For decades, the human appendix was dismissed as a useless evolutionary leftover – a small, worm-shaped pouch that caused nothing but trouble. Appendectomy (surgical removal) became one of the most common emergency surgeries worldwide. But what if the appendix actually helps us live longer? A new study on mammals suggests exactly that.
https://onlinelibrary.wiley.com/doi/10.1111/joa.13501
What the researchers did
Scientists collected data from 258 mammalian species (a “short dataset”) to analyze the relationship between the appendix and maximum lifespan. Among these species, 39 had an appendix and 219 did not. A larger dataset of 317 species (the “long dataset”) was used to study evolutionary history.
Because species are not independent – they share evolutionary relationships – the researchers could not use ordinary regression. Instead, they applied a special statistical method called Phylogenetic Generalized Least Squares (PGLS) , which accounts for the “family tree” of species.
To find the best explanation for the data, they tested 15 different statistical models . Each model was evaluated using the Akaike Information Criterion (AIC) , a tool that compares how well models fit the data while penalizing unnecessary complexity.
Key finding 1: Appendix = longer life
The best model (the one with the highest AIC weight) included two independent variables: body weight and presence/absence of an appendix – but no interaction between them. In this optimal model, species with an appendix showed a significantly higher intercept (a statistical measure of baseline lifespan) than species without one.
In plain English: after accounting for differences in body size and evolutionary relatedness, mammals that have an appendix tend to live longer than those that don’t. The correlation is real.
Key finding 2: Gaining an appendix is common – losing it is rare
Using maximum parsimony reconstruction (MPR) on the evolutionary tree, researchers traced how the appendix appeared and disappeared over millions of years. The result was striking:
- 16 to 17 independent “gains” (evolutionary origins) of the appendix
- Only 1 clear “loss” – in the grey bamboo lemur (Hapalemur griseus )
Even with a method called ACCTRAN, which deliberately favors losses over gains, the numbers still showed far more gains (18–20) than losses (4–5). This suggests that once an animal evolves an appendix, it tends to keep it.
Key finding 3: A strange evolutionary divide
The appendix’s distribution across mammals is extremely uneven. Among two major groups of placental mammals:
- Euarchontoglires (which includes primates, rodents, and rabbits) experienced 19 transitions (gains or losses) of the appendix.
- Laurasiatheria (which includes bats, whales, dogs, cows, and horses) experienced zero transitions .
In other words, the entire branch of Laurasiatheria – hundreds of species – never changed its appendix status. This suggests that deep evolutionary history, not just ecology or diet, strongly influences whether a lineage keeps or loses this organ.
What does this mean for humans?
The study does not say that removing your appendix will shorten your life. It says that across evolution, species with an appendix consistently live longer – possibly because the appendix plays a role in immune function or maintaining beneficial gut bacteria during times of infection.
For doctors and patients, this challenges the old “useless organ” view. Perhaps we should be more careful before scheduling an appendectomy. The appendix may be small, but evolution has kept it around for a reason.
Bottom line:
Science now shows that the appendix is not a mistake. It is an organ that has been gained dozens of times in mammal evolution, almost never lost, and linked to longer life. Next time someone calls it “vestigial,” you can tell them: the data say otherwise.