Stomachs evolved some 450 million years ago, but after giving this organ a test drive, on 18 separate occasions ancestors of contemporary animals switched back, reports Ed Yong on National Geographic. A team of scientists at the University of Porto found that platypuses, spiny echidnas and around 25 percent of fish species are among those animals that have returned to a pre-stomach state.
Stomachs break down food, and just as the organ originally evolved to digest larger blocks of proteins, its de-evolution was driven by diet, researchers think. Yong explains how this might have transpired:
We know that animals evolve very different sets of pepsinogen genes to cope with the proteins in their specific diets. Perhaps the ancestors of stomach-less species shifted to a different diet that made these enzymes worthless. Over time, they built up debilitating mutations, and were eventually lost.
Pepsinogens work best in acidic environments, so if they disappear, you don’t need an acidic chamber any more. Gastric pumps need a good deal of energy to keep the stomach acidic, so if they are no longer needed, they would eventually be lost too.
Once an animal loses its stomach, it's unlikely to go back. In all of the stomach-less species the researchers tested (save for pufferfish), the genes that normally code for stomachs were completely missing from their genome. In other words, not only did those animals toss their stomachs out with the evolutionary garbage, they burned the genetic instructions, too.
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