title_body: What are common origins of accusative case markers?
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”).