You made me curious as well. Part of the issue is that even among native English speakers, the pronunciation of R varies widely. A quick search of English R pronunciation on youtube turns up several different ways of pronouncing R, all by American speakers. The Irish, British, Australian, etc Rs are distinctly different as well. The Old English R was trilled: http://acunix.wheatonma.edu/mdrout/grammarbook2007/ch3.html You'll hear the Old English R at the very end of this video, in the very last phrase: http://www.youtube.com/watch?v=AcfqgR4dHgg I'm not sure exactly what happened in the development of the English Rs, but I guess there was some influence from the continent (mostly from French speakers) so the trilled/rolled R mutated, softened, and became the liquid consonant that is used today (in its varying forms). Compare the first two R examples on this page and you'll hear how the second sounds like a lazy attempt at the first: http://en.wikipedia.org/wiki/R (Even then, I don't pronounce my standard R in that way.) I think the habit of "dropping" the R at the end of a syllable, as found in a number of British dialects (plus standard British English) as well as in Australian English, comes from the French and German Rs. English speakers probably found the throaty fricative R too rough and softened it until it became almost invisible. Standard British pronunciations of words like "beer" and "fear" will give you an idea of where that final R went. Hope that helps a bit.

A very interesting question. You could probably find a lot of informative websites by googling “rhotic consonants” or “rhotic and non-rhotic accents,” or maybe “guttural R.” Wikipedia has a number of articles of general information on the subject of phonology

Good question! http://medievalwriting.50megs.com/scripts/letters/historyr.htm http://www.learner.org/courses/worldhistory/audio_glossary_R.html In the year 1066 the French invaded England and killed King Harold and for the next 300 years everybody in England spoke French and the official documents were written in Latin. King Henry I became king in English but the language had been transformed by then with a lot of French words and ways to pronounce words.