This question may be a better fit on linguistics.SE, but it pertains specifically to English fillers. Also, the question may have a more straightforward answer than what I'm expecting.
TL;DR: Are these fillers mimetic or trace to other languages? Where are they first recorded in English?
First off, although I've always pronounced them differently, the Wiktionary entry for er has the pronunciation for non-rhotic dialects (/ɜ, /ə in ODO), comparing it to uh (/ʌ, /ʌh). Would this be a very similar sound? I'm skeptical as to whether or not this points to a common heritage. Furthermore, the usual sources show er occurred more recently than uh or um.
I run into confusion when Etymonline states that uh is "attested c.1600" whereas ODO puts its origin in the 1960s. Perhaps it's because ODO is focused on modern usage. All sources I've seen ascribe the words to natural utterances. However, if they are purely mimetic, this doesn't explain why filler sounds differ so widely across languages (i.e. why doesn't every language have these fillers or vice-versa).
Based on the fillers of other languages, one might conclude these English fillers were influenced by the Welsh ym or German äh (/ɛː/) or hm. Or is this a case of false cognates because "m" is easily produced?
Either way, how do we know uh and um go back to the 17th century since I'd hazard a guess that "realistic dialogue" (recorded on the page) is relatively "new" (no earlier than 1800s)?