"In northern latitudes throughout the Pleistocene era, the main determinant of animal and vegetable existence was the advance and retreat of the glaciers. These frequently covered large parts of Europe, Asia and North America with impenetrable ice-sheets, locking up huge quantities of sea water, and reducing average temperatures by 10-12 degrees centigrade and ocean levels by over 350 feet- far below those of modern times. Only when they shrank back, allowing the northward spread of oak and spruce forests, and the sub-Arctic vegetation on which the mammoths and reindeer browsed, was it possible for early humans to live much outside the equatorial regions; and even then it required the discovery of fire, and the ability to sew warm clothing, before they could survive a winter in the rich, but frozen, hunting grounds."