The appendix is a useless leftover

What actually happened

Darwin proposed the vestigial reading, and it was the textbook answer for over a century. The evidence against it accumulated on two fronts. First, evolutionary: a 2009 analysis by Heather Smith, William Parker, and colleagues found the appendix has evolved independently at least twice, and likely far more times, across mammals, and has persisted for tens of millions of years. Genuinely useless structures do not get reinvented repeatedly, and they do not survive that long against the metabolic cost and the mortality risk of appendicitis.

Second, mechanistic: the 2007 biofilm paper from Randal Bollinger and Parker at Duke proposed that the appendix's shape, a narrow cul-de-sac off the main flow, is exactly what you would design to shelter a bacterial community from being swept out. Studies since found that people without an appendix are roughly two to four times more likely to have recurrent Clostridioides difficile infection, which is what you would predict if the reserve is gone.

The practical point is unchanged: if it is inflamed, it comes out, and you will be fine without it. But the confident classroom line that it does nothing at all is obsolete. Treatment has shifted too, with trials including CODA in 2020 showing antibiotics are a reasonable alternative to surgery for many uncomplicated cases.