prehistoric humans also chose to do the opposite of what wolves and other predators do, risking their lives by targeting the healthiest, fattest, and sassiest animals in a herd. This demanded strategic thinking, communication, collaboration, and pushing ourselves to the limit—all things that unleashed our true, best, and unique human potential and rapidly grew our brains. And this was all made possible because our brains were supplied with the energy and the brain-essential nutrients found in animal fat.17