I have heard that the comparative of 2-syllable adverbs ending in -ly are formed with more (e.g. more slowly), whereas 2-syllable adjectives ending in -ly have -er at the end (e.g. sillier, earlier). Why then 'early' as an adverb is 'earlier' and not 'more early' (e.g. he came earlier than her)? Thank you for your help.