Around 10,800 BCE, something happened to the Earth.

The climate had been warming for thousands of years, coming out of the last Ice Age. The great glaciers were retreating. Then — in the span of a few decades — the warming stopped. Temperatures dropped by as much as ten degrees Celsius (50 degrees Fahrenheit). The ice sheets stabilized. This lasted twelve hundred years. Then the warming came back, permanently, and the world we live in began.

This is the Younger Dryas. It is not disputed. It is documented in ice cores from Greenland to Antarctica.

What triggered it is the question. The Younger Dryas Impact Hypothesis argues a comet struck the North American ice sheet — leaving nanodiamonds, meltglass, and a platinum spike in a global geological layer at exactly that moment. The scientific mainstream is genuinely divided in a way  ...