Metchosin said above "Any repetitive backbreaking work elicits this form of music from the human soul"
I totally agree with that. Any rhythmic activity that doesn't demand too much concious thought will allow you to sing, and will encourage the mind to freewheel a bit & often result in a new song of the not too deep & meaningful variety.
The fact that these songs then reinforce the rhythm of the work is damn handy. So I'd go for the idea that work songs & chants will have arisen all over the place and at all times.
That sea shanties in particular were heavily influenced by the African call & response charting sounds very feasible indeed. Stan Hugill mentions that the African sailors were the best 'yippers' & that the Europeans could not quite achieve the same wild yells (which were integral parts of the shanties). Still - its always hard to find out the absolute truths of history (or even the present), su b*ggered if I know....

Kris