According to dictionaries 'as long as' means "during a period of time" (as long as you live), but it also means "on the condition that" (I will attend the conference as long as I can arrive on time).
I'd like to know, does the second usage comes from the first, original one or does "long" have a different meaning here?
The two senses you mention are adverbial. In sense 1,
as long as you live,
as long as expresses an 'amount of relative duration'. In sense 2,
as long as I can arrive on time,
as long as expresses 'on the condition that', 'provided that', or 'if only'.
These senses are summed up by definition 1b of long, adv.1 in OED Online:
In the comparative and superlative, or preceded by advs. of comparison (as, how, so, thus, too, etc.), the adv. indicates amount of relative duration. (Cf. long adj.1 8a.) so (or as) long as: often nearly equivalent to ‘provided that’, ‘if only’. Also, long as, ellipt. for so (or as) long as.
c900 tr. Bede Eccl. Hist. iv.xxv. (Schipper) 496 Ic..þe..ætywde..hu lange þu on hreowe awunian sceole.
971 Blickl. Hom. 169 Swa lange swa ge ðisdydon ðara anum ðe on me gelyfdon.
["long, adv.1". OED Online. December 2015. Oxford University Press. http://www.oed.com/view/Entry/109979?redirectedFrom=as+long+as (accessed February 18, 2016). Bold emphasis mine.]
In the first quote shown, from Bede's Ecclesiastical History, which dates to sometime before 950, "hu lange" translates as 'how long' (Latin quamdiu, see the fourth bullet at the Bosworth-Toller Anglo-Saxon Dictionary entry for 'hreów': quamdiu pœnitentiæ insistere, which may be translated as "how long [a time] to continue your penance"--translation from Saint Bede, The Complete Works of Venerable Bede, in the original Latin, collated with the Manuscripts, and various printed editions, and accompanied by a new English translation of the Historical Works, and a Life of the Author).
In the second quote shown, from Blickling Homilies, which dates to 971, "Swa lange swa" translates literally to 'as long as', used in your second sense, that is, it means 'provided that' or, more precisely in this case, 'on account of the fact that':
...as long as you did this to only one who believed in me...
(From Blickling Homilies: Edition and Translation, Richard J. Kelly, Bloomsbury Publishing, Jul 15, 2010, p. 119.)
Image from the untranslated page:
(From The Blickling Homilies of the Tenth Century from the Marquis of Lothian's Unique Ms. A.D. 971, Volume 1, Richard Morris, Early English text society, 1880, p. 169. See p. 168 for the translation.)
Thus,
Additionally, the etymology given for these adverbial senses of 'long' (adv.1, definition 1b) suggests a derivation from adjectival uses of 'long' (adj.1). The earliest attestations provided for the adverbial senses and the adjectival senses are, however, from the same work, dated to around 888 (Ælfred tr. Boethius, De Consol. Philosophy).
The cross-reference in the OED adverb definition 1b to definition 8a of the adjective for "amount of relative duration", but not for 'provided that', does suggest the lexicographers see a closer relationship between that sense of 'amount of relative duration' and the sense of the etymological source adjective.
The cross-referenced adjectival sense of 'long' is this:
Having (more or less, or a specified) extension serially or temporally.
["long, adj.1 and n.". OED Online. December 2015. Oxford University Press. http://www.oed.com/view/Entry/109975 (accessed February 18, 2016).]
The evidence presented, however, does not support a conclusion that one of the adverbial senses was the source or "original" form of the other adverbial sense.
It is clear that the adverbial sense of 'long' in the phrase 'as long as', meaning 'amount of relative duration' refers to temporal extension. That is not clear in the case of the adverbial sense "nearly equivalent to 'provided that'". For 'provided that' and similar meanings, I prefer logical and thus serial extension to temporal extension. In this context, serial refers to "...forming part of...a series, in respect...of conceptual order" (OED Online, serial, adj. and n., emphasis mine).
Thus, while the base meaning of 'long' may be the same in both cases, that is, it conveys the idea of extension, in the first case, (your sense 1) 'long' refers to a temporal extension, and in the second case (your sense 2) 'long' refers to a serial extension.
In a word, yes. Long is an adjective used to describe the length of something (duh). In both those examples, it references (in weird ways) the length of time, specifically the length of time that something is true. You can think of it as a logic argument.
As long as you live, they will never leave you alone.
The condition: said person being alone.
If them being alive is true, them being left alone is false.
Otherwise if them being alive is false, them being left alone is true.
These conditionals happen immediately and as far as we know apply forever. As soon as that person has died, they will be left alone. The property of them being alive lasts for however long they are alive, however long that is. "As long as" is generic enough to where we don't have to specify much to have a point with which the conditionals can change. In this example that point is the end of that persons life.
I will attend the conference as long as I can arrive on time.
The condition: that persons presence at the conference.
If they can guarantee arrival before the conference starts, them being present at the conference is true, otherwise it is false.
Written another way this can be "as long as me arriving on time is true, I will attend the conference." We don't know the exact time it can stay true. We just know that if it stays true they will attend the conference, otherwise they won't.
As long as the conditionals match, the outcome will be the expected outcome.
EDIT:
@Lawrence It's difficult to explain without using 'as long as'. Here's a to the point answer. I said "In a word, yes."; yes being an answer to the question on the last line. The first use that jumps out to me is the usage in wedding vows. Wikipedia claims that a 1549 prayer book included:
Wilte thou have this woman to thy wedded wife, to live together after Goddes ordeinaunce in the holy estate of matrimonie? Wilt thou love her, coumforte her, honor, and kepe her in sickenesse and in health? And forsaking all other kepe thee only to her, so long as you both shall live?
This is the first example. Assuming that extra piece of information in the comments on the second usage is true, I'd say that if the second usage dates to the 1800's, and from the workings described above, I would say that yes the second usage is derived from the first. Possibly becuase most people don't enter into life long agreements or anything as much anymore.
Both work with conditionals. The difference is the first does rely on some kind of time frame, the second relies mostly on the conditionals themselves while hinting at the time (either future, ongoing, or past) that the proper conditions are met. If that makes sense. So I would say that the second is derived from the first. It isn't too hard to make a jump from first meaning to the second one. Time just plays a backseat role in the second usage.
Expanding on the second example, the person will attend the conference if, up to the point the conference starts, they are able to assure that they will arrive on time. That point during which it can change at any time is the time frame, but we are more worried about the conditionals.
In the OED under "As", section B.2.a, there's an example that has both senses although it uses "so long as" - which I think is equivalent.
1842 Longfellow in Graham's Mag. Oct. 176/1 "So long as you are innocent fear nothing."
It can be taken to mean "for the period of time" or "provided that". It seems to argue that the second sense developed from the first which goes way back.
1567 Gude & Godlie Ball. (S.T.S.) 27 "Als lang as I leue on this eird."
Giving a phrase which is primarily to do with time another meaning is not uncommon.
The regular co-occurrence of two phenomena leads the observer to think that one causes the other, that the other will only appear provided the one manifests itself: if A lasts as long as B but ceases to exist when B does, then it seems natural to conclude that B is the condition of the existence of A.
The conjunction 'while', also to do with time, primarily, can take on the meaning of contrast, with or without a cause and effect relationship.
While there is sun, the weather is not very warm.
(contrast with an inconclusive cause and effect relationship)
The north of the country is mountainous while the south is flat.
(contrast without any idea of a cause and effect relationship).
I disagree with superpentil.
The question as you phrase it is two-parted:
a) does the second usage come from the first original one? b) has "long" a different meaning here?
Now, granted, superpentil responded only to the phrases you put forth. And in that limited sense, he tried to make an argument that they are both related to time. But I believe your question is about the two meanings, not simply the two phrase examples you cited.
The answer to b) is that, in fact, "long" does not actually have a meaning in the "provided that" definition. There is no actual meaning to the term "as long as" when it means "provided that;" the only meaning resides in the idiom itself.
Let's take a different example for the conditional usage. How about this sentence? "As long as you say 'Bob's your uncle,' I'll give you a present."
Is there a time component to the condition?
No, there is not. The recipient of the present can say 'Bob's your uncle' quickly, slowly, immediately or after a while, and in all cases, he would qualify to receive the present. Further, though it's implied that he say the phrase prior to receiving the present, technically he can say it either before or after he gets the gift.
By using this alternative example, it's clear (at me at least) that the phrase has nothing to do with time, in any way. This completely collapses superpentil's argument (again, out of respect to him, it's important to note that he made the argument about the specific example you posed).
Further, I can think of a million ways in which condition would have nothing to do with time…and I'm guessing you can as well. And since we can do that, it becomes obvious that the definition of "long" here is ONLY idiomatic.
However, in the first usage ("as long as you live"), the word "long" clearly has the meaning we associate it with--length. And in that idiom, the word "long" means just what we know it to mean: length of time. The same idiom could also be used to describe physical length ("Rapunzel had hair as long as the Tower of London").
The answer to a) appears to be more difficult, especially since you said you wanted an empirical source.
Both the Oxford Dictionary and the American Heritage Dictionary of Idioms indicate that the usage of the phrase which revolves around length of time dates back to the 1400s: 1. For the period of time that, as in You may keep the book as long as you want, that is, keep it for whatever time you wish to. [Early 1400s]
However, the idiom referring to condition dates back to some 400 years later: 3. Also, so long as; just so. Provided that, as in As long as you don't expect it by tomorrow, I'll make the drawing, or So long as sales are greater than returns, the company will make a profit, or You may have another cookie, just so you don't take the last one. [Early 1800s]
I will continue to research and see whether I can find out more.
UPDATE: After another day of research, including talking to a former high-school English teacher of mine on Facebook, we found nothing further. I will fall back on the Oxford and American Heritage dictionary date citations, which indicate the terms were coined in vastly different eras; this would potentially explain why the word 'long' has no meaning in the conditional idiom, which was coined much later than the other.
I think I can point out something that has not been mentioned:
1) As long as you live
2) As long as you can make it
In both of these sentences the term "As long as" has the same precise meaning. In both cases we are dealing with a conditional. This conditional is binary and the options are:
1)live /cease to live 2)can make it / cant make it
The final part is what seems to change the meaning but really its just a different conditional. This asserts the current value. For example:
1)As long as I live: I am currently living
2)As long as I can make it: I can make it
2b)As long as I can not make it: I can not make it
So both cases refer to the length of time that the assertion which follows the term remains true.
