Ancient pictures show something cooking over fire; the sound was borrowed for grammar. In classical Chinese, 者 turns any verb into 'the one who does it' — the original -er suffix.
记者 journalist (one who records), 作者 author (one who makes), 读者 reader, 学者 scholar. Laozi wrote 知人者智，自知者明: 'one who knows others is clever; one who knows himself is enlightened.'
Add 者 to a verb and you get the DOER — the original '-ER'.
Buddhist translators used 行者 'walker' for wandering monks — Sun Wukong's nickname 孙行者 in Journey to the West.