To get it back to 'call and response'... I'm going out on a limb here, but I seem to recall from my old college theatre classes that traditional Greek plays had the Chorus, with the Choral Leader. While not singing, they were working in a call and response medium, with the leader intoning his lines, and the chorus responding en masse. One instructor I had theorized that this was the start of the traditional chorus in music, and went further (without any backing but theorizing) that this development came out of the first agrarian societies as a way of passing the time.
So there. Shanties might just go back to when the first proto-human stopped in mid-chase and said 'blow this for a lark. I'm gonna plant beans and stop chasing those bloody great beasts with horns and all. Who's with me?'
That could also be why it's so hard to pin down a specific reference from where shanties started - if work songs DID originate before the dawn of history, who's to say WHERE they came from? If you accept point *a*, then point *b* which follows would be that work songs are universal, and that any attempt to track down their genesis will get muddled over the intervening thousands of years...