The question is wrt to Humans and for the sake of this question let’s define vegetarians as non meat eaters. Since we know that humans eating meat (and bone marrow) was a crucial factor in the evolution of the large human brains, when did human communities (or groups) start turning to vegetarians? I believe it must be after humans took up farming.
Also is there any way to tell from skeleta/other remains if the person was vegetarian? If so, what’s the oldest vegetarian remains discovered as yet ?
Pythagoras and his followers didn’t eat meat or fish and that goes back to the 6th century BC.
The Jains were also ancient vegetarians (and they’re still around) but it’s hard to give an objective historical date for when Jainism began. But there’s good evidence the Jains were around as early as 8th century BC.
How do we know what they ate
The isotope composition of your teeth reflects the diet as you grew up, the isotope composition of the bones the diet close to the time of death.Isotope analysis not only indicates diet, but can also provide information on where an individual grew up and where it lived, whether they ate mainly fruit and leaves or grains (C3 from C4 plants), fish or meat, legumes or non-legumes; Stable isotopes and diet: You are what you eat (pdf)
Whoever they were, they were probably not vegetarian by choice, but because meat was too expensive and/or difficult to obtain. And there were undoubtedly many cases of people who only ate meat a few times in their lives, so it’s arguable whether they would be considered vegetarians.
Even today, there are an awful lot of folks who eat almost no meat, and who would be very easy to mistake for vegetarians by first-world standards. Take the amount of meat that a typical American eats in a day, and chop it up real fine, and put it in the big stewpot (along with some sort of grain and legumes) that represents the entire week’s meals for a family of 8. Would you call a person for whom that’s a typical diet “vegetarian”?