Appendix May Not Be Biological Afterthought


Charles Darwin was one of the first scientists to theorize on the function of the appendix, which in his day had been identified only in humans and other great apes. He hypothesized that the distant ancestors of these animals survived on a diet of leaves, and so they required a large cecum, a portion of the gut that houses bacteria that can break down stubborn plant tissue. Later, he speculated, these ancestors shifted to a largely fruit-based diet that was easier to digest. A large cecum was no longer necessary, and it began to shrink; today our cecum is tiny. Darwin thought the appendix, which juts off of the cecum, is one of its former folds that shriveled up as the cecum shrank. Consequently, he thought it carried no function.

But some scientists have challenged the idea that the appendix serves no purpose.

It’s been clear for about a century that the structure contains a particular type of tissue belonging to the lymphatic system. This system carries the white blood cells that help fight infections. Within the last decade, research has shown that this lymphatic tissue encourages the growth of some kinds of beneficial gut bacteria. What’s more, careful anatomical study of other mammals has revealed that species as diverse as beavers, koalas, and porcupines also have a structure jutting off of their guts in exactly the same place as our appendix — in other words, the feature is much more common among mammals than once thought.

Now, an international team of researchers that includes Heather Smith, an evolutionary biologist at Midwestern University in Glendale, Ariz., and William Parker, a surgeon who studies the immune system at Duke University Medical Center in Durham, N. C., says it has the strongest evidence yet that the appendix serves a purpose.



By plotting the dietary information onto the evolutionary tree, the researchers could work out whether the appendix appears when a particular group of mammals changes its diet. In most cases, there was no sign of a dietary shift, suggesting appendix evolution doesn’t necessarily proceed as Darwin thought. He may have correctly identified the origin of the ape appendix, though, which the analysis confirms did appear when our ancestors switched diets.

Randolph Nesse, an evolutionary biologist at the University of Michigan, is impressed by the new study. “I salute the authors for creating an extraordinary database,” he says. “The conclusion that the appendix has appeared 32 times is amazing. I do find their argument for the positive correlation of appendix and cecum sizes to be a convincing refutation of Darwin’s hypothesis.”

“I agree with the general assertion that the appendix evolved numerous times in mammals, but I think the exact count is still up for debate,” adds Olaf Bininda-Emonds, an evolutionary biologist at the University of Oldenburg in Germany. There is some uncertainty over whether all 50 species considered to have an appendix really do possess one. When just the clear-cut cases are included, the appendix evolved 18 times, he says.

Even that figure suggests the appendix performs a useful function, and the hunt is now on to identify what that function is. The research team may already have the answer. In 2007, Parker and his colleagues suggested that the appendix has an immunological role, acting as a “safe house” for beneficial gut bacteria. These bacteria help train the immune system and can prevent diseases by outcompeting dangerous pathogenic bacteria-but there are times when the dangerous microbes gain the upper hand and overrun the gut.


