Complex sentence: The Fort Ancient culture was once thought to be an expansion of the Mississippian cultures, but is now accepted as an independently developed culture that was descended from the Hopewell culture (100 BCE-CE 500), who were also a Mound Builder people.
Split into simpler sentences: The Fort Ancient culture was once thought to have been an expansion of the Mississippian cultures. It is now accepted as an independently developed culture descended from the Hopewell culture (100 BCE-CE 500), also a Mound Builder people.