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  He gave this a try in Wyoming, where there are plenty of antelope. He found he could indeed single out an animal from the herd and track it and chase it long distances, but just as the chosen animal was beginning to tire, it would circle back to the herd and get lost in the crowd, and Carrier would be stuck on the trail of a fresh animal ready to run. Finally, though (and by chance), Carrier learned of tribesmen in South Africa who still practiced this form of hunting. He went to Africa and learned the trick, and it did indeed involve endurance running, but it also involved a sublime knowledge of the prey species and its habits, a knowledge bordering on a supernatural ability to predict what the animal would do. The running itself was meaningless without a big brain. This connection is a track worth following, but the success of the bushmen in Africa at least allowed Carrier, Bramble, and Lieberman to close their case. Humans are indeed Born to Run, to cite the title of Christopher McDougall’s popular book, which summarized their work.

  End of the trail? Not really. In our conversation, Carrier mentioned almost none of this, and in fact took issue with some work by Bramble and Lieberman that says the human gluteus maximus buttresses the case that we are born to run. He says that the muscle in question, the butt muscle, plays almost no role in running but does show up in a host of other activities, and it is those other activities that have his attention now. He launches into a line of thought drawn from a concept pivotal in the original research—an enigma, really: a notion called cost of transport.

  It’s a relatively simple concept that gets straight at the efficiency of locomotion. Imagine a graph, with one axis showing speed and the other axis graphing energy expended by the creature in motion. For most species this graph forms a U-shaped curve, and the bottom of the U is a sweet spot. At this speed, the animal in question covers the most distance with the least energy, just as a car might get its best gas mileage at, say, fifty-five miles per hour. It marks the point of maximum efficiency, the best speed in terms of units of energy expended. The very existence of the U shape says that most animals have bodies meant for a given speed, a point where energy use is minimized.

  Humans match the rule, but only when walking. That is, human walkers lay out a curve with maximum energy efficiency of about 1.3 meters per second. That speed uses the least amount of energy to cover a given distance. But running, at least for humans, does not produce a similar curve with a defined sweet spot; it yields a flat one. We have no optimum speed in terms of energy spent. Meanwhile, all other running animals—horses, dogs, deer—do produce a U-shaped curve when running. So if humans are born to run, where’s the sweet spot? Evolution likes nothing so much as energy efficiency. Species live and die on this issue alone, so why isn’t human running tuned for maximum efficiency?

  Further, the whole question offers a parallel line of inquiry, not among species but within the human body itself. That’s where Carrier is headed with this, but he first notes that the flat cost-of-transport curve for human running appears only when you summarize data for a number of humans. On the other hand, looking at data for each individual does indeed produce a U-shaped curve, but the sweet spot is in a different place for each human. That’s not true for other species, so right off, this suggests that there is far more variability in humans, and it has much to do with individual conditioning and experience.

  But more interestingly, this whole line of reasoning can be and has been examined not just between species and among individual humans but among individual muscles within a given body. Muscle recruitment and efficiency vary according to activity, even with running. Running uphill requires one set of muscles, downhill another, on the flat or side hilling different ones still. So does running fast or running slow. But further still, so does jumping. And throwing, pushing, punching, lifting, and pressing.

  Carrier says that the research on this shows no favoritism, no sweet spot according to any one activity, no real specialization, and this result is counter to what’s found with any other species. For other species, one can make a categorical statement like “born to gallop,” but for humans, no. Born to run? Yes indeed, but also born for doing other activities as well. Humans are the Swiss Army knives of motion.

  “This is not a surprise to the vast majority of people who think about what humans do, but I think it is a surprise to the folks who are so focused on the running hypothesis. We are an animal that needs to do a variety of things with our locomotive system,” Carrier says. “We do more than just walk economically and run long distances.”

  All of this movement dictates a couple of fundamental conditions of our existence: we need to take on enough nutrients (not just energy but nutrients) to power all of this motion, and we need outsize brains to control diverse types of locomotion. Thinking, creating, scheming, mating, coordinating—all those activities also require big brains, but locomotion alone is enough to seal the deal. The evolution of our unique brains was locked into the evolution of our wide range of movement. Mental and physical agility run on the same track.

  FUEL

  There is a paradox at the center of human nutrition. All the other parts of our body seem very good at what they do, are standouts in the animal kingdom, but we are truly lousy at digestion, which is limited and puny. Literally so, because we have to be lousy at it. First off, digestion is an energetically demanding process, so why burn the calories just to take on calories if there is a better solution? But second, if we are going to be able to move around rapidly upright, we need small guts, and small guts mean short intestines, less real estate for digestion. This bit of elemental engineering is a consequence of a number of design features, but the counterrotation we talked about with running is a good case in point. Unlike all the other apes, which are quadrupeds, we have a significant vertical gap between the bottom of our ribs and the top of our pelvis, the territory of the abdominal muscles. These muscles effect the leverage necessary to keep us reliably upright and control the twist of running, so we need a light, tight abdomen, or tight abs, which restricts room for intestines.

  This anatomical adjustment explains much in human makeup and behavior, but start with a simple and profound fact: our short guts mean we can’t eat grass, and this is no small thing, especially if you consider that two million years of evolutionary history occurred in savannas and grasslands. Grasslands are enormously productive in biological terms; that is, they efficiently convert solar energy into carbohydrates. But that energy is wrapped in the building block of all grasses, cellulose, and humans cannot digest it, not at all.

  Our primary method for overcoming our inability to digest is to outsource the job. Our prey animals, the ungulates—grazers and browsers, largely—happen to be very good at digesting cellulose. These quadrupeds can handle such tasks as chewing cuds, patiently feeding and refeeding wads and tangles of grass into a labyrinth of intestines contained in a monumental bulge of a gut.

  There is no ambiguity in the fossil record, in paleoanthropology or anthropology, in everything we know about the human condition, past and present. Humans are hunters and meat eaters. There is no such thing as a vegetarian society in all the record. Eating meat is a fundamental and defining fact of the human condition, at the gut level and bred in the bones.

  Discussion about this has generally been cast in terms of protein. Essential amino acids—proteins—are necessary building blocks for that highly adapted body. The only complete source of those amino acids is meat. True as that may be, it misses some essential points, as have anthropologists and nutritionists in trying to do the calculations that explain our continued existence. When we think of meat today, we think of, well, meat, defined as muscle tissue. We disregard the rest, all those other tissues of the animal body. It’s not a new mistake.

  In the nineteenth century, when Europeans were exploring North America, a few explorers and fur trappers made contact with the nomadic Indians of the northern plains, a people who, like many hunter-gatherers, lived almost exclusively off animals. The Europeans of necessity adopted that di
et and soon found themselves quite ill, even to the point of sprouting open, running sores on their faces. They were like we are today and ate only muscle meat. But then the Indians showed them the choice parts, the bits of liver and spleen, bone marrow and brain and the fat, especially the fat. The Europeans ate as they were told and got better because the organ tissue contained some essential micronutrients lacking in the muscle meat.

  The basic energetics of an animal diet involve not just protein but also and especially fat and micronutrients and minerals, a matter of bioaccumulation. Grazers store excess energy as fat, in and of itself a dense, rich source of calories to fuel our demanding bodies; but in doing so, they bioaccumulate a rich storehouse of elements like magnesium, iron, and iodine that the deep roots of grass pull from mineral soil. This is also an important factor. Certainly we could (and do) get many of these by eating plants directly, but they are far more concentrated in meat. To get everything we need from plants, we would have to eat far more than we literally have the stomach for. Further, these minerals and micronutrients tend to be unevenly distributed on the face of the planet, as any miner for magnesium, iron, or iodine will tell you. But the big grazers tend to be migratory and range over vast areas, thereby averaging out conditions and balancing geology’s uneven hand. Over time, grazing animals accumulate a full range of nutrients as no stationary plant can, and we take advantage of that life history as stored and accumulated in an animal’s body.

  Yet our need for variety and diversity in diet also shows up in our omnivorous habit. Humans have for all human time eaten a wide array of plants and wandered far and wide to gather them, and this, too, is more than a simple matter of energetics. Diversity ensures the range of micronutrients to support the complexity of the human body, the importance of which will emerge in detail as we develop this story. All of this gets greatly aided by our cultural adaption involving the use of fire, which allows cooking and so further aids in concentration and digestion. Add to this our microbiomes, which are another way of outsourcing to compensate for our poor digestive abilities. Our guts are loaded with thousands of species of bacteria that break down food and add value—a lot more than we think.

  By and large, though, these patterns—nomadism, bipedalism, and omnivory—are defining for our entire genus and have accrued over the course of two million years of hominid history. Yet there is a variation in this theme that illustrates its refinement and gets to our more central question: the difference between Homo sapiens and all other hominids, now extinct. The general approach to food outlined here is true of all the species of hominids, even Neanderthals; yet recall that our basic question is why the single species of humans, modern humans, beat out people like the Neanderthals.

  Neanderthals were indeed hunters—in fact, highly skilled hunters—and, if anything, they were more selective to very large prey animals than Homo sapiens were, meaning that Neanderthals had the skills and social organization necessary to kill elephants with spears. They had big hunks of protein and fat, the very thing that gave all hominids the edge. Neanderthals had bodies that were as upright and graceful as ours. They had plenty big brains. What they did not have, compared with the Homo sapiens of their day, was fish. More to the point, they had not learned how to tap this source of nutrition that was all around them.

  Their chief competitors, Homo sapiens, had. Evidence of fishing first appears in Africa, but only in Homo sapiens. When our species showed up in Europe and Asia about forty thousand years ago, fishing of marine and freshwater sources was widespread and important on both continents.

  This is not to argue that fish gave Homo sapiens the edge that wiped out Neanderthals, Denisovans, and Homo floresiensis, the other hominid species already in Asia and Europe then—although it’s possible. But it does signal something important to modern nutrition, especially in the case of salmon. Remember: we can prove that those ancient Homo sapiens ate fish because of chemical signatures, which is to say that some elements not present in terrestrial species were present in fish, and those elements accumulate in human bones, the fossil record. Further, anyone who has ever witnessed a salmon migration, even in today’s relatively impoverished conditions, understands that collecting this protein took almost no effort, as it was an almost unimaginable abundance. Forget persistence hunting: salmon eaters need only sit at streamside and rake it in, literally tons of high-quality protein. But each of those salmon, one of the world’s most peripatetic species, has ranged thousands on thousands of miles across diverse marine and aquatic environments during its short life cycle. That is, each fish has sampled and bioaccumulated a diverse collection of micronutrients lacking in a terrestrial diet. Remember the value of diversity realized by nomads hunting across diverse environments. Nomads eating a nomadic marine species takes that idea up a notch: nomadism squared.

  EMPATHY

  The message here is diversity, and we will hear it again. But this is a small element of the larger success of humans. The details remain somewhat in dispute, but from such evidence paleoanthropologists have through the years assembled a list of traits they believe defined us as humans. In a recent book, the British scholar of humanity’s roots Chris Stringer offered one such list, as good as any:

  Complex tools, the styles of which may change rapidly through time and space; formal artifacts shaped from bone, ivory, antler, shell, and similar materials; art, including abstract and figurative symbols; structures such as tents or huts for living or working that are organized for different activities (such as toolmaking, food preparation, sleeping, and for hearths); long-distance transport of valued materials such as stone, shells, beads, amber; ceremonies or rituals, which may include art, structures, or complex treatment of the dead; increased cultural “buffering” to adapt to more extreme environments such as deserts or cold steppes; greater complexity of food-gathering and food-processing procedures, such as the use of nets, traps, fishing gear, and complex cooking; and higher population densities approaching those of modern hunter-gatherers.

  It is a long list that accounts for much, but its elements, the traits, are derivative. They certainly derive from how we move, our athleticism, and what we eat and how we get it. But there are activities in here that do not derive from simple biological energetics, how we translate energy into life. Symbols (and remember: words are symbols, so this includes language)? Art? Music? Ritual? Clearly this list is telling us that something important and unprecedented has happened in our brains, something well beyond bipedalism, tight guts, voracious appetites, salmon, and the big brains that were characteristic of the hominid line for the preceding two million years.

  The biologically unprecedented structures in the brain that enable these abilities don’t leave much of an impression in the fossil record, so there is no hard evidence of when they appeared. We have come to know them only recently through neuroscience, an exploding field that continues almost daily with discoveries that illuminate the complexity of the brain. Yet a couple of structures, a class of cells or parts of the brain we’ve known about for some time, give us some hint as to why human abilities exploded on the scene fifty thousand years ago. For instance, since the 1920s, we’ve known about spindle neurons—a uniquely shaped set of cells that first showed up in ape brains, and to a lesser extent in dolphins, whales, and elephants, all animals known for having unique abilities. Humans have many more of them in very specific areas of the brain, and they are involved in complex reactions like trust, empathy, and guilt, but also in practical matters like planning. (You might ask why empathy and planning run together. Good question. Answer coming.)

  Add to that a related and even more wondrous set of cells that neuroscientists call “mirror neurons,” first discovered in the 1980s and ’90s by a group of scientists in Italy. These get more to the point of empathy. The term “mirror” is apt. If we monitor a monkey’s brain while the monkey is eating a peanut, the readout shows a set of firing neurons associated with activities like using a hand to pick up the peanut, chewing, and reg
istering the satisfaction delivered by the food. But if a monkey watches another monkey eat a peanut, that same set of neurons—the mirror neurons—fire in his brain, as if he himself were the one eating the peanut. This is a major part of the circuitry of empathy, which is defined as a notch up from sympathy. More than simply realizing the feelings of another, we also literally feel them ourselves.

  It would be hard to overstate the importance of this in social cohesion, but a bit of reflection shows how far this extends. It gives us some sense of another person’s story, ascribing consciousness to other beings. It allows us to understand that they do not see the world as we see it, the importance of which is best understood by observing people who do not have this ability. For instance, people who have autism are notoriously altered in this very circuitry and these abilities, which is why they don’t lie. They don’t see the point of lying, because they think everyone else knows exactly what they know.

  This consciousness of another’s point of view is exactly what enables the more elegant and refined form of lying so valuable to all humans: storytelling. It allows abstraction and conceptualization, which in turn allows language. It allows a concept of the future, which in turn opens the door to planning and scheming and is why planning is related to empathy. But it also gives us a sense that others see us, and hence body adornment shows up in the archaeological record. So does art, which is an extension of adornment but also a mode of storytelling, a symbolic representation of the world external to us.

  All of this, on the other hand, comes at a great cost. As we have said, the brain is a costly organ in terms of the energy required to keep it humming along. Any additions simply increase that load, but these are more than simple additions, more than a few more cells tucked away in a discrete corner. The activities associated with spindle and mirror neurons are characterized not by the firing of a few cells but by the assembly of networks of cells all firing in concert, a glow of energy humming around the entire brain. These, unlike many of our more mundane tasks, are whole-brain activities, heavy calculation loads. This load translates into a requirement for even more calories to support it.