A cluster of closely related peoples on the lower reaches of the Congo River and the adjacent coast to the north and south aroused the astonishment of the early Portuguese explorers because of the complexity of their political institutions and their social life. (p. 292)
See Kongo people (Wikipedia)
Read Chapter 38. “Expansion of the Bantu. Central Bantu — Kongo Cluster” (Africa. Its Peoples and Their Culture History. G. Murdock, 1959) on webAfriqa.