Appendix

I've never been confident in the claim that the appendix is a useless vestigial organ. Everything else is doing something, sometimes many somethings at once. Why waste energy growing something useless? Maybe it is a bacteria reservoir?

They discovered that the appendix has evolved independently in several mammal lineages, over 30 separate times, and almost never disappears from a lineage once it has appeared. This suggests that the appendix likely serves an adaptive purpose.