Dirt: The Erosion of Civilizations - Kootenay Local Agricultural Society

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Ice Age was not a single event. More than twenty major glaciations repeatedly
buried North America and Europe under ice, defining what geologists
call the Quaternary—the fourth era of geologic time.
At the peak of the most recent glaciation, roughly 20,000 years ago, glaciers
covered almost a third of Earth’s land surface. Outside of the tropics
even unglaciated areas experienced extreme environmental changes. Human
populations either adapted, died out, or moved on as their hunting
and foraging grounds shifted around the world.
Each time Europe froze, North Africa dried, becoming an uninhabitable
sand sea. Naturally, people left. Some migrated south back into Africa.
Others ventured east to Asia or into southern Europe as periodic climate
upheavals launched the great human migrations that eventually circled the
world.
Judged by the fossil evidence, Homo erectus walked out of Africa and
ventured east across Asia, sticking to tropical and temperate latitudes
about two million years ago just after the start of the glacial era. Fossil and
DNA evidence indicates that the initial separation of Neanderthals from
the ancestors of genetically modern humans occurred at least 300,000
years ago—about the time Neanderthals arrived in Europe and western
Asia. After successfully adapting to the glacial climate of northwestern
Eurasia, Neanderthals disappeared as a new wave of genetically modern
humans spread from Africa through the Middle East around 45,000 years
ago and across Europe by at least 35,000 years ago. People continued
spreading out across the world when the Northern Hemisphere’s great ice
sheets once again plowed southward, rearranging the environments of
Europe, northern Africa, and the Middle East.
During the most recent glaciation, large herds of reindeer, mammoth,
wooly rhinoceroses, and giant elk roamed Europe’s frozen plains. Ice covered
Scandinavia, the Baltic coast, northern Britain, and most of Ireland.
Treeless tundra stretched from France through Germany, on to Poland and
across Russia. European forests shrank to a narrow fringe around the
Mediterranean. Early Europeans lived through this frozen time by following
and culling herds of large animals. Some of these species, notably
wooly rhinos and giant elk, did not survive the transition to the postglacial
climate.
Extreme environmental shifts also isolated human populations and
helped differentiate people into the distinct appearances we know today as
races. Skin shields our bodies and critical organs from ultraviolet radiation.
But skin must also pass enough sunlight to support production of the
rivers of life