If the appendix is so useless, why do we still have it?

The appendix is a blind-ending pouch attached to the caecum.  Although there are dissenters, majority opinion is that the appendix is a vestigial structure - it's a remnant of structure that once did have an adaptive function.  Specifically, it is hypothesised that the appendix was once much larger, and housed a huge collection of bacteria that were capable of breaking down cellulose from the plant material we ingested.  (We humans can't digest cellulose.)  As our diets changed over evolutionary time to become more omnivorous and carnivorous, the value of the appendix declined, and so the evolution 'decided' to cut back on the appendix's size, thereby saving on costs. Evidence for this view comes from looking at other mammals who digest great great quantities of cellulose-rich food: their appendices are much larger and more functional.  The final appendiceal insult comes from the fact that individuals who lack an appendix don't seem to have any physiological deficiency thereafter.

If that's the case, then why do we still have the thing at all?  George Williams and Randalph Nesse came up with a brilliantly plausible explanation.  Could it, they wondered, be paradoxically maintained by appendicitis?

The long, thin shape of the appendix makes it vulnerable when inflammation causes swelling that squeezes the artery to the appendix and cuts off its only blood supply.  When filled with bacteria, an appendix without a blood supply cannot defend itself.  Bacteria grow rapidly and eventually burst the appendix, spreading infection and toxins throughout the abdominal cavity.  A bit of inflammation and swelling is less likely to disrupt the blood supply of a large appendix than that of a long, thin one.  Natural selection gradually reduces the size of the useless appendix, but any appendix narrower than a certain diameter becomes more vulnerable to appendicitis.  Thus, deaths from appendicitis may paradoxically select for a slightly larger appendix, maintaining this less-than-useless trait.  Selection is also almost certainly very slowly making the appendix shorter, but in the meantime the appendix may be maintained by the shortsightedness of natural selection.

Thus according to this theory, the appendix hasn't completely gone away because past a certain point, any gradual effort in this direction results in more appendicitis.  Back in the day, appendicitis was often deadly, and thus those people with a small, but not too small, appendix were at the greatest survival advantage. 

The quote, by the way, is from Nesse and Williams' book, "Why We Get Sick", which is such a brilliant fusion of evolution and medicine that it should be prescribed reading for every undergraduate.  We'll have more from it at a later date.