Various theories have been proposed to explain how this social complexity developed and why it developed in some areas and not others, but archaeologists and historians have not formulated any one satisfying explanation.
Nick Brooks, a climate-change researcher at the University of East Anglia in England, offers his idea in the latest issue of Quaternary International.
"The emergence of complex societies coincided with or followed a period of increased aridity," which began 8,000 years ago but intensified periodically in subsequent millennia, he said.
That's been one of the main "prime movers" of explanations involving cultural complexity for quite a while. But then one has to consider the New World where the earliest true complex cultures didn't develop until several thousand years later.