It is not easy to overemphasize the importance of climate in human history, particularly in the earliest times when man was merely a numerically unimportant parasite in the total fauna. With changes in climate, he was forced to migrate with the animals and plants on which he lived, and at the hunting and gathering of which he was adept. The only alternative was to stay on and adapt his culture to a new food supply, which would need new implements and new methods. On the whole, it was easier to move, even if some of the oscillations were, like those in recent times, rather rapid.

The ponderous ebb and flow of the glaciers caused climatic changes which affected the entire world. With the gathering of vast quantities of ice near the poles, zones of climate shrank inward, converging on the equator. At times of maximum glaciation, wide belts of land bordering the glaciers became treeless, frozen tundras, like the northern rims of Siberia and North America today. During the last glaciation, such a zone included the whole of Europe north of the Alps and Pyrenees, and much of Siberia. Below this stretched temperate forests, with zones of willow and birch, of pine, and of hardwood, and beyond these, temperate, grassy plains, watered by cyclonic rain belts. Still farther away, near the equator, stood tropical forests. The present deserts had shrunk to narrow patches between the grasslands or had disappeared.

As the glaciers retreated, the zones of tundra followed, constantly shrinking as the ice cap thinned. The forest encroached on the tundra belt, and the grasslands likewise moved inward; at the same time the tropical forest shrank, and the land in between two belts of grassland became desert. What had once been the optimum home for food gathering man now became bare and sterile, and remained virtually unoccupied until the rise of pastoral nomadism, with ass and camel, once more made it habitable.

The centers of Pleistocene glaciation were not located exactly on the poles. In the northern hemisphere, the center was in the north Atlantic, with land nuclei in Scandinavia, northern Britain, and Greenland, so that northwestern Europe and northeastern America were covered, while territories of higher latitudes, in eastern Europe and Siberia, and in Western North America, were left bare. In Europe, the ice covered, at its maximum, all of the British Isles but the southwestern tip of Great Britain; most of Belgium, Holland, northern Germany, the Baltic States, and Finland, as well, of course, as Scandinavia. Secondary centers of glaciation, based on altitude rather than latitude, lay in the Alps, Pyrenees, and Caucasus, in the Himalayas and Pamirs, in the mountain skeleton of Siberia, and in the Atlas mountains of North Africa.

These ice caps, and the surrounding zones of cold, acted as barriers to the naked hunters of the Early and Middle Pleistocene. In Europe, no sure instance has been established of a Lower or Middle Palaeolithic find in a glacial context; before the first Würm glaciation, human beings and related primates gave the ice a wide berth.

During the entire span of the Pleistocene up to the fourth or Würm glaciation, bands of human beings, probably including both sapiens and non-sapiens forms, shifted slowly from continent to continent with the changes of climate. During the fourth glaciation, the parts of Europe and Asia immediately south of the ice sheet, and in the tundra belt, were for the first time, under such conditions, inhabited. This was by Neanderthal man, who lived in caves, warmed himself over fires, and could, judging by his tool kit, dress skins, although, in default of needles, he was probably a poor tailor. The European branch of this species was a marginal, primitive form, and barely survived the fourth ice. During the Laufen interglacial, Neanderthal was replaced in Europe by pure and mixed sapiens men coming from the east in several waves. With the last major ice advance, Würm II, sapiens man stayed on, for by now he had developed the knowledge and skill to make warm clothing, as numerous skin-working tools and fine bone needles attest.

In the meanwhile, other sapiens men must have lived in more favorable climates, as much on vegetable food as on meat. Some of these developed the microlithic cultural technique, which involved striking off small blades for composite instruments, and this spread to Europe north of the Pyrenees only after the retreat of the last ice. These sapiens men were, as we shall see, quite different from those in the North. The post-glacial movements of human groups completely changed the racial complexion of much of the habitable earth.