The Soso occupied the western and southern parts of the Futa Jallon, where they lived in small groups and maintained a comparatively isolated existence. Their language belonged to the Mandingo family but was shaped by contact with Mel-speaking peoples. This linguistic and cultural position placed them between the Mandingo-speaking interior and the coastal societies farther south.

Soso political organization was less highly structured than that of the Jallonke. The Simo initiation society held an important place in their social order and was described as Temne or Baga in origin. Baga and Nalu populations were once numerous in the valleys of the Futa Jallon, before leaving the region during the Fulani jihad of the eighteenth century.

For a long time the Soso were peasants and hunters who remained largely unfamiliar with Islam. Their isolation was disrupted by two forces: Fulani incursions from the fifteenth century onward and the arrival of Portuguese traders on the coast. From the end of the fifteenth century, Djakana caravans crossed their territory, linking the Bambugu and Bure goldfields with the coastal river systems.

Commercial change encouraged Soso movements toward the coast. They pushed Baga and Temne groups toward the Rio Pongo and Benna, where those populations had arrived by the middle of the sixteenth century. Islamic influence became much stronger only at the end of the seventeenth and beginning of the eighteenth centuries, producing a broad cultural and social transformation.