If the existence of a writing system is a condition, then it cannot go beyond the fourth millenium BCE. So what about Gobekli Tepe and a few other CITIES in Anatolia? and that excludes all cities in Black Africa because they did not have any writing system. And what about Asia? When was a writing system devised there? And what about the various scribbling artifacts next to the paintings in the various caves in Europe or Indonesia? They are not a writing system, are they? What is a writing system? A way to record some information and to transmit it to other people on some more or less durable medium. Sumerian is nowadays known as not being dated back to 3,500 BCE but it is going a long way before this date, and as a linguist I will say any representational or non-representational code is a recording code for information and its transmission, hence is a WRITING system. It CANNOT be a criterion for the existence of cities. Childe either is not a linguist (which is probably true), or he is just incompetent on this criterion.