earlier vs in the beginning

earlier is an adjective but in the beginning is not an adjective.

Word	Noun	Adjective	Verb	Adverb
earlier	No	Yes	No	Yes
in the beginning	No	No	No	Yes

As adverbs, earlier and in the beginning are synonyms defined as:

Other synonyms of earlier include in the first place, originally, to begin with.

earlier (adjective)	in the beginning (adjective)
(comparative and superlative of `early') more early than; most early

earlier (adverb)	in the beginning (adverb)
earlier in time; previously	with reference to the origin or beginning
comparatives of `soon' or `early'	before now
before now