At least, Laboratory Equipment says, in ancient Egypt. A new study of carbon-levels in mummies has found that until recently, when people settled down to build cities, they stopped eating (too much) meat:
All carbon atoms are taken in by plants from carbon dioxide in the atmosphere by the process of photosynthesis. By eating plants, and the animals that had eaten plants, the carbon ends up in our bodies.
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The researchers reported their findings in the Journal of Archaeological Science. They measured carbon-13 to carbon-12 ratios (and also some other isotope ratios) in bone, enamel and hair in these remains, and compared them to similar measurements performed on pigs that had received controlled diets consisting of different proportions of C3 and C4 foodstuffs. As pigs have a similar metabolism to humans, their carbon isotope ratios could be compared to what was found in the mummies.
Hair absorbs a higher rate of animal proteins than bone or teeth, and the isotope ratios in hair of the mummies corresponded to that found in hair of modern European vegetarians, confirming that the ancient Egyptians were also mainly vegetarians.
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The real mystery is the fish. Most people would probably expect the ancient Egyptians living along the Nile to have eaten loads of fish. However, despite considerable cultural evidence, there seems to have been little fish in their diet.