The correct answer to this open question is the following.
Although the question is incomplete because you did not attach references to the lesson, we can say the following.
One Mesopotamian contribution that I think had the biggest impact on human history was the development of agriculture techniques and the knowledge to know about the flooding of the Tigris and Euphrates Rivers, so Sumerians could use it for growing crops and to avoid some catastrophes.
The Sumerians were the oldest civilization on planet Earth. They settled in the middle of the above-mentioned Rivers in the Middle East region, modern-day Iraq. They developed great knowledge to built impressive buildings called Ziggurats and formed important city-states such as Uruk, Kish, Ur, and Lagash. Agriculture techniques were a big reason they could settle in these regions.
