Grammaticalization is a linguistics term that is used to refer to the emergence of new grammatical structures such as verb declensions and case affixes. The rise of new adpositions is also grammaticalization. This is a key component of language change and occurs in mainly two ways. Language internal grammaticalization is an innovation in the grammar that happens based on the internal dynamics of tha language. Contact induced grammaticaluzation on the other is what happens when one language sort of copies a grammatical structure from another language. This copying usually happens in a semantic level. For instance, language A is using prepositions derived from certain verbs. Language B that is a neighboring language with no genetic affinity, could also start using its own verbs with the same meaning as prepositions with the same job.
In any case, we know today that grammaticalization is a widespread process that has occured countless times throughout the history of human language and it is happening as we speak. We know this because several grammatical items in whatever languages we are speaking can be identified to have origins in nouns or verbs. The most common example of such items are the adpositions that have not quite their original phonetic elements and can still be identified with existing nouns or verbs. For example, with some thought, you can figure out that the english preposition "below" has something to do with "low" as both words tell us about something that is down there. The grammaticalization of an adjective into an adposition has not been completed in this case but in a few centuries, it should be expected that everyone other than linguists will forget about the origin of this word and it will be completely grammaticalized as a preposition and with time, it could even become a case affix. though that is a position only a few adpositions can attain.
It is very important to study and understand the process of grammaticalization because this certain topic in historical linguistics has great potential to provide insight into how the languages develop and change and if there are sort of premises in human mind that lead to the same types of changes in different and distant languages. The existing research gives hope to linguists that there are actually rules of grammaticalization that can be discovered by comparing attested languages. There are indeed certain types and paths of grammaticalization that has occured over and over in geographically and linguistically diverse languages and this tells us something about the way human mind is inventing and reinventing the language. One very frequent pathway is the grammaticalization of adpositions. This with other pathways of grammaticalization has been studied extensively by Tania Kuteva and Bernd Heine, who show in their many books that adpositions can also go into further grammaticalization to become case affixes. As said before, this sort of grammaticalization has been studied extensively but there is also quite a bit of material about grammaticalization in Iranian Languages such as Persian that deserves to be studied. The Iranian Language family is one of the oldest attested language families in the world and we are able to see how the many languages in this group changes over the centuries.