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Stomachs are thought to have first evolved about 350 million years ago. At about the same time our vertebrate ancestors developed jaws that made possible the relatively rapid consumption of bulky meals. The evolution of acid secretion by the stomach therefore provided a novel solution to the problem of limiting the growth of ingested microorganisms while food is temporarily retained prior to delivery to the small intestine for breakdown and absorption. The evolutionary success of the stomach is attested by the fact that what is easily recognisable as the same organ is found in virtually all living vertebrates above the Agnatha (although curiously a few species do manage to do without it). Nevertheless, the secretion of hydrochloric acid in high concentrations comes at a price. It is energetically expensive, there is an obvious need for elaborate defence mechanisms to resist auto-digestion and there …
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