Because so many of the today's dietary choices have only been popularized in recent years (think gluten-free, paleo diet, etc.) many people think vegetarianism is also a newer fad. But not eating meat has an ancient history. As far back as 3,200 B.C. in Egypt, certain religious groups that believed in karma and reincarnations not only avoided eating meat, but also wouldn't wear clothing made from animal materials.
Pythagoras (the developer of the Pythagorean theorem from math class) was a progressive, open-minded thinker, accepting women as equals in his intellectual circle and arguing that the world was a sphere, not flat. Pythagoras argued in 580 B.C. that animals should be treated well, and argued against killing them for food. He believed killing animals this way "brutalized" the human soul, and that meat-eating was connected to war-like thinking. While some of his pupils followed in his footsteps, other Greek philosophers, namely Aristotle, believed that animals existed for humans to use — and were equivalent to slaves. The Romans followed this belief, which showed up in the Christian Bible, forming the foundational thinking about animals — as useful things rather than individual creatures. This belief is followed by most countries that have a Christian background.
According to an article from the Vegetarian Society website, "Pythagoreans were despised as subversives, with many keeping their vegetarianism to themselves for fear of persecution. However, the term 'Pythagorean' was to become synonymous with 'vegetarian' and vegetarianism was to spread throughout the Roman Empire from the 3rd to 6th centuries among those influenced by Neo-Platonist philosophy. Such authors included Plutarch (c.CE46) whose 16 volume work 'Moralia' includes the 'Essay on Flesh Eating,' Porphyry (c.CE232) who wrote 'On Abstinence From Animal Food' and Apollonius who was a well traveled healer and strict vegetarian who spoke out against deliberately imposed grain restrictions."
Several Asian religions were either partially or fully vegetarian, most notably Buddhism, but also Hindus, Jains and others where non-violence was idealized. Indian king Asoka (264-232 B.C.) ended animal sacrifice during his reign and much of India converted to vegetarianism at that time. When famine and disease were at their high points during the Middle Ages, vegetarianism was at a low point. It was considered by some in the Renaissance, and some people stood up for what they believed was right, including Leonardo da Vinci, who was known for openly denouncing red meat. Later, during the Enlightenment, intellectuals, writers and artists turned back to the classic texts of earlier eras and once again started promulgating the ideas of vegetarianism.
Benjamin Franklin was a vegetarian for a time. According to a wonderfully detailed article on PBS by food historian Tori Avey, "In his autobiography, Franklin describes preparing ... boiled rice or potatoes and hasty pudding. He found that the diet had its economic advantages. His food expenses were decreased by half, affording him the opportunity to purchase more books for his collection. Franklin soon became an advocate of animal rights, which easily fit in with his anti-slavery and political rights agenda."
Using the word "vegetarian" to refer to someone who doesn't eat meat didn't evolve until 1839. It was popularized by the Vegetarian Society in Manchester, England, when it was founded in 1847. Similar societies appeared in Amsterdam and Germany around the same time, and by the 1880s, there were even inexpensive vegetarian restaurants in London. Some vegetarians at that time chose the diet specifically to fight obesity, which was, at that time, was being linked to many health issues. These ideas soon made their way to America, to John Harvey Kellogg, Mahatma Gandhi and the modern era.