history - Who broke the parshiyot into aliyot and how?

The Torah readings on Shabbat are broken into 7 aliyot. I know that there are rules for the minimum number of pesukim for each aliyah but I am wondering who set up the exact placement of the aliyah divisions and when (for an annual reading cycle).

This answer posits that the divisions had to do with setting up a learning cycle but that doesn't give any history or specific guidelines. One practical question which develops is, in the case of reading a double parsha, why isn't the seventh aliyah of the double identical with what would be the seventh aliyah of the second parsha when it is read on its own? The determination that it should be "seventh" was already made -- why change that when the 2 parshiyot are combined? (I note that for a variety of reasons, aliyot are not always balanced and even in terms of length so "balance" would not, on its own, appear to be a persuasive answer.)

Answer

In terms of the history of division of Aliyot, you should see Ilana Katzenellenbogen's survey article in Sinai 119 (1998), pp. 224-45. She looks at 33 different division customs from the last 1000 years from around the Jewish world and compiles a ~10 page chart with all the different variants from "our common custom" (OCC). Her conclusions are (summarized from p. 225-226):

1. 
   
   

   The divisions in various biblical manuscripts are different from OCC and from each other.

   
   


2. 
   

   The divisions in various lists of customs are different from OCC and from each other, but the more recent they are the more they begin to resemble OCC.

   
   


3. 
   

   Her inspection of printed bibles revealed the following strata:

   
   
   
   
   • From 1517 to 1568: 17 editions, none of which had any division markings.
   
   
   • From 1574 to 1702: 22 editions, 13 of which had no division markings and 9 with divisions unlike OCC and unlike each other.
   
   
   
   • From 1702 on: over 100 editions all of which are very similar to OCC (the few differences are found mostly in dividing Parshat Bereishit).
   
   
   
   

She posits from this that the divisions were originally 'regulated' by local Chazanim and Baalei Keriya usually in separate books of customs, and only in the last 300-400 years has there been a gradual fixation of the division (probably partially influenced by mass printing).

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While I'm here I'll note that in footnote 5 she collects the relevant Talmudic dicta about where one may make a division (not 1 or 2 verses before or after a parsha break, etc.) including one related to content (Yerushalmi Megilla 3:7):

זה שהוא עומד לקרות בתורה צריך שיהא פותח בדבר טוב וחותם בהדבר טוב
He who comes up to read from the Torah needs to open with a good thing and close with a good thing.

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Finally I note there were Achronim (notable among them the Vilna Gaon (Maaseh Rav 132)) who generally opposed respecting the printed division of OCC.