Most people know only two things about the appendix: You don't need it — and if it bursts, you need surgery fast.

That basic story traces back at least to Charles Darwin, the English naturalist who developed the theory of natural selection. In "The Descent of Man," he described the appendix as a vestige: a leftover from plant-eating ancestors with larger digestive organs. For more than a century, that interpretation shaped both textbook and casual medical wisdom.

But the evolutionary story of the appendix turns out to be much more complicated.