use of the interjection "but lo' ..."

In an article I tried to understand (the german understanding) of:

(...) we’re outside the part of C where the standard Dirichlet series actually converges. But lo’ we can ask what’s the Ramanujan summation (...)

Here the interjection lo' got my interest. I found, for instance lo and behold in leo.org and thus I think I understand the meaning here, too.

But is there some root of that interjection (semantic, etymological,...)? I mean, the author has added an apostrophe so something should be behind it; Leo.org does not help here.

I have never seen it with an apostrophe. I'm guessing the author is under the impression that it is short for "look!". It isn't

According to the OED, in Middle English there are two distinct words "lo" or "loo" which have fallen together; one of them is indeed derived from a form of "look", but the other "lá, an exclamation indicating surprise, grief, or joy".

From TheFreeOnlineDictionary:

lo Used to attract attention or show surprise.

You don't need the apostrophe. In fact, don't use it. you can use an exclamation point, however, even in the middle of a sentence.

In older English usage you will also find the word lo used with other conjunctions such as and, as in the expression, "And lo, there was...".