The vermiform (“wormlike”) appendix is an extension of the human large intestine. It is homologous to a part of the caecum of other mammals. Humans (and cats) are among the most finicky eaters in the mammal world. Humans use intelligence to choose foods that are safe and nutritious to eat, at least we did so under the prehistoric circumstances in which our bodies evolved. But most mammals, particularly herbivores and omnivores, just eat stuff, and rely on their intestines to process the food. Dogs just gobble things down. As for cats, which are not necessarily any more intelligent than dogs, I do not know why they are finicky. (One of our cats once ate a ribbon off of a Christmas tree, not an impressive example of intelligence.) An important part of intestinal food processing in most mammals is the caecum, in which bacteria break down many otherwise indigestible food materials. Humans have big brains and small caecums; most mammals have smaller brains and larger caecums; once again, I have no explanation for cats. In humans, part of the caecum has degenerated into the appendix, which really does look like a worm, and is not large enough for any significant amount of food to enter. It is a dead end tunnel off the side of the intestine.
The human vermiform appendix is largely considered to be vestigial and worthless. But it turns out to not be entirely worthless. Sometimes disease bacteria multiply in your intestine, and drive out the good bacteria that normally live there. Then the disease bacteria eventually die away, if you are lucky. Eventually the good bacteria return to their intestinal home. But where do they come from? Apparently, many of the good bacteria hide in the little appendix corridor, and emerge after the bacterial war is over. The appendix is therefore a refuge for good bacteria. This is particularly important for modern people who take oral antibiotics, which devastate the bacterial populations in the main part of the intestine, but not on the appendix.
But even though the appendix is not worthless, it is still vestigial. It is reduced in size and function compared to our mammalian ancestors. In humans it no longer serves a digestive function. It has degenerated over evolutionary time, though not far enough to have become worthless. It remains today, as in Darwin’s day, an excellent example of a vestigial organ.
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