When the period of written history began in the sixth century B.C., Europe had already been inhabited by members of the human species for a million years or more. During this long span of time they had seen the ice sheets advance from the north and then melt away at least four times. They had been obliged to adjust, both physically and mentally, to this changing environment, and in doing so had gradually raised the level of their own skills and of the control which they were able to exercise over their surroundings. Nevertheless, their levels of cultural development varied from one part of the continent to another. Most of the significant advances in man's material culture, like agriculture and the smelting of metals, had been made in the Middle East and had entered Europe through the Balkan peninsula. From here they had been diffused northwestward to central Europe and then to western. There was always a steep cultural gradient between the more developed regions, like Greece and the Aegean, and the least, such as Scandinavia and the Atlantic periphery of Europe. Such differences, it might be thought, would be bridged in time, as the more advanced cultures spread outward like ripples from their hearth in the Aegean region. But such a leveling up did not, and indeed could not, take place. The receptivity of Europe to new cultures and new techniques itself varied too greatly.