As far as their not being prepositions, I also seem to remember reading somewhere that there is a substantial body of linguists who believe that the modern prepositions derived from what amounted to particles of phrasal verbs.

Many of the prepositions / verbal particles in Modern English derive from propositions in Old English (via Middle English). Some are from adjectives and adverbs: e.g., near and next are the comparative and superlative forms of the archaic adjective nigh. There is an extensive literature on prepositions / verbal particles in Old to Modern English, and if anybody is interested you can contact me at zmjezhd at-sign gmail dot com (who doesn't care who knows), and we can discuss it ...

Ceci n'est pas un seing.