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Researchers from the Midwestern University Arizona College of Osteopathic Medicine in the US gathered data on the presence or absence of the appendix and other gastrointestinal and environmental traits for 533 mammal species.
It should be very well understood that God has not created any organ in the human body which is not useful.
They mapped the data onto a phylogeny (genetic tree) to track how the appendix has evolved through evolution, and to try to determine why some species have an appendix while others do not. They discovered that the appendix has evolved independently in several mammal lineages, and almost never disappears from a lineage once it has appeared. This suggests that the appendix likely serves an adaptive purpose.
They found out that species with an appendix have higher average concentrations of lymphoid (immune) tissue in the caecum. Lymphatic tissue can stimulate growth of some types of beneficial gut bacteria.