I might admittedly be remembering this wrong, but I seem to recollect that Postgres had a couple of (unofficial) contrib modules to support JSON back in 2008 or so, and that (unfortunately buried in Postgres vs Mongo articles) the initial release of Mongo was actually a fork of Postgres for all practical intents.

The relevant data structures and indexing mechanisms (hstore, tsvector, gin, gist) were all available -- if only experimentally -- before Mongo launched.