In the ancient world, most people couldn’t afford to eat a lot of meat or fish or eggs, so they got most of their protein from beans and cheese. But it’s really not that hard to get enough protein to live. If you had nothing to eat but bread, and you had enough bread to stay alive on, you would also get enough protein just from the bread.
Bibliography and further reading about protein:
Nutrition: What’s in the Food We Eat, by Dorothy Hinshaw Patent and William Munoz (1992). Easy reading.