I got into a discussion today about this issue.
Which was the first popular song that could be classified as ‘rap?’
The guy I was talking about said Aerosmith’s originial “Walk this Way” from the '70s was early rap. I don’t think it was, but our perception of it is colored by the later version of it with Run DMC.
The first I can remember hearing was “Rappers Delight” by the Sugar Hill Gang from the late 70s.