What does the appendix actually do?
More than it was long credited with. The appendix was described for most of the twentieth century as a vestigial organ with no function — Darwin himself suggested it was a remnant of a larger caecum used for digesting plant material — but the evidence has shifted substantially.
The current leading hypothesis: a bacterial safe house. Proposed by researchers at Duke University in 2007, the argument is that the appendix serves as a reservoir for beneficial gut bacteria.
Its structure supports this. It is a narrow, blind-ended tube branching off the large intestine — a cul-de-sac largely protected from the bulk flow of intestinal contents. Its walls contain substantial lymphoid tissue producing immune factors that help maintain a biofilm of beneficial bacteria on its lining.
After an event that flushes the gut — severe diarrhoeal illness such as cholera or dysentery — the appendix can repopulate the colon with a healthy microbial community. In environments where such illnesses were common and repeated, that would be a substantial survival advantage.
Supporting evidence:
Evolutionary history. Comparative analysis indicates the appendix has evolved independently many times across mammal lineages, and has rarely been lost once acquired. Structures that are genuinely useless do not evolve repeatedly and persist.
Immune tissue. Its concentration of gut-associated lymphoid tissue suggests an immune role, particularly in early life.
Some studies have found higher rates of recurrent Clostridioides difficile infection in people without an appendix, consistent with the reservoir hypothesis — though findings are not uniform.
What this does not mean. People live entirely normally without one, particularly in settings with clean water and modern sanitation, where the repopulation function is rarely needed.
Appendicitis remains a genuine emergency — an obstructed appendix becomes inflamed, can perforate, and requires urgent treatment. Some uncomplicated cases are now treated with antibiotics rather than surgery.