Marking the passage of time has played a role in every great civilization, and as civilizations have continued to evolve, they have also developed a need for more precise timekeeping. The Sumerians, in early Mesopotamia, were content to track the months and years—as early as 3500 BCE—while the Egyptians, a few centuries later, used giant obelisks to track the time during the day. Even within civilizations, the marking of time continued to advance. The Egyptians moved from obelisks to more precise and portable sundials that divided days into 12 parts and were used to track working hours. Shadow clocks allowed Egyptians to track time day and night, and water clocks were used to track time irrespective of the sun.