This is about Iranian Languages sub-family and how the grammar of these languages developed and changed through history.
Iranian languages sub-family is not only one of the oldest attested language groups in the world, they are also one of the most diverse ones on the entire planet. This page is about the origin and development of Iranian Languages and some of the major changes that took place in the grammar of the languages in this group during the development. First, let use see what are the Iranian Languages and where they came from.
Iranian languages are some forty to fifty languages spoken by well over two hundred million people around and in the preset Iran. All of these languages are related to the Persian language, which is the official language of Iran. Iranian Languages are also distantly related to the Indian Language and together they all belong to the Indo-European Languages family, which included virtually every language spoken today in Europe and America. Iranian Languages have a history that goes back thousands of years ago. Thanks to Avesta, the thousands of years old holy book of Zoroastrians, we know what the ancient Iranian Language actually looked like. What is better is that there are many extant royal inscriptions and other stuff that shows us exactly how the Iranian Languages developed the carious stages of grammaticalization that took place through millennia. Iranian languges changed and diversified as centuries passed and today we have languages like Pashtu, which is related to Persian but not quite intelligible to Iranians. There are also others like Mazandarani in the North and Balochi language in the South. As time passed, different groups of people invented different ways to express themselves to language was split into several branched, some of which are spoken by only a limited number of people.
Major changes happened as Iranian Languages passed from one stage to another. For instance, we know that the ancient proto-Iranian language had case inflections. You could know whether a certain noun was in dative case, accusative case or ablative case just by looking at one word because the word was inflected depending on its role in the sentence. That was all gone by the time, the language entered the Middle Iranian era. The case inflections are nowhere to be seen the extant texts of Middle Persian and its relatives. Instead we see many adpositions, some of which are similar in different Iranian languages while others are quite different. Interestingly, most of the newly acquird adpositions appear to be derived from nouns. Indeed, there is an entire academic paper on Adpositions Derived From Nouns In Iranian Languages, which you can also view hereScience Articles, that tells in detail how the languages developed these prepositions and postpositions.