The Appendix
There have been many ideas over the years regarding which bodily function our appendix serves. We know that if it becomes inflamed and bursts that it's a serious medical emergency, but why do we even need it?
Charles Darwin thought that it's presence had something to do with diet. One study examined over 300 mammals and found an appendix present in ~50, and concluded that there were at least 32 evolutions among the 50 species. In observing those 50 more closely, it seems that the organ adapts based on many factors, the least of which is diet.
Diseases such as Cholera and Dysentery are able to completely flush everything out of our intestines. Since we need different types of bacteria in our intestines (think pre- and probiotics), an earlier studylogically suggested the appendix to be a type of holding cell. If we ever lost all our good bacteria, our bodies would have a small amount in reserve to get proper digestion started again.
If you're wondering why many people are able to live without an appendix, one possible explanation is that we generally don't need it anymore. The prevalence of diseases, in North America, that are severe enough to necessitate a 're-boot' of our digestive flora is minimal.
Dan