I read an interesting theory a couple of years ago ~ in a work of fiction ~ that may or may be true. There's no way we'll ever know, but it's interesting to speculate.

This was in a historical novel by either Jimmy Breslin or (more likely) Pete Hamill ~ I get 'em mixed up because they're both Irish-American New Yorkers who came from Catholic working class roots to become prominent newspaper columnists. There's a scene where the newly-arrived Irish immigrant protagonist and his buddies befriend a group of black men with whom they are working on the docks.

As a result of their late-night socializing, the uniquely American art form of tapdancing emerges as a kind of hybrid between Irish step-dancing and African-American dance.

The author makes a more persuasive case than I am making here, and of course the idea is presented dramatically, as an adjunct to the storyline, not didactically like I'm doing here. I found the idea very plausible, and an interesting possible example of the "folk process" at work.