The fact that all southern chinese dialects don't have 'r' sound implies strongly that original middle chinese doesn't have 'r' sound.
I read that the manchurian language is similar to japanese so we can infer from the characteristics of the japanese language. (as they both belong to same language family)
The japanese language has 'r' sound and has no endings such as l,s,k,t,p. (as far as I know).
Mandarin today is a combination of middle chinese language simplified to suit the tongue of the manchurians by introducing the 'r' sound and eliminating the endings 'k,t,p'.
As the hakka migrated from the north during the later sung dynasty, it is presumed that mandarin acquired the 'r' sound from the 'yuan' dynasty onwards.
Since the manchurian language is almost extinct , it is difficult to research ...
That is my hypothesis anyway and is open to debate.
Edited by xng, 06 January 2006 - 11:52 AM.