The human appendix is widely considered a vestigial organ, a structure that has lost its original function through evolution.

In many herbivores, such as rabbits and horses, the caecum is a large, functional organ that houses bacteria specialized in the digestion of cellulose from plant matter. The human appendix is a remnant of a larger, cellulose-digesting caecum found in our distant herbivorous ancestors. As the early human diet shifted away from raw plant material, the need for a large caecum diminished, and it shrank over time, leaving the appendix as a reduced structure.