During the last 2 million years the Earth has experienced four long periods of cold climate known as ice ages. During each ice age, the average temperatures around the world has droped dramatically. While the middle latitudes near the equator stay warm, the higher latitudes both in the North, and in the South get very cold, making life in these regions more difficult.
During these periods, massive glaciers form, which can cover thousands of square miles. As ice sheets spread across the landscape they freeze out plants and animals. As more and more water freezes the sea level decreases by as much as 300 feet. As sea levels drop, land bridges appear between continents and islands.
Cooler temperatures forced change on early hominids. They either had to adapt to their new environments, migrate, or perish. Land bridges allowed hominids to migrate to new lands such as the Americas, Japan, Malaysia, and Australia. Cooler temperatures spawned the development of clothes, and the discovery and mastery of fire.
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