We humans evolved about 130,000 years ago in East Africa; at that time, the last of the Pleistocene's warm interglacial periods was underway. As a result, for the first 50,000 years of our history, humans were confined to Sub-Saharan Africa, hemmed in by vast deserts to the north and deep seas to the east, south and west.
Then, about 80,000 years ago, the climate started to cool and the Wisconsin Glaciation began. As the Continental and mountain glaciers enlarged, sea levels fell and a cool, wet climate caused vegetation and active streams to appear across the deserts. In concert, humans spread from Africa, moving along the southern coast of Asia and into the Middle East; Pleistocene land bridges opened as sea levels fell, providing access to southern Indonesia, Japan and the Americas. By the peak of the Wisconsin Glaciation, some 20,000 years ago, man had colonized all Continents except Antarctica.
From that nadir of the global climate a steady warming occurred, glaciers retreated and sea levels rose. Accelerated by human activity, especially by our use of fossil fuels, global warming is now threatening polar ecosystems and will soon have dramatic effects on island nations and coastal regions across the globe. As sea levels rise by 25 feet between now and the end of the Century, lowlands will be inundated by the oceans and saltwater will spread far into estuaries, deltas, coastal wetlands and our larger rivers. Humans living in coastal areas will be displaced inland, triggering the second major climate-related migration in our history; there they may face other challenges imposed by global warming, including droughts, severe storms and food shortages. Mass extinction, induced by the alteration or destruction of natural habitats, will unfold as well, placing additional stress on human populations.
