For me the interesting parts of science are often prefixed with 'comparative'. I worked through the noughties in a Comparative Immunology lab; Comparative Anatomy gives us the handle on endless forms most beautiful - why, and how, a whale's leg is so different from that of a hippopotamus. Comparative Development tracks through the biological miracle that, although all vertebrates start from a single celled zygote, they all grow up uniquely different from each other. You look more similar to your siblings than to your cousins, David Bowie or Koko the signing Gorilla. Further out we have a lot in common with monkeys, less with dogs and horses and less still with hawks and vultures. In evolutionary terms, some differences are 'trivial' and some profound. Habitat, for example, is not key to understanding where we came from: we have more in common with some flying [bats] and swimming [dolphins] creatures than we do with many terrestrial vertebrates [lizards, ostriches]. It's pretty clear that the ancestral vertebrates developed in the sea and millions of years later one of these creatures crawled ashore and went forth to multiply on dry land. But several times, some of these land-dwellers have reverted to the ancestral location and slipped back into the ocean: whales, seals, penguins, sea-snakes, marine iguanas, manatees. The mammals in the list represent three independent flips back to flipperhood.
I was reflecting on this because of a io9 report of a study investigating the presence of an appendix in a range of mammals. The human appendix is called 'vermiform' because it is a (fat) worm-like appendage to the gut where the small intestine (going down) morphs into the ascending loop of the colon.It is quite variable: effective absent in about 1/100,000 and ranging in size from 2-20cm in length. In some human populations, it tends to sit in front of the gut and in others it is generally tucked behind. For years, the appendix was regarded as a vestigial caecum with no obvious function and an annoying tendency to get inflamed. Appendectomies became routine as soon as minor surgery became safe. Later, larger, epidemiological studies found that an intact appendix conferred some immunological benefit and the rate of whipping the things out declined. Those without an appendix, for example, are more likely to get recurrent Clostridium difficile [prev] infections. About 10 years ago, researchers at Duke U came up with the theory that the appendix was a safe house for 'good' bacteria in the intestinome. After your finely adjusted intestinal flora gets flushed down the toilet with that dodgy curry or is given a good drubbing by a course of antibiotics, then the hold-outs in the appendix come out of the bunker and recolonise the intestine.
The comparative anatomy study is neat because it is real old fashioned science, the researchers gathered truck-loads of data about appendix size and shape, other aspects of the intestine for each species and then information about habitats and feeding habits, food quality, weaning age, and even rates of precipitation. They then lashed all the data into a big digital hopper and shook it to see what correlated with the presence and absence of the appendix. What came out was the observation that the appendix is associated with the caecum! But also associated with the presence of caecal 'lymphoid' tissue. Lymphoid tissue is all about immunity and immune function; which in turn is all about finding an adaptive balance between good and bad microbes. When they dig further they report an inverse correlation between caecum length and habitat breadth . . . but no correlation between appendix length and habitat breadth. They do correct for multiple testing but nevertheless there is no obvious ecological correlate with appendix size which convinces me that we/they have found a 'cause' for the appendix.The paper is also nice because, being published in Comptes Rendus Palevol, all the figure legends are in both English and French. Thus:
contrasts taxa differing in the state of the independent character
un contraste entre taxons différant dans l’état de caractère indépendant
. . . reading scientific French is easy, it's the small words that cause all the problems.
I guess the bottom line is that an appendix can be handy to have in certain circumstances but you're not going to be totally banjaxed if you're missing one. That's what we know from 100 years of human appendectomies and the redundancy seems to be true across a wide range of mammals.