ecome quite controversial as researchers have learned more about the phenomenon. I'll say more later about why even the "true tails" in humans don't deserve that name. For now, here's a crucial fact: even such so-called "tails" aren't anything like those found in tailed mammals. That is for the simple reason that "true tails" in humans entirely lack vertebrae -- or any kind of bone, cartilage, notochord, or spinal cord. As the aforementioned paper in the Journal of Neurosurgery explains:
In all reported cases, the vestigial human tail lacks bone, cartilage, notochord, and spinal cord. It is unique in this feature.2Other prominent medical research journals agree:
- A 2013 paper in the Journal of Child Neurology states: "True tails are boneless, midline protrusion usually attached to the skin of the sacrococcygeal region and capable of spontaneous or reflex motion. They consist of normal skin, connective tissue, muscle, vessels, and nerves and are covered by skin. Bone, cartilage, notochord, and spinal cord are lacking."3
- A paper from the Journal of Pediatric Surgery states: "The human vestigial tail lacks bone, cartilage, notochord, and spinal cord. It contains a central core of mature fatty tissue divided into small lobules by thin fibrous septa. Small blood vessels and nerve fibers are scattered throughout. Bundles of striated muscle fibers, sometimes degenerated, tend to aggregate in the center."4
- An article in the British Journal of Neurosurgery explains: "A true tail in humans is vestigial and never contains vertebrae in contrast to other vertebrate animals."5
- Most striking of all, perhaps, are the words of a famous paper on tails in The New England Journal of Medicine: "When the caudal appendage is critically examined, however, it is evident that there are major morphologic differences between the caudal appendage and the tails of other vertebrates. First of all, the caudal appendage does not contain even rudimentary vertebral structures. There are no well-documented cases of caudal appendages containing caudal vertebrae or an increased number of vertebrae in the medical literature, and there is no zoological precedent for a vertebral tail without caudal vertebrae."6
- Finally, an article in Human Pathology explains: "In humans a true tail, is vestigial, however, and never contains vertebrae. ... Bona-fide cases of human tails containing bone have not been documented."7
But what about "pseudotails" -- can't they contain bone? Yes, sometimes they can, but pseudotails don't contain vertebrae (as all other mammalian tails do), and they're not located at the base of the coccyx, where a "true tail" ought to be -- they are found in various other places along the lower back, and may even be off to the side from the backbone. Moreover, "pseudotails" are often found associated with other types of defects, and are obvious deformations, as multiple papers have recognized:
- "The pseudotail is an anomalous prolongation of the coccygeal vertebra, lipoma, teratoma, chondrodystrophy, or parasitic fetus."8
- "The pseudotail has no embryological relationship to human tail development, but is any variable abnormal caudal tail-like structure or protrusion."9
- "Pseudotail has no relationship to human tail development; it is a tissue that has been accidentally located in the lumbrosacrococcygeal area."10
- "Pseudotails are varied lesions with only a superficial resemblance to the true vestigial tail. The most common cause of a pseudotail is a prolongation of the coccygeal vertebra"11
- "The pseudotail is often short, stump-like, and occasionally bulging. It may be composed of teratomatous elements, adipose tissue, or cartilage."12
Nonetheless, it's quite reasonable to wonder why humans have these things called "true tails" -- which have a non-trivial superficial resemblance to a real tail, despite their significant differences -- in the first place? And intriguingly, why are humans occasionally found with "pseudotails" -- growths that also superficially resemble tails, but are clearly deformations, defects, and abnormalities? Cou