There is no universally 'natural' way of harmonizing. In fact if you were in, say, Indian music-culture you might consider all harmonizing to be wrong and 'noise'. Every hear the famous Bulgarian singing? They think minor second intervals sound like good harmony. Style of harmonization varies according to cultural aesthetics/ local custom.

The style of harmony selected by The Young Tradition to sing a shanty, or by a Polish group, or by a choir like Fishermen's Friends, sounds different the style used by The Barouallie Whalers.

Let's leave aside the always available option to sing whatever way you want. Let's say you want your style to be "true to" historical practice. Given that the evidence suggests harmonizing was done by Blacks or by Whites in imitation of Blacks, it seems reasonable to look to African-American harmonizing styles as a model. Again, that is if you want to go about this methodically.