In India, there's a large proportion of vegetarians, and I believe this is often attached to religion. How did this start? E.g. was it more ethics towards animals, or fear of tainted meat, or religious purity, or something else?
That's the only example I know of, but if there are/were other similar cultures, I'd love to hear how they started – I'm interested in the general question, not India specifically.
In ancient India, the brahmin ("priest") caste engaged in vegetarianism out of religious belief. The killing of animals was associated with bad karma, so brahmins maintained their diets as a sign of karmic purity. It's also important to note that the lush Indian subcontinent was uniquely suited to sustaining a vegetarian lifestyle, due to its wide biodiversity. A vegetarian diet would not be possible in Alaska, for example.
Religious vegetarianism began to wane as Buddhism spread. Contrary to popular belief, the Buddha allowed his followers to eat meat, so long as the animal was not killed specifically for the follower in question. The Buddha wanted his teachings to be available to all castes, not just brahmins. This allowed Buddhism to spread to arid Tibet and China even as it died out in its country of origin, India. Even the Dalai Lama is not a vegetarian!
I am not aware of vegetarianism existing in any large scale prior to Upanishadic India, but I may be wrong.
Sources:
The Bloodless Revolution: Radical Vegetarians and the Discovery of India by Tristam Stuart
In This Very Life by Sayadaw U Pandita (RIP)
The Upanishads, Penguin Classics, translated by Juan Mascaro