3.8 to 2.5 billion years ago
During the Archaean, the landmass that was to become North America began to form. The ancient rocks of the Canadian Shield, which makes up most of northern Quebec and Ontario, were formed as fragments of continental crust collided and grew. This "heart" of the continent extends into northern Minnesota. There are no rocks of this age present in the Ozarks.
Fossil blue-green algae and spheroidal bacteria are found in rocks as old as 3.0 to 3.5 billion years. Life had gained a foothold on earth.