… is from pages 163-164 of Matt Ridley’s superb 2010 book, The Rational Optimist:
To argue, therefore, that emperors or agricultural surpluses made the urban revolution is to get it backwards. Intensification of trade came first. Agricultural surpluses were summoned forth by trade, which offered farmers a way of turning their produce into valuable goods from elsewhere. Emperors, with their ziggurats and pyramids, were often made possible by trade. Throughout history, empires start as trade areas before they become the playthings of military plunderers from within or without.