Isotope evidence for agricultural extensification reveals how the world's first cities were fed.

This study sheds light on the agricultural economy that underpinned the emergence of the first urban centres in northern Mesopotamia. Using δC and δN values of crop remains from the sites of Tell Sabi Abyad, Tell Zeidan, Hamoukar, Tell Brak and Tell Leilan (6500-2000 cal bc), we reveal that labour-intensive practices such as manuring/middening and water management formed an integral part of the agricultural strategy from the seventh millennium bc. Increased agricultural production to support growing urban populations was achieved by cultivation of larger areas of land, entailing lower manure/midden inputs per unit area-extensification.