For the last 12,500 years since the end of the Würm Ice Age, we have been living in a warm period, but during the Ice Ages, temperatures dropped dramatically and sheets of ice advanced. All of Great Britain was covered in a single sheet. New England too was subsumed in ice, and when the glaciers finally retreated, they left a rime of sand and rubble that we know today as Cape Cod.
With so much ice, ocean levels dropped. One could walk from France to England, or from Siberia to Alaska (and hence, into the Americas). But even as flora and fauna changed remarkably - reindeer replaced horses as the hunters' primary prey - Magdalenian man survived.
The retreat of the Würm Ice Age coincides with humanity's move from stone to bronze and the dawn of agriculture. Had the glaciers not retreated when they did, we might still be living in caves, chipping flints, and dying at twenty-five.