Complex sentence: The Fort Ancient culture was once thought to have been an extension of the Mississippian culture though scholars now believe Fort Ancient culture developed independently and was descended from the Hopewell culture (100 BCE--500 CE), also a mound builder people.
Split into simpler sentences: The Fort Ancient culture was once thought to have been an extension of the Mississippian culture. But, scholars now believe Fort Ancient culture developed independently and was descended from the Hopewell culture (100 BCE--500 CE), also a mound builder people.