A new understanding of how our bodies work, how to keep them healthy, and how our biological diversity unites us rather than divides us How does the body work--and why does it seem to work so differently for each of us? Why do we grow tall or short, obese or slim? Why do some of us stay healthy despite our bad habits while others who do all the right things fall ill? When we look around the planet, why do people vary in skin color, facial features, stature, body proportions, and disease risk? ; ;;The answer is both simple and powerful: We’re different because we’re adaptable. Over the past 100,000 years, as humans expanded into every biome on the planet, our bodies were fine-tuned to our local environments. Adaptability is at the heart of being human and the engine of our diversity – our species’ original superpower. As an evolutionary anthropologist working with human populations around the globe, Herman Pontzer has conducted research that embraces our incredible diversity, documenting the connections among lifestyle, landscape, local adaptations, and health. 1
So Simple a Beginning

How old are you?

If you're reading this book the answer probably comes easily. Your birthday was one of the first dates you ever committed to memory, the promise of presents and cake cementing the coordinates into your internal calendar. As an adult that day might be less meaningful, or even cause for despair. But you know it. You know exactly how old you are.

It might surprise you, then, that many people around the world today don't have a precise idea of their age. Much of my research takes me to communities like these, that don't keep calendars or track time the way we do in the industrialized world. It was a bit of a shock the first time I encountered it.

I was living and working with the Hadza community in the dry, wide-open savanna of northern Tanzania. It's a small and tight-knit population. Only about 1,200 people consider themselves Hadza and speak the language. The Hadza are hunter-gatherers, living in small grass houses clustered in camps among the acacia and baobab trees, making their living off of the wild landscape around them. Every morning, women leave camp in groups, on foot, to forage for berries or wild tubers. Men head out, usually alone, to hunt for antelope, giraffe, and other wild game using bows and arrows they make themselves. In a world that has rapidly moved to farms, then towns, and then to industrialized cities, the Hadza have maintained their traditional way of life for generations, perhaps millennia. There are no domesticated crops or animals in a Hadza camp, no electricity or plumbing, no guns or cars. And no calendars.

Settling into camp life on my first trip there, I would see kids running around, hanging out with mom and dad, sharing a meal around the fire. Trying to make a bit of friendly conversation, with the best Swahili (our common language) that I could manage, I'd ask a mom or dad how old their son or daughter was. The usual answer was a shrug.

Who doesn't know how old their kids are? I would wonder. Then I encountered it again when we interviewed the adults and asked their age. No one knew. Our research team spent days each season working to figure out everyone's age.

It's a common theme in fieldwork with nonindustrial communities. I've encountered the same issue working with traditional pastoralists in northern Kenya, and my collaborators have had similar experiences all over the world. Accurate calendars that track our planet's orbit around the sun are a relatively recent invention of farming cultures that needed to know precisely when to plant their crops and when to harvest and prepare for winter. Lunar calendars are more common, and potentially more meaningful. Their cycle seems more in tune to the rhythms of daily life and our attention spans, and they determine how light the nights are in cultures without electricity. We've even kept them embedded in our Western calendars, though we've stretched them into months.

It's easy to be a bit smug about the tools we have to track time, as though by measuring it accurately we might exert some control. But do you really know how old you are, even with your calendar? You know when you were born, of course, but what about all the stuff that happened before that? The day you were born, your body had already been under construction for months. In some ways, it's been under construction for more than a billion years.

Endless Forms Most Beautiful

Watching the body develop is a bit like taking a walking tour of the history of life. You watch a single cell become a multicellular cluster, then fold itself into a simple tube with a gut and basic nervous system, grow a head with eyes and a mouth, and sprout limbs. We see vestiges of ancient structures as the embryo develops, hints of old designs that were once the norm but have since been abandoned or changed beyond recognition. A yolk sac forms in the first few weeks only to dissolve away. The jaw, ears, and throat grow from fleshy arches that would have become the gills of our fishy ancestors.

There's an old-fashioned idea that we pass through each stage of evolution as we develop. "Ontogeny recapitulates phylogeny" is how I learned it. That's not quite right-you don't pass through a fish stage, or a reptile stage. But the layered nature of evolution, each generation tweaking and adding to what came before, is obvious from our embryos.

About nine months before birth, somewhere near the outer reaches of one of mom's fallopian tubes, a sperm breaks through the wall of a recently ovulated egg, fertilizing it. (If you're not clear on how we get to this step, you might check out chapter 7 or schedule an awkward talk with your parents.) DNA carried by the sperm and egg, packaged in tightly wrapped chromosomes like skeins of yarn, will combine to become the embryo's genome, unique among all life, with the recipe for a human written in a four-letter code some 3 billion units long. After some gymnastics to package the twenty-three chromosomes from the sperm and the twenty-three from the egg into a single forty-six chromosome nucleus, it's time to grow.

That one cell divides into two, which each divide to make four, then eight, sixteen, thirty-two . . . all the while floating lazily along the fallopian tube, toward the uterus. At around sixty cells big, the cluster of cells fills with fluid, like a new soccer ball that somehow learned to inflate itself. Around five days after fertilization this fluid-filled ball of cells lands upon the uterus wall and burrows in, implanting. The cells embedded in the wall of the uterus will grow into the placenta, nestling up against the blood-rich walls of the uterus to pull in oxygen and nutrients. Only a tiny clump of cells on the inner wall will grow into the body.

At about three weeks after fertilization, that clump of cells grows and organizes itself into three distinct layers (picture a stack of three slices of bread). This three-layer sandwich curls to form a roll, and the layers form concentric tubes. The bottom layer becomes the innermost tube, the top layer becomes the outer tube, and the middle layer rests between them. That inner tube will become your digestive tract, from mouth to butt. And that's the basic body plan for fish, birds, reptiles, and mammals like us. We are all just elaborate tubes.

At the same time your main body tube is forming, a second tube forms down the middle of the outer layer. A deep groove develops along the surface, marking the track that will become your spine. The sides of the groove rise and crash into one another like cresting waves. They fuse together where they meet, converting the open gully into a closed "neural tube." As the name suggests, the brain and spinal cord will fill the neural tube eventually. Production of neurons, the cells that store and…