The earliest known language of Mesopotamia is Sumerian. Sumerian is considered to be one of the oldest written languages in human history and was spoken in ancient Mesopotamia, which is now modern-day Iraq and parts of Iran and Syria. Here is an explanation of why Sumerian is considered the earliest known language of Mesopotamia:

- Sumerian civilization: The Sumerians were one of the earliest civilizations in Mesopotamia, dating back to around 4000 BCE. They developed a complex society with advanced agricultural practices, city-states, and a written language.

- Cuneiform script: The Sumerians are credited with the invention of cuneiform, one of the earliest writing systems in the world. Cuneiform was initially used for accounting and record-keeping purposes and eventually evolved into a full-fledged writing system for various purposes, including literature, law, and religion.

- Sumerian literature: Sumerian texts, written in cuneiform, provide valuable insights into the language and culture of ancient Mesopotamia. These texts include epic poems like the Epic of Gilgamesh, hymns, myths, and legal and administrative documents. The preservation of these texts has allowed scholars to decipher and understand the Sumerian language.

- Influence on other languages: Sumerian had a significant influence on later languages in the region. As neighboring civilizations interacted with the Sumerians, they adopted and adapted aspects of the Sumerian language and writing system. For example, Akkadian, a Semitic language, used cuneiform to write both Sumerian and its own language.

- Extinction and legacy: The use of Sumerian declined over time, and it eventually became extinct as a spoken language around the 18th century BCE. However, Sumerian continued to be used as a literary and religious language in Mesopotamia for several centuries. Its influence on subsequent Mesopotamian cultures, such as the Babylonians and Assyrians, is evident in their literature and religious practices.

In conclusion, Sumerian is considered the earliest known language of Mesopotamia due to its development by the Sumerian civilization, the invention of cuneiform script, the preservation of Sumerian texts, its influence on other languages, and its lasting legacy in Mesopotamian culture.