The appendix in the human body, which is a thin pocket that ventures off the cecum in the stomach, associated with the digestive system, may have a function that humankind did not know about until recently. New research suggests that its main purpose, aside from getting inflated and turning into appendicitis, is related to releasing useful gut bacteria.
While many other mammals have the appendix, little has been known about its major functions in the body, and for a long time it has been seen as a remainder of the evolutionary process. However, studying its functions in the context of evolution, researchers have a new theory about its usefulness.
The Appendix As A Safe House For Gut Bacteria
The study, published in the January-February edition of the journal Comptes Rendus Palevol , was led by researchers at Midwestern University Arizona College of Osteopathic Medicine with the joint efforts of an international team.
The scientists gathered data on 533 species of mammals and assessed whether they have or do not have the organ. They also studied a series of environmental and gastrointestinal characteristics of these species. Then, they created a genetic tree to better understand the evolution of the appendix among the mammals they investigated, as an attempt to understand the reason why it is present in some species and absent in others.
What they discovered is that the mysterious organ has evolved in 30 different directions, and most of the time it does not disappear along with the evolution of a species, which further underlines the existence of an important underlying function that it serves.
The researchers analyzed a series of factors, including climate, diet or ecological conditions, managing to reject a series of hypotheses proposed by previous studies on the matter.
Additionally, the team found one common trait of the mammal species which do have the appendix, which is a higher average concentration of lymphoid tissue in the cecum, suggesting it serves an immunity-related purpose. Lymphatic tissue is also associated with the growth of useful gut bacteria, which further proves that this organ may, in fact, be a safe house for mammals' gut bacteria.
"A correlation between appendix presence and concentration of cecal lymphoid tissue supports the hypothesis of an adaptive immune function for this complex. Other new findings include an inverse correlation between relative cecum length and habitat breadth, and positive relationships between cecum length and mean group size, and between colon length and weaning age," noted the research.
The team also documented the likelihood of animals to have an appendix depending on the shape of their ceca. The research suggests that mammals with a spiral-shaped type of ceca were more prone to having an appendix compared with the ones with cylindrical or round cecum. This led them to the conclusion that the appendix did not evolve independently, but in the context of a "cecoappendicular complex" which includes the organ as well as cecum.
A Useful Organ
Previous research on the matter has proven that a cluster of immune cells allow the appendix to keep humans' digestive systems healthy, suggesting that innate lymphoid cells (ILCs) play a crucial role in the protection against bacterial infection of patients with weakened immune systems, thus assessing the evolutionary importance of the appendix among humans.