The Appendix/Cave Fish Eyes/Etc. are (NOT) vestigial

[Bill_II]

@jpm

Why do you think it disappeared? Not a whole lot of fossil appendixes (?) around. Are you saying it independently evolved 32 times? References?

In a paper referenced by the paper @Ashwin_s linked above it does indicate that the appendix has appeared 38 times and been lost 6 times. An interesting paper by the way.

An analysis of the phylogenetic distribution and morphological features of the appendix strongly suggest that the appendix can be described as a ‘‘recurrent phenotype’’ among Euarchontoglires and perhaps among Diprotodontia.

The Cecal Appendix: One More Immune Component With a Function Disturbed By Post‐Industrial Culture

https://onlinelibrary.wiley.com/doi/full/10.1002/ar.21357

And from the Conclusion

The cecal appendix appears to be a ‘‘recurrent’’ trait, variably expressed in Euarchontoglires and in Diprotodontia. Changes in selection pressure which influence the presence or absence of the appendix could involve changes in such factors as diet and social behavior that decrease the impact of intestinal pathogens on the health of the population, but the exact nature of such factors remains unknown.

So even if Darwin got it wrong in general (he was only aware of the appendix in humans) it still supports evolution as the appendix appears as needed.

It is thus possible that the appendices found in Euarchontoglires and perhaps also in Diprotodontia are homologous within their respective clades, due to common descent from an ancestor which had either developed a cecal appendix or perhaps a ‘‘constitutional responsiveness’’ (inherited ability to respond to environmental variation) (Darwin, 1859; West-Eberhard, 2003), which facilitates the relatively rapid evolution of a cecal appendix.

Yes @Ashwin_s I know you will argue that this just means evolution is not falsifiable. I prefer to think that the theory of evolution is robust enough to account for new data.