Sub Saharan Africans in Roman army

[Eleatic Guest]

I think it would be impossible to dismiss the seaborne movement of people and goods up the west coast of Africa, although I know very little about the archaeology of this.

I am not so sure. Sails were AFAIK unknown on the African west coast and even the lateen-rigged Portuguese caravelle found it difficult to sail past Cape Borador opposite of the Canary islands. Its circumnavigation after several failed attempts in 1430 was hailed as a major breakthrough of Portuguese exploration. Dugout canoes are another possibility, but the Atlantic Sea is so rough and the Saharan coast so barren that I doubt that any substantial seaborne movement from (sub)tropical Africa northwards ever occurred.