IE languages have prepositions (actually they started as postpositions which can still be seen in Vedic Sanskrit). PIE is reconstructed with case. I've never seen a language that didn't have preps. Anyway, another idea is that since most languages have numerous elements of redundancy built in, it's not too tramatic for one of the redundant bits to go away or be replaced. Latin had cases, which as any Latin student is told, made word order optional. But spoken and prose Latin tended to have a default word order, and that word order became more fixed in the Romance languages when cases started to be confused with one another (for historical-phonological reasons). Could be the same with prepositions and cases. When cases collapse, the prepositions take on the meaning of the expression rather than spreading it across case plus preposition. It's just those cases (no pun) wherein case alone determined syntactic relations of nouns to verbs (e.g., dative for indirect object or accusative for direct object) that need to be grammaticalized. It's interesting that in English for indirect objects a prep was used (sometimes not always), but for the direct object word order filled in. The ablative absolute got replaced with conjunctions (when/while) plus the phrase.
Anyway, not dogma just what I've thought over the years. And are you suggesting that the nest determined that chickens lay eggs?
