What does the appendix do? Biologists explain the complicated evolution of this inconvenient organ

Most people only know two things about the appendix: You don’t need it – and if it bursts, you need surgery quickly.

This basic history goes 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 remnant of herbivorous ancestors with larger digestive organs. For more than a century, this interpretation has shaped both classical and informal medical wisdom.

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

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 discovered an organ that evolution has continually reinvented, more interesting than most people imagine.

How did the appendix evolve?

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

Some species, notably certain primates such as humans and great apes, have a long cylindrical appendage. In others, including several marsupials such as wombats and koalas, the appendix appears shorter or funnel-shaped. Still others, notably certain rodents and rabbits, have structures of different proportions or branches. This structural diversity suggests that evolution has modified the organ under different ecological conditions.

This 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 study found that the appendix evolved separately at least 32 times in 361 mammal species.

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

In other words, the appendix is ​​unlikely to be an unnecessary evolutionary accident.

What is the annex for?

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. Early in life, this tissue exposes developing immune cells to gut 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 mucosa and other mucosal surfaces. Lymphoid follicles produce antibodies, such as immunoglobulin A, to neutralize pathogens.

The researchers also proposed that the appendix acts as a microbial refuge. Some have suggested that biofilms – communities of thin, structured bacteria – line the appendix. During severe gastrointestinal infections that eliminate much of the gut microbiome from the colon, beneficial bacteria harbored in these biofilms can survive and help repopulate the gut afterward. These beneficial microbes aid digestion, compete with pathogens, and interact with the immune system in ways that reduce inflammation and promote healing.

These hypotheses motivated a question explored by our team: if the appendix helps preserve microbial stability, could its removal subtly affect reproductive capacity?

Older clinical concerns suggested that appendicitis or appendectomy could impair fertility by causing inflammation and scarring – known as tubal adhesions – in the fallopian tubes. Such scars could physically obstruct the passage of the egg to the uterus. But several large studies have since found no decrease in fertility after an appendectomy — in some cases, researchers have seen a slight increase in pregnancy rates.

The appendix appears to have multiple functions, including immune and microbial. However, the impact on fertility does not appear to be one of them.

Evolutionary significance and modern life

Although the appendix has an interesting past, and evolution continually reinvents it, its modern significance is modest at best. Darwin underestimated the history of this organ, but his instinct was not far from the medical present: certain parts of human biology mattered more in the environments in which people evolved than in the lives they lead today.

Early humans lived in environments with little sanitation and high social contact – perfect conditions for outbreaks of diarrhea-causing pathogens. An appendix that quickly restores the microbiome after infection could significantly improve survival. But over the past century, clean water, improved sanitation and antibiotics have significantly 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 maintaining the appendix – including appendicitis – remain. Modern surgery usually treats an infected appendix by removing it. A structure that was once a global evolutionary advantage is now more of a medical liability.

This discrepancy between past adaptations and present environments illustrates a fundamental principle of evolutionary medicine: evolution optimizes survival and reproduction in ancestral environments, not health, comfort or longevity in modern environments.

Evolution occurs at the population level over generations, favoring traits that increase average reproductive success, even if these traits sometimes harm individuals. Medicine works the other way: it helps people thrive in the present world rather than survive in the past world.

The annex is not an IKEA spare part included “just in case”, but neither is it essential today. Human biology has many features that were once beneficial, now marginal – and understanding them allows medicine to make better modern decisions.

This article is republished from The Conversation, an independent, nonprofit news organization that brings you trusted facts and analysis to help you make sense of our complex world. It was written by: Phil Starks, Tufts University and Lilia Goncharova, Tufts University

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The authors do not work for, consult, own shares in, or receive funding from any company or organization that would benefit from this article, and have disclosed no relevant affiliations beyond their academic appointment.