A never‐ending pain in the ...: That annoying appendix

Although the first description of the human appendix was published in 1521, its function remains the subject of debate. The human appendix has variably been classified as a vestige and as a synapomorphy uniting hominoids. The vestigial hypothesis was largely based on the lack of a known function for this structure, and the observation that patients are able to thrive after an appendectomy. However, recent studies indicate the mammalian appendix functions to maintain commensal bacteria in the gut, particularly after bouts of intestinal distress. This function may no longer be readily apparent due to modern medical care and hygienic environments. In fact, a comparative anatomical analysis suggests appendices have evolved independently in a number of mammalian groups, including marsupials, lagomorphs, rodents, and primates. Among the primates, appendices have been documented in a number of strepsirrhines, platyrrhines, and cercopithecoids, as well as all hominoids. The distribution of the appendix in primates suggests that this structure may have arisen early in this lineage, likely as a safe house for symbiotic microbial biofilms. Thus, the presence of an appendix in primates, including humans, is a derived character state, and may represent an adaptation or an exaptation in these species. Funded by the Fannie E. Rippel Foundation, Duke Cancer Center, and the NIH (RO1 HL60232‐03 and P30 DK34987).