Western music’s whole habit of stacking one melody against another has a first surviving example, and it sat unrecognised in a manuscript until 2014, when St John’s College PhD student Giovanni Varelli — now a musicologist at Pavia — saw what he was looking at. Cambridge calls it the earliest known practical example of polyphony: two melodic lines moving together, from around 900 AD. It is short, and you can listen to it.