10 Worst Adaptations in the Animal Kingdom

An inflamed human appendix gets evicted. iStock/Thinkstock

Like a lot of the so-called "worst adaptations," the appendix gets a bad rap. Sure, it can kill you by bursting and there's basically little consequence for taking it out like yesterday's trash. And in Charles Darwin's day -- when finding evolutionary reasons for adaptations was pretty important to further his survival-of-the-fittest theory -- Mr. Darwin himself was stymied by the appendix. He thought maybe it was a vestigial part of the cecum, a section of the gut full of bacteria that was used to break down leaves. As we chomped less greenery, the cecum shrunk and our appendix became just a sad, shriveled reminder of our big-cecum days.

Scientists now see that the appendix could possibly have a function after all, although they're not entirely sure what it is. They think it still might have something to do with gut bacteria: It's possible that good bacteria can migrate to the appendix when an infection occurs, keeping them safe from the bad bacteria, thus allowing a healthy repopulation after the infection is over [source: Barras]. But not every mammalian species has an appendix, which might make the "worst adaptation" not for those with the appendix – but for those without.