Complex sentence: In the 17th and 18th centuries, some of the Coushatta (Koasati) joined the emerging Creek Confederacy, where they became known as part of the "Upper Creeks" and said to be closely related to the Alabama Indians.
Split into simpler sentences: In the 17th and 18th centuries, some of the Coushatta (Koasati) joined the emerging Creek Confederacy, where they became known as part of the "Upper Creeks". They were said to be closely related to the Alabama Indians.