How did the stomach evolve?

Answer 1

The stomach is not a recent evolutionary adaption. The first creatures to have stomachs developed billions of years ago in the early oceans. These were simple-celled organisms with specialized organelles that broke down the food absorbed from the environment and expelled the waste. (These are comparable to the human stomach and digestive track, just much simpler.) As these organisms became more and more complex through the forces of natural selection, so did their digestive systems. They eventually split and gave rise to aquatic insects (crustaceans), fish, octopi, mollusks, etc. The first creature with a stomach to walk on land was the aquatic insect, the ancestor of today's insects. Then came fish, led by the lobe-fin, which also had rudimentary lungs and limbs capable of bearing weight. Terrestrial fish gave way to amphibians, which gave way to reptiles, which gave way to birds and mammals. Mammals are vertebrates like cows, dogs, lions, horses, bears, and apes. Humans are descended from a line of proto-apes. Our modern stomach is therefore the result of billions of years of evolution.

There are many overall similarities between the stomachs of various animals. This points to common ancestry. For more on the similarities (and differences) between fish, amphibian, and mammalian stomachs, see the paper linked below.