The past six hundred million years have been mostly a span of relentless heat, during which plants and then animals first climbed up and colonized Earth’s great, empty landmasses. Extreme heat was the backdrop for the rise and fall of the dinosaurs, and equally the setting for the subsequent ascendance of mammals. The heat reached its greatest extremes some fifty million years ago, with carbon-dioxide levels nearing 2,000 ppm (versus ~414 ppm today) around the time when our tiny, early primate ancestors were just starting to spread and diversify through the world’s forest canopies. Those early primates arose in the heat, adapted for the heat; but Earth continued to change, and the climatic conditions that gave rise to Homo sapiens would be very different. – 3 Quarks Daily