title_body: What are common origins of accusative case markers? What are the primary way(s) accusative case markers can develop? 
  
  The primary way of development of oblique case markers seems relatively straightforward, with attachment of adpositions (which can already be considered "case markers" under some definitions and which can in turn come from nouns or verbs) on the noun, then possible generalisation onto all elements of the NP as agreement. Ergative cases are a little less straightforward, but they very frequently have an oblique double function, hinting at origins such as reinterpretation of passive constructions or the generalisation of usage of e.g. instrumental or ablative case markers as markers of unexpected agency (e.g. via the occasional omission of agents in sentences like "[the man] opened the door with the key").
  
  The emergence of accusative cases seem less straightforward to me though, and while reinterpretations of antipassives are a readily availible source that doesn't to me seem like a plausible origin for the majority of accusative markers, nor does it provide a way to go from no case marking to accusative (which seems like a thing that should be able to happen), and I'm not aware of any similar broad tendency of oblique double-function as seen in ergatives which could otherwise hint at origins. As such I particularly would like to know what oblique cases could reasonably be extended to accusative, and under what circumstances the original extension (from which further generalisation could be done) may occur under, and additionally if there are any reasons accusatives seem to have fewer double functions (even if this is just a result of me looking in the wrong places).
upvoted_answer: This is merely a marginal answer and I’m sure there’s a lot more data that might prove valuable, but in multiple Romance languages (at least Spanish and Romansh) the preposition a has developed into an accusative marker for particularly animate objects. This preposition derives from Latin ad “to(wards)” and is in both languages somewhat equivalent to English to or at in usage (in Romansh it is commonly used to mark the dative as well, as in French).
  
  In general I would not be surprised to see allative or dative adpositions turn into accusative markers, there is quite a bit of overlap in the meanings (compare “I hit towards you” and “I hit you”).