The stomach is one of the oldest and known stable.

The need to nurture is one of the most primitive and hence almost every kind of animal has had a body for that purpose. However if we look at the different types of stomachs to which we are able to move from the histological point of view, we find some interesting information.

Ciona intestinalis, a living fossil, with a known age of 525 million years has a thick glandular stomach walls secrete digestive enzymes. It also has a pyloric gland formed by a system of tubules that originate in the intestinal wall and draining into the stomach, so far unknown function. Here we find a first information of interest, some anatomical structures are from the primitive stomach intestine, which leads us to believe that the digestive system was primitive gut without stomach.

Later in the most primitive marine mollusks found a pouch with ciliated grooves that rate the food, the substances are the stomach small and large, the intestine. This information reinforces the hypothesis that primordial digestion was performed in the gut and made necessary a classification structure of food. In the back, near the intestine, is a blind pouch, the bag style, hair that has walls, and produce enzymes.

The stomach already presents modern features:

a) Has a structure that classifies food: improved mechanism found much later in the booklet of ruminants.

b) Produces digestive enzymes in the rear: mechanism of pepsinogen precursor

350 million years ago amphibians appear with a stomach very similar to the present in higher vertebrates. An innovative feature is the presence of a central production of pepsinogen and hydrochloric defined by a cardiac and a pyloric valve.

The most interesting information, to interpret the evolution, is that the larval forms of amphibians have stomach without digestive function only as storage areas.

This supports the hypothesis (the larvae are a reflection of previous evolutionary stages) that the digestive function was initiated in the gut and subsequent specialization allowed the development of actual stomach, first as a storage area and then as an area of digestion. It also explains the reason why the stomach of some modern mammals has no digestive function in early age.

From this point the evolution of the stomach has been minimal except in ruminants have specialized four areas with different functions (booklet as a range, a grid to absorb water and jell as a true stomach and rumen fermentation chamber)