I read somewhere that before the advent of paper and books, ancient civilisations would keep epic stories in their heads. After we started writing things down in larger volumes, committing entire "books" to memory was not needed, so it became less prevalent and eventually stopped altogether.

I guess smartphones may further reduce what we need to commit to memory, which will probably have some implications in terms of neuro-plasticity - i.e. it may well shrink (or at least change) parts of our brains.