Comparative anatomy and phylogenetic distribution of the mammalian cecal appendix.
A recently improved understanding of gut immunity has merged with current thinking in biological and medical science, pointing to an apparent function of the mammalian cecal appendix as a safe-house for symbiotic gut microbes, preserving the flora during times of gastrointestinal infection in societies without modern medicine. This function is potentially a selective force for the evolution and maintenance of the appendix, and provides an impetus for reassessment of the evolution of the appendix. A comparative anatomical approach reveals three apparent morphotypes of the cecal appendix, as well as appendix-like structures in some species that lack a true cecal appendix. Cladistic analyses indicate that the appendix has evolved independently at least twice (at least once in diprotodont marsupials and at least once in Euarchontoglires), shows a highly significant (P < 0.0001) phylogenetic signal in its distribution, and has been maintained in mammalian evolution for 80 million years or longer.
Smith
H F
HF
Department of Basic Medical Sciences, University of Arizona College of Medicine-Phoenix, in Partnership with Arizona State University, Phoenix, AZ, USA.
Fisher
R E
RE
Everett
M L
ML
Thomas
A D
AD
Bollinger
R Randal
RR
Parker
W
W
D000818:Animals; D002432:Cecum / Q000033:anatomy & histology* / Q000382:microbiology; D008322:Mammals / Q000033:anatomy & histology* / Q000145:classification*; D010802:Phylogeny