Literature reports indicate that their prevalence varies across populations. Some, such as human tails and branchial cysts may be clinically symptomatic. Atavistic and vestigial structures are found throughout the head, neck, and spine. Pertinent anatomical structures are numerous and include human tails, plica semilunaris, the vomeronasal organ, levator claviculae, and external ear muscles, to name a few. We define atavistic and vestigial structures and employ these definitions consistently when classifying anatomical structures.

We review the literature on atavistic and vestigial anatomical structures of the head, neck, and spine that may be encountered in clinical practice. We provide an overview of important atavistic and vestigial structures in the head, neck, and spine that can serve as a reference for anatomists and clinical neuroscientists.

While physicians are cognizant of their existence, atavistic and vestigial structures are rarely emphasized in anatomical curricula and can, therefore, be puzzling when discovered incidentally. In addition, the literature is replete with examples of the terms atavistic and vestigial being used interchangeably without careful distinction between them. Atavism is when an ancestral trait reappears after loss through an evolutionary change in previous generations, whereas vestigial structures are remnants that are largely or entirely functionless relative to their original roles. Resultant atavistic and vestigial anatomical structures have long been a source of perplexity. Organisms may retain nonfunctional anatomical features as a consequence of evolutionary natural selection.

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