Food historians tell us the history of soup is probably as old as the history of cooking. The act of combining various ingredients in a large pot to create a nutritious, filling, easily digested, simple to make/serve food was inevitable. This made it the perfect choice for both sedentary and travelling cultures, rich and poor, healthy people and invalids. Soup (and stews, pottages, porridges, gruels, etc.) evolved according to local ingredients and tastes. New England chowder, Spanish gazpacho, Russian borscht, Italian minestrone, French onion, Chinese won ton and Campbell’s tomato…are all variations on the same theme.
Soups were easily digested and were prescribed for invalids since ancient times. The modern restaurant industry is said to be based on soup. Restoratifs (wheron the word “restaurant” comes) were the first items served in public restaurants in 18th century Paris. Broth [Pot-au-feu], bouillion, and consomme entered here. Classic French cuisine generated many of the soups we know today.
Advancements in science enabled soups to take many forms…portable, canned, dehydrated, microwave-ready. “Pocket soup” was carried by colonial travellers, as it could easily be reconstituted with a little hot water. Canned and dehydrated soups were available in the 19th century. These supplied the military, covered wagon trains, cowboy chuck wagons, and the home pantry. Advances in science also permitted the adjustment of nutrients to fit specific dietary needs (low salt, high fiber, etc.).
“Cereals, roasted to make them digestible and then ground and moistened or diluted with water to make a paste, either thick or thin, did not become gruel or porridge until people had the idea and means of cooking them. They may initially have been cooked by hot stones in receptacles of natural substances, and then in utensils which could go straight over the fire. Soup, in fact, derives from sop or sup, meaning the sliced of bread on which broth was poured. Until bread was invented, the only kind of thick soup was a concoction of grains, or of plants and meat cooked in a pot. Gruel or porridge was thus a basic food, a staple from of nourishment, and long held that place in Western countries, for in practice bread was a luxury eaten only in towns. A thick porridge of some kind is still the staple food of many peoples, and it is not always made of cereals, but may consist of other starch foods: legumes, chestnuts or root vegetables.”
—Food in History, Maguelonne Toussaint-Samat, translated by Anthea Bell [Barnes & Noble Books:New York] 1992 (p. 177)
“Soup…This category included liquid foods for invalids, such as beaten egg, barley and emmer gruel…and the water from boiling pulses, vegetables or other foods…soups or purees made from vegetables or fruits…broth made with meal of legumes or cereals with added animal fat…and soup in the usual modern English sense, based on meat and vetetables…Medicinal spices and herbs might be added to these various soups, especially if they were intended for invalids as part of a prescribed diet.”
—Food in the Ancient World from A to Z, Andrew Dalby [Routledge:London] 2003 (p. 307)
“Soups. General Observations. The culinary preparations included in this section are of fairly recent origin in their present form, dating from only the early part of the 19th century. Soups of the old classical kitchen were in fact complete dishes in themselves and contained, apart from the liquid content and its vegetable garnish, a wide variety of meat, poultry, game and fish. It is only the liquid part of these classical dishes which has retained the name of soup. Examples of old style of soup which still survive are the Flemish Hochepot, the Spanish Oilles and the French Petite Marmite…On this point as on many others, culinary art owes much to Careme….”
—The Complete Guide to the Art of Modern Cookery, A. Escoffier, first translation of Le Guide Culinaire  by H.L. Cracknell and R.J. Kaufmann [John Wiley & Sons:New York] 1979 (p. 65)
[NOTE: Escoffier’s notes regarding soup classification and serving are also contained in this book.]