The following is taken from Endless Forms Most Beautiful, the New Science of Evo Devo by Sean B Carroll, published in 2005, at page 260:
“About 2.3 million years ago, there began a global shift toward a cooler and drier climate. This caused the forests of Africa to shrink and to be replaced with a drier savannah. While great apes stayed in more stable rain-forest habitats, hominins adapted to more variable habitats. After a period of relative stability, in the last 700,000 years the Earth’s climate has, on average, been colder than during any other period since the extinction of the dinosaurs 65 million years ago. Abrupt fluctuations in temperature have occurred many times, with some major shifts taking place in the course of just several years. The changing climate and its effects on food and availability, water, hunting, and migration may have selected for hominins better adapted to such constantly changing conditions. Under the changing climate, brain size has roughly doubled in a million years,encompassing perhaps 50,000 hominin generations.”
This has been your climate awareness missive for the day. You may go now.