When and why did Torah-reading "aliyot" begin?

Question

I was able to locate only one reference discussing how public Torah reading was done (i.e. - who read it):

Originally the Torah was read in public once every 7 years during Hakhel

Mishneh Torah, Laws of Festival Offering (Chagigah)3:3 excerpt from Sefaria:

אֵימָתַי הָיוּ קוֹרִין. בְּמוֹצָאֵי יוֹם טוֹב הָרִאשׁוֹן שֶׁל חַג הַסֻּכּוֹת שֶׁהוּא תְּחִלַּת יְמֵי חֻלּוֹ שֶׁל מוֹעֵד שֶׁל שָׁנָה שְׁמִינִית. וְהַמֶּלֶךְ הוּא שֶׁיִּקְרָא בְּאָזְנֵיהֶם.

When would they read? At the end of the first-day holiday of the festival of Sukkot, which is the beginning of [the festival's] intermediate days, on the eight year [of the sabbatical cycle]. And the king would be the one to read in their ears,

Jerusalem Talmud Megillah 29a mentions that Ezra establish public reading on Mon., Thurs. and Shabbat Mincha. However, it does not mention there who read it or that there were aliyot. The only inference I could find is that in Ezra chapter 8, we see that Ezra, himself, did the public reading.

Mishnah Megillah ch 4:1 and 2 discuss the number of aliyot to be given on weekdays, Shabbat and other holidays.

Apparently, by the time the Mishnah was written, the concept of aliyot had already occurred.

When and why did this method change to giving aliyot? What purpose do these "aliyot" accomplish? I.e., if one person read it, what was "wrong" with it that the method was changed?

Answer 1

Rambam (Hilchot Tfila 12:1) writes that Moshe Rabenu instituted public Torah reading on Shabbat, Monday and Thursdays. According to his reading Ezra instituted 3 alyot during the week.

Moses, our teacher, established the rule for Israel that they should read the Law publicly on Sabbaths and also on the second and fifth days of the week, during the morning service, so that three days shall not elapse without hearing the Law. Ezra established the rule that the Torah should be read at the afternoon service, every Sabbath for the benefit of those who would otherwise spend the day vacuously. He also ordained that on the second and fifth days of the week, three persons should be called to the reading of the Law and that they should read not less than three verses each.

probably based on the gemara in Bava Kamma 82a (although the gemara mentions prophets at the time of Moshe). That gemara also mentioned that already at the time either one man read three verses, or three men read three verses corresponding to Kohanim, Leviim and Israel.

Initially they instituted that one man read three verses; or alternatively, that three men read three verses (Meiri: one verse per person). Either way, the number three corresponds to the three types of Jews: Priests, Levites, and Israelites. Ezra later came and instituted that three men always read, and that ten verses altogether be read by them, corresponding to the ten idlers in a city, i.e., the ten men who are paid to spend their time dealing with synagogue and communal matters.