Grinding stones--first used by Plano foragers around 7500 BC-were soon adopted by indians in many locations to grind seeds as well as meat, for people across the continent were resorting by necessity to a more varied diet. It included sundry wild plants as well as small game such as deer and rabbits. This shift reflected a change of far-reaching significance in the North American environment-the dwindling and ultimate disappearance of the big Ice Age mammals. The extinction of the mammoth was just one instance of this mysterious trend. Not long thereafter, mastodons vanished from the forests. By 6000 BC two-thirds of all New World species weighing more than 100 pounds at maturity had become extinct, including the giant beaver and giant ground sloth, and carnivores such as the dire wolf, saber-toothed tiger, and short-faced bear. On the Great Plains, no herbivore heavier than the bison survived the transition. Two venerable grass eaters with links to the Old World-the horse and the camel-were also lost from the American landscape.
For the horse, it marked the second such extinction in its remarkable existence. Like the camel, it had first evolved in the New World millions of years earlier and spread to Eurasia over the land bridge during an epoch of glaciation. Its descendeants there had then roamed back across Beringia to repopulate the New World after the North American horse died out for the first time. After the second extinction around 9000 BC, more than ten millienia would pass before the horse was reintroduced to the Americas by European explorers. It would be a homecoming of great significance, for the horse was destined to transform the lives of the Plains Indians.