The OP says, "I am looking for more detail, specifically about the differences in the causal and temporal meanings of 'since', and how they came about," or how the two meanings of since developed.
First, in brief, since has been around a long time, in both senses. The OED includes definitions and quotations relating to each from Middle English sithen, an adverb, preposition, and conjunction:
Forms: α. OE sioððan, OE–ME siððan, ME siððon, ME siððen; OE siðþan,
siþþan, siþþon, ME siþþen (ME Orm. siþþenn), ME siþthen, ME sythethyn,
sitthen; ME siðen (ME siden, syden), ME siþen (ME siþan), ME syþen, ME
siþin; ME–16, 18 sithen (ME sithenne, sithin, sithyn, sithun, 16
sithene); ME–15 sythen (ME sythene, sythin, sythinne, sythyn, sython,
sythun). β. OE–ME seoððan, ME seoð-, seodþen ( soððen); OE seoþðan, ME
seod-, ME seoðþan; OE seoþþan, ME seoþþen (ME soþþen); ME seððan, ME
seððen (ME sed-), ME seþþen; OE seoðan, ME seoðen, seoþen; ME seðen,
ME seiþin, ME seþen, seþin; ME–15 sethen (ME sethenne), ME sethyn,
ME–15 sethin (ME sethinne, sethun). γ. OE syð-, ME sydþan, OE–ME
syþðan, syððan (ME syððon), ME suððen (ME suden), ME swo-, swu-,
souððen; OE syþþan ( syþan), ME suþþen, sut(t)hen.
Etymology: Old English siþþon, siþþan, etc. (see above), for earlier
*síð þon (þan) ‘subsequent to that’ (compare æfter þon, and German seitdem),
with shortening of the first vowel. The Middle English
siþen, sithen, however, may also represent Old Norse síðan (Danish
siden), of the same meaning.(Show Less) Obs.
There are far more definitions relating to time than to causality.
Under sithens the OED lists one definition as a conjunction:
2a. Seeing that; = sith adv. 2. Now arch.Very common from c1550 to
1377 Langland Piers Plowman B. xix. 15 Why calle ȝe hym
cryst,..sithenes iuwes calle hym ihesus?
1898 E. W. Hamilton Mawkin of Flow xiii It's our plain duty to
bide by it, sithens we was sent by Buccleuch to be guided by the
One definition of since from the OED is:
II 4a. Because that; seeing that; inasmuch as. 1577 B. Googe tr. C.
Heresbach Foure Bks. Husbandry i. f. 7v Sins it is not yet dinner
tyme, let vs walke about.
Turning to time, one OED definition (under B, preposition) is:
Definitions under B (preposition) include:
- Ever or continuously from (a specified time, etc.) till now.
1734 tr. C. Rollin Anc. Hist. I. 255 It was now five months and a half
since his first setting out.
1780 Mirror No. 103 I inherited a law-suit, kept alive by various
means, ever since the year thirty~three.
1861 Thackeray Four Georges iv. 168 He sleeps since thirty years.
This last--"he sleeps since thirty years"--would be an error today, and is the mistake most common among English language learners.
Native English speakers do not have a problem figuring out which meaning of since is meant, and @Clare's point that only sentences in isolation are likely to be confused is one with which I agree. But it's possible to write sentences likely to be confusing to readers, who have to backtrack to figure out what is meant. The solution is to recast (rewrite) a sentence that can be misunderstood.
That said, the best discussion of since I've found is from English Grammar Today:since
We use since as a preposition, a conjunction and an adverb to refer to
a time, and as a conjunction to introduce a reason. Since: time We
use since to refer back to a previous point in time. We use since as a
preposition with a date, a time or a noun phrase: It was the band’s
first live performance since May 1990. (since + date) I have been
happily married for 26 years, since the age of 21. (since + noun
phrase) We also use since as a conjunction to introduce a subordinate
clause: It’s so long since I saw them. (since + clause) Lenny had
slept most of the way since leaving Texas. (since + clause) He’s been
back to the office a few times since he retired. (since + clause)
Since and tenses
When since introduces an action or event at a point of time in the
past, we can use the past simple or present perfect after since and
the present perfect in the main clause:
They haven’t received any junk mail since they moved house.
They haven’t received any junk mail since they’ve moved house.
We can use the past simple, present perfect or past perfect after
since with the expression it + be + time + since:
It’s been years since I rode a bike. (it’s = it has)
It’s been years since I’ve ridden a bike. (it’s = it has)
It’s years since I rode a bike. (it’s = it is)
It’s years since I’ve ridden a bike. (it’s = it is)
It’d been years since I’d ridden a bike. (it’d = it had)
It’s been years since … is more common in American English than It’s
years since …. When since introduces a state in the past that is still
continuing in the present, we use a present perfect form of the verb
after since and a present perfect form of the verb in the main clause:
Since I’ve been back at work, I’ve been feeling great.
Since + -ing
We can use since + -ing form to refer to time when the subject of the
verb is the same in the main clause and the subordinate clause:
Since leaving school, he has had three or four temporary jobs. (Since
he left school, he has …)
Since moving from a Chicago suburb to southern California a few months
ago, I’ve learned how to play a new game called Lanesmanship. (Since I
moved …, I’ve learned …) Since, since then
We can use since or since then as an adverb of time when the time
reference is understood from the context:
His father doesn’t talk to him. They had an argument a couple of years
ago and they haven’t spoken since. (since they had the argument)
They bought the house in 2006 and they’ve done a lot of work on it
since then. (since 2006)
We use ever since as a stronger form of since or since then:
When I was young, I had a little collie dog, but one day he bit me
really badly. I’ve hated dogs ever since. Since: reason
We use since as a subordinating conjunction to introduce a subordinate
clause. We use it to give a reason for something:
Sean had no reason to take a taxi since his flat was near enough to
Since her husband hated holidays so much, she decided to go on her
They couldn’t deliver the parcel since no one was there to answer the
Since: typical errors
• We don’t use since with extended periods of time. We use for:
She was waiting for four hours.
Not: She was waiting since four hours.
• We use since, not from, with a clause referring back to a point in
I’ve been swimming since I was three years old.
I’ve been swimming from the age of three.
Not: I’ve been swimming from I was three years old.
• We use since, not once, to introduce a reason:
I think I should have my money back since I didn’t have what was
promised in the brochure.
Not: I think I should have my money back once I didn’t have …
• We use since, not ago, after ‘it’s a long time’ when we refer back
to a point in time:
It’s a long time since your last letter.
Not: It’s a long time ago your last letter.
In summary, the evidence suggests that the two main senses of since developed concurrently, and it's a word easy for non-native speakers to misuse.