Becoming Human: Evolution in Action

Humanity traces back to East Africa more than seven million years ago, when some early primates—mammals like humans, monkeys, and apes—began adapting to life outside the rainforest. The thick canopy of trees that made up their habitat was shrinking as the climate in many parts of Africa became cooler and drier. They eventually became suited to living on grasslands called savannas. The small genetic adaptations help explain how humans evolved over time.

Life in the savanna was hotter, and food and water were less abundant. Even more dangerous, they now lived in more open land where there were few trees to escape to. These primates were not particularly fast or muscular, and they lacked the mental abilities of modern humans, but some were better suited to these conditions and survived at a slightly higher rate over time.

Over thousands of generations, our earliest ancestors began to walk upright. Standing taller allowed them to walk long distances to safety and food. It also allowed them to see over tall grass. And with their hands now free, they could carry food, infants, and tools. Fossil evidence from four to six million years ago shows a transition toward bipedalism. Their knees could fully straighten, allowing them to stand taller and to support their full weight on only two legs.

This upright position came at a cost. Bipedalism slowed movement and made climbing trees harder. It also put more stress on the spine, hips, and knees. Today, older adults frequently suffer from arthritis, a consequence of our upright stance.

Dense fur traps heat, and in open environments like the savanna, cooling becomes essential. Our ancestors began to lose most of their fur, except for the tops of their heads, which still faced the sun directly. Cooling instead came from sweat glands, which cool the body as moisture evaporates from the skin.

On the savanna, the diet of these early humans changed. The roots and tough leaves were less plentiful, so they supplemented their diet with meat. They were not powerful or fast enough to compete with other predators, so they sometimes scavenged what other animals left behind.

Early humans learned to control fire about one million years ago. Cooking meat over a fire helps kill harmful bacteria, making it safer to eat. Cooked meat also meant their digestive tracts could shrink, leaving their bodies with energy to support an even greater evolutionary adaptation.

As their jaws became smaller and digestion required less energy, more energy was available for the brain to grow. The brain consumes a tremendous amount of energy, so the former scavengers slowly became more active hunters. But the brain could only grow so large and still allow childbirth.