Descartes' skepticism was certainly a pillar of modern individualism, but Martin Luther and the Protestant Reformation preceded him. Plato wasn't so proto-modern, I think. His elitism fit with the old world traditions, although he redefined the primitive dominance hierarchy in intellectual terms. Like Jesus, Plato implied that the last should be first (intellectuals should rule from behind the throne). But sure, ancient Greek and Roman naturalists and humanists delved into the same godless waters that made for modern thought.

The origin of modernity depends on how we define this kind of progress. If it's defined as the confrontation with our true identity, without delusions, we can identify the advent of behavioural modernity (of art, symbolism, culture) tens of thousands of years ago as the start of this progress.