Baku, March 12, AZERTAC

Most people know only two things about the appendix: You don't need it – and if it bursts, you need surgery fast, according to Science Alert.

That basic story traces back at least to Charles Darwin, the English naturalist who developed the theory of natural selection. In The Descent of Man, he described the appendix as a vestige: a leftover from plant-eating ancestors with larger digestive organs.

For more than a century, that interpretation shaped both textbook and casual medical wisdom.

But the evolutionary story of the appendix turns out to be much more complicated.

Along with our colleague Helene M. Hartman, a student preparing for a career in health care, we combined our expertise in behavioral ecology, biology, and history to review the scientific literature on the appendix, expecting a simple answer.

Instead, we found an organ that evolution kept reinventing, more interesting than most people imagine.

How did the appendix evolve?

The appendix is a small pouch branching off the first section of the large intestine. Its shape and structure vary widely across species – a clue that evolution may have tinkered with it more than once.

Some species, including certain primates such as humans and great apes, have a long, cylindrical appendix. In others, including several marsupials such as wombats and koalas, the appendix appears shorter or more funnel-shaped.

Still others, including some rodents and rabbits, have differently proportioned or branching structures. This structural diversity suggests that evolution has modified the organ under different ecological conditions.

That suspicion is supported by evolutionary analyses. Comparative studies show that an appendix-like structure evolved independently in at least three distinct lineages of mammals – marsupials, primates, and glires, a group that includes rodents and rabbits.

A broader evolutionary survey found that the appendix evolved separately at least 32 times across 361 mammalian species.

When a trait evolves repeatedly and independently, biologists call this convergent evolution. Convergence does not mean a structure is indispensable. But it does suggest that, under certain environmental conditions, having that structure provided a consistent enough advantage for evolution to favor it again and again.

In other words, the appendix is unlikely to be a useless evolutionary accident.

What does the appendix do?

The appendix supports the immune system. It contains gut-associated lymphoid tissue – immune cells embedded in the intestinal wall that help monitor microbial activity in the gut.

In early life, this tissue exposes developing immune cells to intestinal microbes, helping the body learn to distinguish between harmless symbionts and harmful pathogens.

The appendix is particularly rich in structures called lymphoid follicles during childhood and adolescence, when the immune system is still maturing. These immune components participate in mucosal immunity, which helps regulate microbial populations along the intestinal lining and other mucosal surfaces.

Lymphoid follicles produce antibodies, such as immunoglobulin A, to neutralize pathogens.

Researchers have also proposed that the appendix acts as a microbial refuge. Some have suggested that biofilms – thin, structured communities of bacteria – line the appendix.

During severe gastrointestinal infections that flush much of the gut microbiome from the colon, beneficial bacteria sheltered within these biofilms may survive and help repopulate the intestine afterward. Those beneficial microbes assist with digestion, compete with pathogens, and interact with the immune system in ways that reduce inflammation and promote recovery.

These hypotheses motivated a question our team explored: If the appendix helps preserve microbial stability, could removing it subtly affect reproductive fitness?

Older clinical concerns suggested that appendicitis or appendectomy might impair fertility by causing inflammation and scarring – known as tubal adhesions – in the fallopian tubes. Such scarring could physically obstruct the egg's passage to the uterus.

But several large studies have since found no decrease in fertility after appendectomy – in some cases, researchers found a small increase in pregnancy rates.

The appendix appears to have multiple functions, including immune and microbial ones. Affecting fertility, however, does not seem to be one of them.

Evolutionary importance and modern life

While the appendix has an interesting past, with evolution continually reinventing it, its modern importance is modest at best. Darwin underestimated the organ's history, but his instinct wasn't far off in the medical present: Some parts of human biology mattered more in the environments people evolved in than in the lives they lead today.

Early humans lived in environments with little sanitation and strong social contact – perfect conditions for outbreaks of pathogens that cause diarrhea. An appendix that quickly restored the microbiome after infection could significantly improve survival.

But over the past century, clean water, improved sanitation, and antibiotics have sharply reduced deaths from diarrheal diseases in high-income countries.

As a result, the evolutionary pressures that once favored the appendix have largely disappeared. Meanwhile, the medical risks of keeping the appendix – most notably appendicitis – remain. Modern surgery typically treats an infected appendix by removing it. A structure that was once a global evolutionary advantage is now more of a medical liability.

This mismatch between past adaptations and present environments illustrates a core principle in evolutionary medicine: Evolution optimizes for survival and reproduction in ancestral environments, not for health, comfort, or longevity in modern ones.

Evolution operates at the level of populations over generations, favoring traits that increase average reproductive success, even if those traits sometimes harm individuals. Medicine works the other way around – helping individuals thrive in the present world rather than survive the past one.

The appendix is not an IKEA spare part included "just in case," but

neither is it essential today. Human biology has many traits that were once beneficial, now marginal – and understanding them allows medicine to make better modern decisions.