66 Million Years Ago - Present Day
The Cenozoic Era began after the impact of an asteroid wiped out the dinosaurs, enabling mammals to proliferate, flourish, and grow in size. One example is the giant beaver, Castoroides. Most fossils of Castoroides have been found in states bordering the Great Lakes, from Illinois to Ohio, but their fossils range from Alaska to Florida, dating from more than one million years ago to 11,000 years ago. Humans arrived in Illinois around 12,000 years ago, in time to hunt megafauna from mammoths to giant ground sloths as Ice Age glaciers receded.