CE 1350. Eventually, the last enclave of purely Plaquemine culture was the Natchez Bluffs area, while the Yazoo Basin and adjacent areas of Louisiana became a hybrid Plaquemine-Mississippian culture. This division was recorded by Europeans when they first arrived in the area. In the Natchez Bluffs area, the Taensa and Natchez people had held out against Mississippian influence and continued to use the same sites as their ancestors. The Plaquemine culture is considered directly ancestral to these historic period groups encountered by Europeans. Groups who appear to have absorbed more Mississippian influence were identified as those tribes speaking the Tunican, Chitimachan, and Muskogean languages. Disappearance Following the description by Jacques le Moyne in 1560, the mound building cultures seem to have disappeared within the next century. However, there were also other European accounts, earlier than 1560, that gives a first-hand description of the enormous earth-built mounds being constructed by Native Americans. One of them was Garcilaso de la Vega (c.1539–1616), a Spanish chronicler also known as "El Inca" because of his Incan mother. He was the record-keeper of the noted De Soto expedition that landed in present-day Florida on May 31, 1538. Garcilaso gave a first-hand description in his Historia de la Florida (published in 1605, Lisbon, as La Florida del Inca) describing how the Indians had built mounds and how the Native American mound cultures practiced their traditional way of life. De la Vega's accounts also include vital details about the Native American tribes' systems of government present in the South-East, tribal territories, and construction of mounds and temples. A few French expeditions in 1560s reported staying with Indian societies who had built mounds also. Diseases Later explorers to the same regions, only a few decades after mound-building settlements had been reported, found the regions largely depopulated with its residents vanished and the mounds untended. Conflicts with Europeans were dismissed by historians as the major cause of population reduction, since few clashes had occurred between the natives and the Europeans in the area during the same period. The most widely accepted explanation today is that new infectious diseases brought from the Old World, such as smallpox and influenza, had decimated most of the Native Americans from the last mound-builder civilization, as they had no immunity to such diseases. The Fort Ancient culture of the Ohio River valley is considered a "sister culture" of the Mississippian horizon, or one of