early on vs early

early on is not an adjective while early is an adjective.

Word	Noun	Adjective	Verb	Adverb
early on	No	No	No	Yes
early	No	Yes	No	Yes

As adverbs, early on and early are synonyms defined as:

early on (adjective)	early (adjective)
	at or near the beginning of a period of time or course of events or before the usual or expected time
	of an early stage in the development of a language or literature
	being or occurring at an early stage of development
	belonging to the distant past
	very young
	expected in the near future

early on (adverb)	early (adverb)
during an early stage	before the usual time or the time expected
	during an early stage
	in good time