You might be worried about climate change today, and you’d be right to be. A shifting climate threatens our food supply. But this is an old story. For our ancestors, climate change wasn’t a future threat but a constant, pressing reality that reshaped their world and, in turn, reshaped them. The emergence of nearly modern human bodies wasn’t a simple, linear progression towards us. It was a radical, messy, and ingenious response to a planet growing colder and drier, forcing a fundamental rethink of the most basic question of all: “what’s for dinner?”
For millions of years, our ape-like ancestors, the australopiths, had gotten by. As their forest homes in Africa shrank, they adapted. Some developed the ability to walk upright, allowing them to forage more efficiently in the thinning woodlands. Others evolved massive jaws and molar teeth, like living grindstones, to crush the tough, fibrous plants that were replacing the soft fruits they preferred. But around 2.5 million years ago, the cooling and drying trend accelerated. The world entered the Ice Age. Vast grasslands, like the set of The Lion King, spread across Africa, creating a landscape of scarcity and opportunity.
In this new world, the old strategies were not enough. A new way of life began to take shape, one that would define the human genus, Homo. This wasn’t just a tweak to the old menu; it was a revolutionary business model called hunting and gathering. It was an integrated system of four interlocking parts: gathering difficult-to-acquire plant foods; acquiring meat through hunting and scavenging; an unprecedented level of cooperation and food sharing; and, crucially, using tools to process food before it even touched the lips. This package of behaviors marks the true beginning of the human story. The key adaptation that made it possible wasn’t a bigger brain, at least not at first. It was a new kind of body.
A world of scarcity, then, forced a revolutionary new strategy for survival. The hunter-gatherer lifestyle was a life on the move. Modern hunter-gatherers walk between 9 and 15 kilometers (about 6 to 9 miles) every single day, and we have every reason to think our ancestor, Homo erectus, did the same. This relentless trekking in the hot, open savanna drove the evolution of a body fundamentally different from that of the australopiths. The most obvious change was in the legs, which grew significantly longer. Longer legs mean longer strides, which dramatically cuts the energy cost of walking. Their feet also became fully modern, with a strong, developed arch—a feature we can see preserved in 1.5-million-year-old footprints that are indistinguishable from our own. This arch acted like a leaf spring, storing and releasing energy with each step, making walking more efficient.
But walking was only half the story. The real puzzle of early human hunting is how our ancestors, who were slow sprinters compared to any four-legged prey, could possibly catch an animal. They were armed with little more than sharpened sticks and rocks. The solution lies in one of our most underappreciated talents: endurance running. You can’t out-sprint a zebra, but you can out-run it. This is the logic behind persistence hunting. Most mammals cool themselves by panting. But they cannot pant and gallop at the same time. Humans, by contrast, cool themselves by sweating from millions of glands all over our nearly hairless bodies. A persistence hunter could track an animal in the midday heat, chasing it just fast enough to force it into a gallop. The animal would sprint away, then stop to pant, but before it could fully cool down, the hunter would appear again, forcing another gallop. After several cycles of this, the animal’s body temperature would rise to lethal levels, and it would collapse from heatstroke. The hunter could then dispatch it safely.
This bizarre, brilliant strategy left its mark all over our anatomy. Many features that seem random are, in fact, elegant adaptations for endurance running. Your gluteus maximus—the largest muscle in your body—is barely active when you walk, but it fires powerfully when you run to keep your trunk from pitching forward. It’s a running muscle. We also evolved a unique piece of anatomy called the nuchal ligament, a kind of biological rubber band that runs from the base of the skull down the neck. It’s absent in apes and australopiths. When you run, it uses the bobbing motion of your arms to passively stabilize your head, keeping your vision steady without conscious effort. Even your nose is a running adaptation. The projecting external nose, another feature that appears with Homo, creates turbulence in the air you breathe. This helps humidify dry air on the way in, protecting your lungs, and recapture moisture on the way out, conserving precious water during a long, hot run. We are, quite literally, born to run.
This new lifestyle, in turn, selected for a body built for endurance. The hunter-gatherer package required more than just legs. It demanded new skills and new tools. The human hand, with its long, opposable thumb and short fingers, is perfectly adapted for a powerful precision grip. This grip allows us to both make stone tools with forceful hammering and use the resulting sharp flakes with delicate control. Archaeologists find the first simple stone tools, called Oldowan tools, appearing around 2.6 million years ago. These weren’t just for butchering animals. They were also for processing tough plant foods like tubers, pounding them to break down indigestible fiber.
This simple act of processing food outside the body had a profound consequence. By pre-digesting their food with tools, early humans off-loaded the work their jaws and teeth used to do. As a result, selection could favor smaller teeth, smaller chewing muscles, and a smaller, flatter face. We are the only primates without a snout, in part because our ancestors invented cutlery.
This dietary shift led to an even more important trade-off, deep within the body. Brains and guts are both metabolically expensive tissues; they burn a lot of calories just to maintain. You can’t afford to have both a large gut and a large brain. The high-quality, easy-to-digest diet of a hunter-gatherer—rich in meat and processed plants—required a much shorter, simpler digestive tract than the diet of a plant-chewing australopith. The energy saved by shrinking the gut was reallocated to fuel a bigger brain.
This wasn’t an overnight change. The brain of Homo erectus, at around 600 to 1,000 grams, was only modestly bigger than an australopith’s, but it was a crucial step. A bigger brain was useful for managing the complex social dynamics of cooperation and food sharing, for remembering landscapes, for tracking animals, and for planning for the future. Once the new diet made a bigger brain metabolically possible, the demands of the hunter-gatherer lifestyle made that bigger brain advantageous. This set in motion a feedback loop that would, over the next two million years, continue to shape the human mind. The story of our evolution is the story of a new relationship between our bodies, our food, and our minds, born from the challenges of a changing world.