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I am a big believer that culture is evolved, so based on that, 

Why vegetarianism has evolved in certain castes in the Hindu faith, especially the Brahmin Caste

For Example, pork is taboo in Islam 

My explanation for that is that at that time, pigs used to roam in the street and eat shit/trash that they had on the street and, also, diseases were rampant in the era and there was no modern medicine and no vaccines, so I think that belief came up because it was a protective instinct and the codified as law in Quran 

So, does anybody have an explanation why vegetarianism evolved in certain castes in the Hindu faith?

Note to Mods: This post is not intended to hurt anyone's religion or cultural customs/ beliefs. Just want to seek truth