TOEFL Reading練習 #029 — The Organ That Keeps Changing Its Story(日本語解説)

The Organ That Keeps Changing Its Story

¶1 For more than a century, the human appendix has stood as medicine's favorite example of an organ that outlived its purpose. Surgeons remove an inflamed one without much debate about function, and general biology courses have long listed it among structures reduced from a more useful ancestral form. That confidence did not collapse all at once. It eroded in stages, and each stage was triggered by evidence the earlier explanation could not account for. First came tissue in the appendix wall that behaved as if it were doing active work. Then came a hypothesis that finally gave that tissue a job to do. Last came evolutionary evidence suggesting that job had been favored again and again in unrelated animals. What began as a tidy case of anatomical leftover has become a slower argument about what a small, easily removed organ is actually for.

¶2 The vestigial explanation traces back to Charles Darwin, who argued in 1871 that the appendix was a shrunken remnant of a larger pouch, the cecum, that plant-eating ancestors had used to digest tough, fiber-rich leaves. As the human diet shifted toward food that was easier to break down, Darwin reasoned, natural selection stopped favoring a large cecum. What remained then shrank into a narrow, closed tube with no digestive task left. An anatomist named Robert Wiedersheim popularized the idea in 1893, publishing a list of eighty-six human structures he judged to be vestigial, meaning reduced from a more functional ancestral version. The appendix ranked near the top. For most of the twentieth century, doctors treated the organ accordingly. Appendectomy, the surgical removal of the appendix, became one of the most common operations performed on otherwise healthy patients. Since no clear pattern of long-term harm followed removal, the outcome seemed to confirm that little of value had been lost.

¶3 The appendix wall itself, however, complicated the story. Anatomists dissecting the organ found it dense with lymphoid tissue, clusters of cells that form part of the immune system, the body's network for detecting and responding to infection. The concentration was highest in childhood and declined steadily with age, a pattern more typical of tissue doing active work than of tissue quietly waiting to disappear. The finding did not overturn the vestigial view on its own. Plenty of tissue persists in the body without providing an obvious survival advantage, and appendectomy patients still recovered without apparent difficulty, which kept the dismissive explanation intact. What the lymphoid tissue was actually doing there remained an open question for decades.

¶4 In 2007, a team at Duke University led by the immunologist William Parker and the surgeon R. Randal Bollinger proposed a specific answer. They found that the appendix regularly contains a biofilm, a thin, structured layer of bacteria and the mucus that protects them, sitting in a narrow pouch off the main path of digestion. Because the appendix sits outside the direct flow of intestinal contents, they argued, it could shelter a reservoir of beneficial gut bacteria. That shelter could hold up even when a severe infection or bout of diarrheal illness washed the rest of the digestive tract clean. Once the crisis passed, the sheltered bacteria could reseed the gut and speed its recovery. Under this "safe house" hypothesis, the lymphoid tissue was not a leftover at all; it was there to monitor and maintain the very bacterial reservoir the organ was built to protect.

¶5 A single hypothesis about humans, though, could not settle whether the appendix reflected genuine evolutionary advantage or a coincidence found in one species. [A] In 2013, the evolutionary biologist Heather Smith addressed that gap by comparing the digestive anatomy of more than five hundred mammal species. [B] She found that an appendix-like structure had evolved independently at least thirty-two separate times across unrelated lineages, and that species with one also tended to carry higher concentrations of lymphoid tissue in the gut. [C] A trait that keeps reappearing on its own, across branches of the mammal family tree that share no recent common ancestor, is not easily dismissed as accidental leftover tissue. [D] Even so, the evolutionary evidence answers a different question than the one doctors face at a patient's bedside. It shows that the appendix has been repeatedly favored by natural selection somewhere in mammalian history; it does not show how much protection it still provides a person today, especially in places where antibiotics and clean water have made the severe intestinal infections it may guard against far less common. More than a century after Darwin's first explanation, physicians still remove the organ without hesitation, and no one has yet measured exactly what, if anything, a person loses when it is gone.