According to wiktionary entry on Yea. From Middle English ye, ȝea, ya, ȝa, from Old English ġēa, iā (“yea, yes”), from Proto-Germanic ja (“yes, thus, so”), fro

According to wiktionary entry on Yea. From Middle English ye, ȝea, ya, ȝa, from Old English ġēa, iā (“yea, yes”), from Proto-Germanic ja (“yes, thus, so”), from Proto-Indo-European yē (“already”) Wiktionary about yes says From Middle English yes, yis, from Old English ġēse, ġīse, ġȳse, ġīese (“yes, of course, so be it”), equivalent to ġēa (“yes", "so”) + sī(e) (“may it be”), from Proto-Indo-European yē (“already”). Compare yea. So "yes", seeming so simple, was once a compound word.