Actually I agree with Cranky Yankee about the probable origin of the shanty being African. I have seen plenty of examples of African laborers working and singing. Be that as it may, it was then taken on board by sailors of many nations and modified to fit the language and customs of those sailors.
One additional source of African sailors was slavery. Sometimes a slave was sold or even rented to a ship captain. For some it became the path to freedom.
Anyway, I wouldn't be at all surprised if the experts were guilty of a little racial prejudice in their research and glossed over the African origin of the custom of using songs with labor.
I think Hugill mentions a work song dating from Elizabethan England. Any more info on that?