Complex sentence: The area around present-day Columbia was originally populated by Native American tribes, most notably the Susquehannocks, who migrated to the area between 1575 and 1600 after separating from the Iroquois Confederacy.
Split into simpler sentences: The area around present-day Columbia was originally populated by Native American tribes, most notably the Susquehannocks. The Susquehannocks had migrated to the area between 1575 and 1600 after separating from the Iroquois Confederacy.