amounts of light that penetrate underground, could explain the persistence of vestigial eyes.
True moles, which are not rodents but insectivores, have independently lost their eyes, retaining only a vestigial, skin-covered organ that you can see by pushing aside the fur on its head. Similarly, in some burrowing snakes the eyes are completely hidden beneath the scales. Many cave animals also have eyes that are reduced or missing. These include fish (like the blind cave fish you can buy at pet stores), spiders, salamanders, shrimp, and beetles. There is even a blind cave crayfish that still has eyestallcs, but no eyes atop them!
Whales are treasure troves of vestigial organs. Many living species have a vestigial pelvis and leg bones, testifying, as we saw in the last chapter, to their descent from four-legged terrestrial ancestors. If you look at a complete whale skeleton in a museum, you’ll often see the tiny hindlimb and pelvic bones hanging from the rest of the skeleton, suspended by wires. That’s because in living whales they’re not connected to the rest of the bones, but are simply imbedded in tissue. They once were part of the skeleton, but became disconnected and tiny when they were no longer needed. The list of vestigial organs in animals could fill a large catalog. Darwin himself, an avid beetle collector in his youth, pointed out that some flightless beetles still have vestiges of wings beneath their fused wing covers (the beetle’s “shell”).
We humans have many vestigial features proving that we evolved. The most famous is the appendix. Known medically as the vermiform (“worm-shaped”) appendix, it’s a thin, pencil-sized cylinder of tissue that forms the end of the pouch, or caecum, that sits at the junction of our large and small intestines. Like many vestigial features, its size and degree of development are highly variable: in humans, its length ranges from about an inch to over a foot. A few people are even born without one.
In herbivorous animals like koalas, rabbits, and kangaroos, the caecum and its appendix tip are much larger than ours. This is also true of leaf-eating primates like lemurs, lorises, and spider monkeys. The enlarged pouch serves as a fermenting vessel (like the “extra stomachs” of cows), containing bacteria that help the animal break down cellulose into usable sugars. In primates whose diet includes fewer leaves, like orangutans and macaques, the caecum and appendix are reduced. In humans, who don’t eat leaves and can’t digest cellulose, the appendix is nearly gone. Obviously the less herbivorous the animal, the smaller the caecum and appendix. In other words, our appendix is simply the remnant of an organ that was critically important to our leaf-eating ancestors, but of no real value to us.
Does an appendix do us any good at all? If so, it’s not obvious. Removing it doesn’t produce any bad side effects or increase mortality (in fact, removal seems to reduce the incidence of colitis). Discussing the appendix in his famous textbook The Vertebrate Body , the paleontologist Alfred Romer remarked dryly, “Its major importance would appear to be financial support of the surgical profession.” But to be fair, it may be of some small use. The appendix contains patches of tissue that may function as part of the immune system. It has also been suggested that it provides a refuge for useful gut bacteria when an infection removes them from the rest of our digestive system.
But these minor benefits are surely outweighed by the severe problems that come with the human appendix. Its narrowness makes it easily clogged, which can lead to its infection and inflammation, otherwise known as appendicitis. If not treated, a ruptured appendix can kill you. You have about one chance in fifteen of getting appendicitis in your lifetime. Fortunately, thanks to the evolutionarily recent practice of surgery, the chance of dying when you get appendicitis is only 1 percent. But before
True moles, which are not rodents but insectivores, have independently lost their eyes, retaining only a vestigial, skin-covered organ that you can see by pushing aside the fur on its head. Similarly, in some burrowing snakes the eyes are completely hidden beneath the scales. Many cave animals also have eyes that are reduced or missing. These include fish (like the blind cave fish you can buy at pet stores), spiders, salamanders, shrimp, and beetles. There is even a blind cave crayfish that still has eyestallcs, but no eyes atop them!
Whales are treasure troves of vestigial organs. Many living species have a vestigial pelvis and leg bones, testifying, as we saw in the last chapter, to their descent from four-legged terrestrial ancestors. If you look at a complete whale skeleton in a museum, you’ll often see the tiny hindlimb and pelvic bones hanging from the rest of the skeleton, suspended by wires. That’s because in living whales they’re not connected to the rest of the bones, but are simply imbedded in tissue. They once were part of the skeleton, but became disconnected and tiny when they were no longer needed. The list of vestigial organs in animals could fill a large catalog. Darwin himself, an avid beetle collector in his youth, pointed out that some flightless beetles still have vestiges of wings beneath their fused wing covers (the beetle’s “shell”).
We humans have many vestigial features proving that we evolved. The most famous is the appendix. Known medically as the vermiform (“worm-shaped”) appendix, it’s a thin, pencil-sized cylinder of tissue that forms the end of the pouch, or caecum, that sits at the junction of our large and small intestines. Like many vestigial features, its size and degree of development are highly variable: in humans, its length ranges from about an inch to over a foot. A few people are even born without one.
In herbivorous animals like koalas, rabbits, and kangaroos, the caecum and its appendix tip are much larger than ours. This is also true of leaf-eating primates like lemurs, lorises, and spider monkeys. The enlarged pouch serves as a fermenting vessel (like the “extra stomachs” of cows), containing bacteria that help the animal break down cellulose into usable sugars. In primates whose diet includes fewer leaves, like orangutans and macaques, the caecum and appendix are reduced. In humans, who don’t eat leaves and can’t digest cellulose, the appendix is nearly gone. Obviously the less herbivorous the animal, the smaller the caecum and appendix. In other words, our appendix is simply the remnant of an organ that was critically important to our leaf-eating ancestors, but of no real value to us.
Does an appendix do us any good at all? If so, it’s not obvious. Removing it doesn’t produce any bad side effects or increase mortality (in fact, removal seems to reduce the incidence of colitis). Discussing the appendix in his famous textbook The Vertebrate Body , the paleontologist Alfred Romer remarked dryly, “Its major importance would appear to be financial support of the surgical profession.” But to be fair, it may be of some small use. The appendix contains patches of tissue that may function as part of the immune system. It has also been suggested that it provides a refuge for useful gut bacteria when an infection removes them from the rest of our digestive system.
But these minor benefits are surely outweighed by the severe problems that come with the human appendix. Its narrowness makes it easily clogged, which can lead to its infection and inflammation, otherwise known as appendicitis. If not treated, a ruptured appendix can kill you. You have about one chance in fifteen of getting appendicitis in your lifetime. Fortunately, thanks to the evolutionarily recent practice of surgery, the chance of dying when you get appendicitis is only 1 percent. But before
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