Our human ancestors had it rough. Saber-toothed cats and woolly mammoths may have been dayto-day concerns, but harsh climate was a consuming long-term challenge. During the past million years, they faced one ice age after another. At the height of the last icy episode, 20,000 years ago, glaciers more than two kilometers thick gripped much of North America and Europe. The chill delivered ice as far south as New York City.