It appears that the stomach has an ancient origin. The stomach first appears in the fish lineage. The prevertebrate chordates do not have a true stomach, whereas the cartilaginous and bony fish do. Although most fish do have a true stomach, some fish species appear to have lost the stomach secondarily. The remaining vertebrate lineages do have a true stomach (at least in the adult animal), although there is great variation in the size and shape of the stomach.[well established][VHOG]
DOI:10.1046/j.1525-142x.2000.00076.x Smith DM, Grasty RC, Theodosiou NA, Tabin CJ, Nascone-Yoder NM, Evolutionary relationships between the amphibian, avian, and mammalian stomachs. Evolution and development (2000)