State of the art:

“A sea shanty, chantey, or chanty is a type of work song that was once commonly sung to accompany labor on board large merchant sailing vessels. The term shanty most accurately refers to a specific style of work song belonging to this historical repertoire. However, in recent, popular usage, the scope of its definition is sometimes expanded to admit a wider range of repertoire and characteristics, or to refer to a maritime work song in general.” [wiki]

More standard narrative:
The Advent and Development of Chanties

and
“...There are also several less-established theories regarding the origins of the sea shanty. Although there is little evidence to support this, some historians argue that the maritime musical form can be traced as far back as Ancient Egypt...”
[Piratical Debauchery, Homesick Sailors and Nautical Rhythms, Reidler, 2017]

The 2400 year gap in evidence and theory is best explained by the modern standard shanty narrative's substitution of a genre label for a work practice. 19th century, English, merchant marine &c are not functional attributes. They are consumer preferences.

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