FORTIFY, FORTIFY: The World’s Oldest Settlements Were Built by a Culture Nobody Expected.

Plus: “Several other Stone Age forts have been found in this region of the world, but none are as old as the Amnya I site. In Europe, comparable sites don’t show up until centuries later and only after the dawn of agriculture. . . . Researchers suspect that a shift in climate roughly 8,000 years ago created the stage for an abundance of seasonal resources in western Siberia, prompting an influx of human migrants.”

This was during the Holocene Climatic Optimum, when the climate was significantly warmer than today.