Why do we still have an appendix? Measuring only 10 cm long and 7 mm wide, it must be one of the most troublesome vestigial organs in the human body. A quarter of a million cases of appendicitis were accounted for in the US in the five years between 1979 and 1984 and your life-time risk of appendicitis is 8.6% for males and 6.7% for females. A paper in The Lancet documents about 16 million cases of appendicitis world-wide in 2013, which resulted in 72,000 deaths globally. This is bound to be an underestimate. Even if appendicitis is only suspected you may well be in for preventative surgery. According to a paper in the American Journal of Epidemiology, an estimated 36 incidental procedures to remove the appendix are performed in the US to prevent one actual case of appendicitis. Acute appendicitis is horribly painful but a ruptured appendix can be fatal.

Given this human cost you might be forgiven for feeling that appendicitis constitutes sufficient selection pressure to drive the appendix extinct. Unless, of course, there is a very good, adaptive reason why we still have a vermiform appendix. I remember reading a paper a few years ago by William Parker, of Duke University, suggesting the appendix had an important immune function and represented a safe-house in the lower gut where “friendly” bacteria could seek refuge in the face of a serious pathogenic onslaught – and from which they could emerge once the carnage was over to re-populate the gut with a benign microbiome. It looks as if he is correct. In a recent paper in Comptes Rendus Palevol titled “Morphological evolution of the mammalian cecum and cecal appendix”, he joins forces with Heather Smith of the Department of Anatomy, Midwestern University, Glendale, Arizona; Sanet Kotze, from the University of Stellenbosch, and Michel Laurin from the Sorbonne. Together they collected data on appendix presence and size, and other gastrointestinal characters, ecological variables, dietary habits, and social characters, all hypothesized to drive appendix evolution, for 533 mammalian species. Their vast dataset reveals that the cecal appendix has evolved a minimum of 29 times, possibly as many as 41 times, throughout mammalian evolution, while it has only been lost a maximum of 12 times. This statistically strong evidence that the appearance of the appendix is significantly more probable than its loss suggests a selective value for this structure, they say, and means they can confidently reject the hypothesis that the appendix is a vestigial structure with little adaptive value or function among mammals.

By tracing back as far as the marsupials and monotremes they found a relationship between the presence or absence of an appendix, and the morphology of the cecum to which it is attached. Marsupials and monotremes with an appendix possessed a long tapering cecum or a small appendix/appendix-like cecum, while monotremes with a rounded or cylindrical cecum were not found to have an appendix. Additionally, they say, several species possessed a possible appendix in the absence of any apparent cecum. So it is sometimes difficult to decide whether the cecum evolved as an expansion of the appendix, or vice versa. The cecum and appendix may be co-evolving. If so, it might be sensible to consider the two structures together, as an evolutionary module – the ceco-appendicular complex.

They specifically set out to test whether or not ecological factors that could affect the incidence of gastro-intestinal infection had influenced the evolution of the appendix. Although they could find no direct link between presence or size of the appendix and a number of dietary, ecological or social characters they did find strong links between ecology and the morphology of the cecum as a whole. They report that relative cecal length correlated with mean group size and was inversely related to habitat breadth. They suspect that species who live in large groups or who use a narrow range within a habitat may be at prolonged risk of communicable diseases and pathogens due to their increased proximity to each other. The preservation of a functional protective microbiome may be more challenging, they say, in such groups. So, the ceco-appendicular complex, which is richly-endowed with lymphoid tissue, they argue, may have evolved as an adaptive immune response to pathogen avoidance in species with large group sizes and/or narrow habitat breadth, as well as a refuge for benign microbes.