Right from tholkappiyam we have glimpses of how muruga worship started among the vettuvars
Muruga was a vettuvar god. He was believed to transcend among those who were into veriyattam. Vel was a representation of this god. No human representation at this stage. No temple of his mentioned in famous silappathikaaram lines pirava yakkai periyon koyilum...
Later, the vedic skanda came. In early pallava period, an amalgamation happened. Tamil velan was merged with skandha or subrahmanya or kaarthikeya. His original wife was valli - fittingly suits a vettuvar god - but now Indira's daughter had to be married to him. Thus came the muruga cult as we see today. He was a very superior god in ancient times - highly adored in sangam literature. He got a place along with thirumal and shiva in pallava trimoorthy temple in mahamallapuram. He was the vital linkage who helped to link shiva with kotravai (somaskantha)
Why brahma - who is believed to be the very creator of this universe - was ignorant of pranava mantra ? it is because he had to be subdued in front of muruga.
Later on when alwars and nayanmars came, shiva and thirumal became superior gods. But the kaumaaram - the original muruga worship - is still vividly maintained in the minds of thousands of tamils.
When tamils reach a foreign land, the very first temple they build is for muruga. The association of murugu worship with tamil hearts is a touching phenomenon which transcends centuries.
Dr.Iyravatham has worked on origins of muruga worship. Dr.Kailasapathy and Dr.Sivathambi write about it.