Answer:
Beginning around 5,500 years ago
Step-by-step explanation:
Sumerians became significant during the start of their civilization.
The Sumerian civilization started at around 400 B.C
This civilization started at the flood plains along the river Euphrates and Tigris.
The Sumerians build cities along rivers in the lower Mesopotamia and also advanced technologies.
They improved transport on land and also excelled in metal work.