I think James Brown is that place in R&B/Soul that shares a genome with rock and roll, especially in that era - it gets vague and no precise boundary. But he’s the godfather, “the king of ‘em all y’all” - definitely a pioneer and highly influential to more indentifiable rock legends.
If you never saw Tales from the Tour Bus, the animated show Mike Judge did about some of the greats in music (animating interviews that are played with the original audio) - it’s awesome. The two volume one they did about James Brown is fantastic.