Most people only know two things about the appendix. It’s not necessary, and if your appendix ruptures, you need immediate surgery.
The basic story goes back at least as far as Charles Darwin, the British naturalist who developed the theory of natural selection. In The Origin of Man, he described the appendix as a remnant of our plant-eating ancestors, who had larger digestive organs. For more than a century, its interpretation has shaped both textbooks and popular medical knowledge.
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Together with my colleague Helen M. Hartman, a student preparing for a career in the medical field, we used our combined expertise in behavioral ecology, biology, and history to review the scientific literature in the appendix and hoped for a simple answer.
Instead, we discovered an organ that evolution kept reinventing, more interesting than most people imagined.
How did the appendix evolve?
The appendix is a small sac that branches off from the first part of the large intestine. Its shape and structure vary widely depending on the species. This is a clue that it may have been modified multiple times during evolution.
Some species, including humans and certain primates such as great apes, have long, cylindrical appendices. In other animals, including some marsupials such as wombats and koalas, the appendix appears shorter or more funnel-shaped. Still other animals, such as some rodents and rabbits, have different proportions and branching structures. This structural diversity suggests that evolution has changed organs under different ecological conditions.
This suspicion is supported by evolutionary analysis. Comparative studies show that appendix-like structures evolved independently in at least three different lineages of mammals: marsupials, primates, and glaeria, a group that includes rodents and rabbits. A more extensive evolutionary study found that the appendix evolved at least 32 separate times across 361 species of mammals.
When traits repeatedly and independently evolve, biologists call this convergent evolution. Convergence does not mean that structure is essential. But it suggests that under certain environmental conditions, having that structure provided a consistent and sufficient advantage for evolution to be advantageous time and time again.
In other words, the appendix is unlikely to be a useless evolutionary accident.
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What does the appendix do?
The appendix supports the immune system. This includes 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 and helps the body learn to distinguish between harmless commensals 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 and help regulate microbial populations along the intestinal lining and other mucosal surfaces. Lymphoid follicles produce antibodies such as immunoglobulin A to neutralize pathogens.
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Researchers also propose that the appendix acts as a hiding place for microorganisms. Some suggest that a biofilm (a thin, structured bacterial community) lines the appendix. During severe gastrointestinal infections when a large portion of the gut microbiome is flushed out of the colon, the beneficial bacteria protected within these biofilms may survive and subsequently help repopulate the gut. These beneficial microorganisms aid digestion, compete with pathogens, and interact with the immune system in ways that reduce inflammation and promote recovery.
These hypotheses raised questions that our team considered: If the appendix helps maintain microbial stability, could removing it subtly affect reproductive fitness?
Old clinical concerns suggested that appendicitis or appendectomy could impair fertility by causing inflammation and scarring within the fallopian tubes, known as tubal adhesions. This kind of scarring can physically prevent the egg from passing into the uterus. But since then, several large studies have found no reduction in fertility after appendectomy, and in some cases, researchers have found a slight increase in pregnancy rates.
The appendix appears to have multiple functions, including immune and microbial functions. However, effects on fertility do not seem to be among them.
The importance of evolution and modern life
Although the appendix has an interesting past and is continually reinvented through evolution, its importance in modern times is modest at best. Darwin underestimated the history of organs, but his instincts were not far behind the present in medicine. Some parts of human biology were more important in the environments in which people evolved than in the lives they lead today.
Early humans lived in environments with little sanitation and strong social contact. This is the perfect condition for the development of pathogens that cause diarrhea. Having an appendix that quickly restores the microbiome after infection could significantly improve survival rates. 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 all but disappeared. On the other hand, the medical risks of preserving the appendix remain, particularly appendicitis. Modern surgery typically treats the 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 the current environment illustrates a core principle of evolutionary medicine. Evolution optimizes for survival and reproduction in the environment of our ancestors, not for health, comfort, and longevity in our modern environment.
Evolution proceeds at the population level over generations, favoring traits that increase average reproductive success, even if those traits are harmful to individuals. Medicine does the opposite: it helps individuals thrive in the present world rather than survive in the past.
This appendix is not a “just in case” IKEA spare part, but it is also not essential today. There are many features of human biology that were once beneficial but are no longer considered important, and understanding them can help medicine make better modern decisions.
This edited article is republished from The Conversation under a Creative Commons license. Read the original article.
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