The first known appearance of the jealous husbands problem is in the medieval text Propositiones ad Acuendos Juvenes , usually attributed to Alcuin (died 804). In Alcuin's formulation the couples are brothers and sisters, but the constraint is still the same—no woman can be in the company of another man unless her brother is present.  , p. 74. From the 13th to the 15th century, the problem became known throughout Northern Europe, with the couples now being husbands and wives.  , pp. 291–293. The problem was later put in the form of masters and valets; the formulation with missionaries and cannibals did not appear until the end of the 19th century.  , p. 81 Varying the number of couples and the size of the boat was considered at the beginning of the 16th century.  , p. 296. Cadet de Fontenay considered placing an island in the middle of the river in 1879; this variant of the problem, with a two-person boat, was completely solved by Ian Pressman and David Singmaster in 1989. 
The Sacred Grove of Oshogbo was one place I had been looking forward to visiting in Nigeria. As prevalent as indigenous religions still are in West Africa, it is often hard to find public expressions of them in towns and cities; the Christianity brought by European slavers and colonialists has taken root and pushed most of these religions out of mainstream life. But in the Sacred Grove shrines honor all the local deities, including Obatala, the god of creation, Ogun, the god of iron, and Oshun, the goddess of water, whose aqueous essence is made manifest by the river running through the trees. The place is unique in the Yoruba religion, and that intrigued me.