David Dalby first made the claim that the particle OK could have African origins in the 1969 Hans Wolff Memorial Lecture. His argument was reprinted in various newspaper articles between 1969 and 1971.[30] This suggestion has also been mentioned more recently by Joseph Holloway, who argued in the 1993 book The African Heritage of American English (co-written with a retired missionary) that various West African languages have near-homophone discourse markers with meanings such as "yes indeed" or which serve as part of the back-channeling repertoire.[1][31] Though Frederic Cassidy challenged Dalby's claims, asserting that there is no documentary evidence that any of these African-language words had any causal link with its use in the American press,[30] one can certainly wonder at the fact that this standard of written proof does not account for the illiteracy in which the West African speakers were kept during the period of slavery in question.