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Ice Ages

During at least five prolonged periods, from as early as 2000 million years ago, the temperature of the earth's atmosphere has fallen and the ice caps of the two poles have extended menacingly to envelop normally fertile regions of the globe.

At the peak of an ice age the ice shelves and glaciers (more extensive and thicker than at other times) trap a much higher proportion of the earth's water - resulting in a fall of about 150 metres in the level of the sea. But ice ages also have warm 'interglacial' periods - in one of which we are living. This particular one began about 10,000 years ago.

Geological evidence suggests that there have so far been five great ice ages, some of them lasting as much as 60 million years. They begin approximately 2000 million years ago, 670 million years ago, 420 million years ago, 290 million years ago, and 1.7 million years ago.

The most recent great ice age, of which we are still a part, has much influenced the evolution and habitat of the animals known to us today. In the warm interglacial periods the hippopotamus, elephant and leopard are at home in north America or northern Europe; in the glacial periods other species (such as the woolly mammoth and the woolly rhinoceros, both probably hunted to extinction by man) occupy the same regions.

The period sometimes referred to as the Great Ice Age, from about 1.7 million years ago, has seen the spread of mankind from Africa and throughout the world. But the colonizing of the last habitable continent, America, does not take place until the most recent glacial period (about 30,000 to 10,000 years ago).

The sudden and rapid development of human society begins after the end of the recent glacial period. The past 10,000 years cover the full span from the first settled farming of the Neolithic Revolution up to the elaborate complexities of modern life. Scientists calculate that we have perhaps 20,000 years before the onset of the next glacial period.

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