The Riddle of the Appendix - New York Times:
Still, I wondered how such a dangerous and disposable organ could survive over evolutionary time. "We consider it maladaptive because we want to live to a very old age," Dr. Fisher said. "But from a strictly Darwinian view, it might not be."
Imagine a trait that helps an animal survive to adulthood, but that also has side effects that can cause trouble later in life. If, on balance, animals produce more offspring with the trait than without it, natural selection will favor it.
Perhaps the appendix lifted the odds that our ancestors could resist childhood diseases and live to childbearing years. Even if it also caused deaths by appendicitis, the appendix might have been a net plus. (It's also possible that appendicitis wasn't such a big problem in the past. Some scientists have argued that modern Western life has made appendicitis more common, either as a result of a change in hygiene or in the foods we eat.)
Dr. Fisher's "net-plus" hypothesis is one of several possible explanations. But they all remain speculation, she said, until scientists learn a lot more about the appendix. "It seems basic, but it's also very hard," she said.
Evolutionary hypotheses guide the search for an explanation of appendicitis, summarized in Carl Zimmer's explanation.