The literature of the Sumerians was not limited to stories, though. In fact, one fact of modern life is still as Sumeria designed it. The Sumerian priesthood developed an early mathematics for commercial and religious purposes, and their number system was based around the number 60. Thus time itself is divided into units of 60 seconds in a minute, 60 minutes in an hour, and so forth. The 24-hour day also originates with Sumerian calendars, as does the 360-degree circle. So the fingerprints of the world's most ancient civilization are still visible today. Sumeria's end came gradually, as barbarians saw what they had achieved and strove to emulate it. About 1,500 years after the first Sumerian cities took form, Amorite nomads settled near Ur in a place called Babylon. The resulting civilization grew into a mighty kingdom in its own right, and when Elamite invaders (from modern-day Iran) sacked Ur, Babylon (which then still styled itself Akkad) saw its opportunity to eclipse its sire. They went in, dispersed the Elamites, and rebuilt Ur in their own image. Sumerian culture and language persisted, but slowly assimilated into the new dominant empire. Sumeria disappeared completely as an autonomous culture by 1900 BC.