early vs early on

early is an adjective but early on is not an adjective.

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

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

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