The increase of complexity in a culture depends basically on the availability of surplus food production (1), allowing part of the community time to invent and to develop technologically and in societal specialisation. The technology, in turn, supports an increase in food production and more populous societies. Food surpluses occur where geographic factors are favourable, including climates suited to cropping and animal domestication. So agriculture first began in the 'Fertile Crescent' between the Tigris and the Euphrates, at a time when the climate there was wetter than it is now, about 8500 BC. Farming instead of hunting-gathering for food led to the establishment of villages and then cities, and thus dense habitation. The concentration of people, as well as the animal husbandry, exposed the inhabitants to epidemics which more than famines and wars controlled the population, be it at irregular intervals. The surviving population became immune to some of the epidemics. Voyagers from the cities inadvertently infected and hence decimated hunter-gatherer societies lacking that immunity . That added to the technological and bureaucratic advances of cultures with surplus food production in enabling them to engulf less fortunate societies. Meanwhile, the climate of the Fertile Crescent became drier, food production there declined and consequently power shifted away to the Mediterranean, which is wetter.
It is interesting that this westward drift of cultural dominance was matched by the trend of climatological studies, which were concentrated first in Egypt, then Greece, then Rome, Italy, France, Germany and then the English-speaking nations (2).
(1) Diamond, J. 1998. Guns, Germs and Steel (Vintage) 480pp.
(2) Linacre, E.T. 1992. Climate Data and Resources (Routledge) p.4