How long ago was the apendix useful?

The appendix is believed to have been useful in our evolutionary past, particularly for digesting cellulose from plant materials. It is thought to have been more prominent in herbivorous ancestors, aiding in the breakdown of tough plant fibers. Over time, as human diets shifted, the appendix became less critical, and its role diminished, leading to its classification as a vestigial organ. While it may still have some immune functions, its usefulness has significantly declined.