Final answer:
The key characteristic of the early Sumerian period of Mesopotamian culture is ziggurats.
Step-by-step explanation:
The key characteristic of the early Sumerian period of Mesopotamian culture is ziggurats. Ziggurats were massive structures in the form of terraced step pyramids with a shrine or temple at the summit. They were built using stacked and piled mud-brick layers. Ziggurats were not places of worship for the general public, but only authorized religious officials were allowed inside. They symbolized the political and religious authority of the gods or goddesses honored in the temples.