It presumably came from the same place as the idea that Jesus was born in a stable and that there were three wise men (neither 'fact' appearing in the bible), that our judges bang gavels and that the Old Bailey statue of Justice is blindfolded.
Just a failure to observe.
I would imagine it came from depictions of Eve taking the fruit; you can't paint something generic, so an apple was chosen (whether for form or colour or whatever) and rapidly became the dominant image, and now it's memetic.
You only have to look at Santa Claus's Coca-Cola designed outfit to know that these things happen.
The idea of it being an apple probably came from misinterpretation. The latin name for apple comes from malus which can also mean 'bad'. Although that is only a hypothesis.
It is more likely to have been a type of grape. But as so many people enjoyed the fruits and products derived from grapes (even Jesus himself had a tipple) it wouldn't have looked so good and would have put people off drinking the vino! Hence the malus domestica was the bad apple, and even today since an episode when I was about 14 I cannot bear the sight nor smell of cider, its evil!
Haven't had real cider.... GS would be the one to ask on that :)
Grapes? ok... quick flick of James bible and yep... grapes... vinegar of...(they obviously had tried Tescos wine) was forbidden..... Jesus didn't read that bit then lol....
Don't quote me on this, but..............The apple, I have been told (although quite by whom escapes me) is, again, a throwback to an earlier myth, i.e. 'The Apple of Discord' which eventually caused the Trojan Wars.........
I suppose the 'good fathers' couldn't find any historical references to naughty oranges or disobedient apricots, etc.
Ain't 'Adam's Apple' supposed to be the chunk that got stuck in his throat while attempting to quickly swallow it down whole when God popped out from behind a bush shouting, "Gotcha!" ???