Seventy-four thousand years ago, humanity nearly went extinct. A super-volcano at what's now Lake Toba, in Sumatra, erupted with a strength more than a thousand times that of Mount St. Helens in 1980. Some 800 cubic kilometers of ash filled the skies of the Northern Hemisphere, lowering global temperatures and pushing a climate already on the verge of an ice age over the edge. Some scientists speculate that as the Earth went into a deep freeze, the population of Homo sapiens may have dropped to as low as a few thousand families.

The Mount Toba incident, although unprecedented in magnitude, was part of a broad pattern. For a period of 2 million years, ending with the last ice age around 10,000 B.C., the Earth experienced a series of convulsive glacial events. This rapid-fire climate change meant that humans couldn't rely on consistent patterns to know which animals to hunt, which plants to gather, or even which predators might be waiting around the corner.

How did we cope? By getting smarter. The neuro­physi­ol­ogist William Calvin argues persuasively that modern human cognition—including sophisticated language and the capacity to plan ahead—evolved in response to the demands of this long age of turbulence. According to Calvin, the reason we survived is that our brains changed to meet the challenge: we transformed the ability to target a moving animal with a thrown rock into a capability for foresight and long-term planning. In the process, we may have developed syntax and formal structure from our simple language.