[The following is an exact transcript of this podcast.]

Two years ago, Duke University Medical Center researchers said that the supposedly useless appendix is actually where good gut bacteria safely hide out during some unpleasant intestinal conditions.

Now the research team has looked at the appendix over evolutionary history. They found that animals have had appendixes for about 80 million years. And the organ has evolved separately at least twice, once among the weird Australian marsupials and another time in the regular old mammal lineage that we belong to.

Darwin thought that only a few animals have an appendix and that the human version was what was left of a digestive organ called the cecum. But the new study found that 70 percent of rodent and primate groups have species with an appendix. And some living animals have a cecum and an appendix. If Darwin had known about species that had both organs, he probably would have revised his views of the appendix, the researchers note.

Ironically, it’s natural selection that keeps the human appendix from shrinking away completely. Because smaller ones are more likely to become infected. And keep your genes out of the pool.

—Steve Mirsky