Plato was alive when Greek philosophers decided the earth was round, and it would be a few hundred years before somebody would make the first real calculation of its size.

Closer to 150 than "a few hundred", don't mind me, just nitpicking a little.

I think geological time, planetary formation & the Big Bang etc would probably be harder to explain and for the greeks to accept than evolution, even though it requires pretty much geological timeframes as well.