Meat, milk and eggs are the best sources of protein. Historically they have been very expensive, especially in the days before refrigeration. In our past, the source of most of our protein has been from the seeds of plants called legumes. The legumes include beans and peas.
Legumes naturally can be a rich source of protein because they grow with friendly bacteria in their roots. These bacteria take nitrogen from the air and the plant uses it as fertiliser. Nitrogen is needed to make protein. These foods are naturally high in protein because of how they grow.
Today beans are sold canned in supermarkets. Older generations used dried beans that needed to be soaked overnight. Often the beans were soaked in water mixed with wood ash from the kitchen stove to soften them. Today we can use sodium bicarbonate. Food cooked with real dried beans has more texture than those cooked with soft beans from cans.
One of the classic bean dishes is Boston baked beans. In this dish, the soaked beans are baked (in water) with some meat and molasses for flavouring. They were often cooked on Saturday and would be left in the oven overnight. On Sunday, religious Christians did not do any work. However their baked beans would be ready to eat.