Are there any features, besides vocabulary, of human languages that only appeared relatively recently?

Written registers are a reasonably recent thing in human language, so the peculiarities of written language would qualify as innovations.

The catch is, the characteristics of written language I can think of are matters of degree, rather than categorical differences from spoken language. But they include things like syntactic complexity, anaphora referring back a long way, intolerance of ambiguity (because of the lack of access to immediate feedback), and notions of periodically structured sentences.