"Rock my Soul" was recorded by Broadway Rastus for ABC Paramount in 1929. It's not a soul record but a gospel tune however the lyrics were often drawn upon by some of the best writers of 60's soul.
Rock my soul in the bosom of Abraham
Rock my soul in the bosom of Abraham
Rock my soul in the bosom of Abraham
Oh, rock my soul
So high I can't get over it
So low I can't get under it
So wide I can't get round it
Oh, rock my soul
These types of recording were often ripped of for lyrical content or just plain plagurised. The best example is probably the traditional spiritual "Lets Praise the Lord" which in the the hands of Ray Charles became "Lets Go Get Stoned" the music and the meter of the lyric is exact just the words change.
The exact change over time of Rhythm and Blues to Soul is impossible to define IMO as is the exact change over from Rhythm and Blues to Rock and Roll or Gospel to Soul or Blues to Rhythm and Blues. To me soul music be it Northern, Modern, Crossover, 70's, Oldies, Newies, Rare or Common is all Rhythm and Blues why do we all feel the need to pigeon hole everything. I don't need a label to tell me what I like or to know that Broadway Rastus isn't gonna be appreciated at an all-nighter.
This is a facinating subject though and two good questions. One may be able to be answered definitavely (1929 is my bid BTW) the other will always be a subjective view.
There must be loads of opinions on this would be good to hear others