The cecal appendix has evolved independently at least 32 times in different mammal lineages

A 2013 study analysing 533 mammal species found that the cecal appendix has evolved independently at least 32 separate times in different lineages, making it one of the most frequently independently evolved structures in vertebrate history. This 'convergent evolution' strongly suggests the appendix has an adaptive function — it keeps being selected for across wildly different animals. The leading hypothesis is immune: the appendix is packed with lymphoid tissue and appears in species with complex gut microbiomes. Far from a vestigial remnant, it may be an evolutionarily favoured organ.

Source: Smith, H.F. et al., 'Multiple independent appearances of the cecal appendix', Journal of Evolutionary Biology (2013)