Complex sentence: The Poverty Point culture may have hit its peak around 1500 BCE, making it the first complex culture, and possibly the first tribal culture, not only in the Mississippi Delta but in the present-day United States.
Split into simpler sentences: The Poverty Point culture may have hit its peak around 1500 BCE. It is one of the oldest complex cultures, and possibly the first tribal culture, in the Mississippi Delta and in the present-day United States.