Silver Surfer served Galactus for many centuries before his Earth encounter right? So since that time frame, Galaztus has gone through like 7 other heralds in less than 10 years?
well im pretty sure big g chose SS when he came to his planet to devour it and the being called norin radd bartered with galactus to spare his planet, and in return he would serve big g, and galactus saw his nobel spirit.......and hence fourth he was transformed into a hearald...
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"I am tired of Earth. These people.
I am tired of being caught in the tangle of their lives."