10,000 years ago North America suffered a perfectly natural climate change

About 10,000 years ago, the North American climate began to change: the world became dried and the great grasslands and forests began to shrink. The continental ice sheets had withdrawn and the melt-water from them filled basins and the shorelines of the continents began to change. These new climatic and geographic conditions altered the animal life and also forced the people into different ways of life. There was a relatively sudden disappearance of many types of animals. Most of these animals were the so-called megafauna, or large mammals such as the mastodons, mammoths, giant ground sloths, and other large-bodied herbivores. As these animals became extinct, so too did many of the carnivores that preyed upon them - the Alaskan lion, the saber-toothed cats, and the dire wolf, to name a few. Other species did not become extent, but underwent rapid selection for smaller forms. For example, the giant bison of the late Ice Age was replaced around 12,000 years ago by forms directly ancestral to the modern bison. And still other life forms, such as small mammals, reptiles, birds, insects, amphibians, fish, and most plants were relatively unaffected. Exactly why these animals died out is subject to debate. At one time, some scientists argued that the ancestral American Indians of the Paleo-Indian period hunted these animals into extinction. But today such an hypothesis receives little support. Instead, modern scientists point to a period of rapid global warming at the end of the Ice Age. As the climate changed, sea levels rose, growing seasons became longer, and snowfall and annual precipitation decreased significantly. While many smaller animals could adapt to these shifitng conditions by modifying their ranges, the larger ones, placing greater demands on their environments, could not cope with the transforming world and were pushed beyond the brink to extinction.