In The Descent of Man, Charles Darwin cited the tailbone (coccyx) as a supposed vestigial feature revealing our descent from tailed ancestors. He wrote: “In certain rare and anomalous cases it [the coccyx] has been known… to form a small external rudiment of a tail.” Thus was born the classical Darwinian view of the human tail, now a full-blown icon of evolution — restated by physicist Karl Giberson in his recent debate with Stephen Meyer, which is why I bring the subject up now. This myth, which I’ll be examining in future posts, holds that the “tail” is a regression to an earlier form, an expression of dormant genes retained from our ancient forebears.

A paper in the Journal of Neurosurgery explains that this view is itself a holdover from recapitulation thinking:

True human tails are rarely encountered in medicine. At the time when Darwin’s theory of evolution was a matter of debate, hundreds of dubious cases were reported. The presence of a tail in a human being was considered by evolutionists as an example that “ontogeny recapitulates phylogeny.”¹

Giberson seems to endorse a similar view of human tails, holding that they arise when vestigial genes are accidentally turned on. He is wrong on multiple levels. Firstly, as far as the medical literature reflects, not a single known human being has ever been born with, as he puts it, a “perfectly formed, even functional tail.”

Human tails are extremely rare, with perhaps only a few hundred cases documented worldwide over the past half-century. Medical researchers who have had the lucky opportunity to study a human tail have divided them into two general categories: “true tails,” which extend from the coccyx (tailbone) where one might expect a so-called “vestigial tail,” and “pseudotails” which are often found in other locations on the lower back, and seem to be obvious aberrations since they are often associated with anomalies.

This distinction is based upon evolutionary assumptions, and in recent years it has become 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.²

Other 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

These observations certainly don’t make it sound like humans can have “perfectly formed, even functional tails.” In fact, it’s difficult to argue that any tail could be called “bona fide” if it isn’t “bone-fied.”

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

In other words, if humans have a bony tail, it’s not a “true tail” — it’s a “pseudotail” because of other abnormalities, but if it’s a “true tail, it contains no bone, cartilage, notochord, or spinal cord. And no human tail contains vertebrae.

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? Could the fact that humans have obviously deformed pseudotails near the same location as “true tails” provide a hint about what causes the formation of “true tails”? Is the distinction between a “true” and “pseudo” tail medically sound?

To help us begin to answer these questions, in my next post I will assess Giberson’s claim that tails result from the simple turning on of vestigial genes.

References Cited:
[1.] Roberto Spiegelmann, Edgardo Schinder, Mordejai Mintz, and Alexander Blakstein, “The human tail: a benign stigma,” Journal of Neurosurgery, 63: 461-462 (1985).
[2.] Roberto Spiegelmann, Edgardo Schinder, Mordejai Mintz, and Alexander Blakstein, “The human tail: a benign stigma,” Journal of Neurosurgery, 63: 461-462 (1985).
[3.] Surasak Puvabanditsin, Eugene Garrow, Sharada Gowda, Meera Joshi-Kale, and Rajeev Mehta, “A Gelatinous Human Tail With Lipomyelocele: Case Report,” Journal of Child Neurology, 28(1) 124-127 (2013) (emphases added). See also Biswanath Mukhopadhyay, Ram M. Shukla, Madhumita Mukhopadhyay, Kartik C. Mandal, Pankaj Haldar, and Abhijit Benare, “Spectrum of human tails: A report of six cases,” Journal of the Indian Association of Pediatric Surgery, 17(1): 23-25 (Jan-Mar, 2012).
[4.] Allan Joel Belzberg, Stanley Terence Myles, and Cynthia Lucy Trevenen, “The Human Tail and Spinal Dysraphism,” Journal of Pediatric Surgery, 26: 1243-1245 (October, 1991) (emphasis added).
[5.] S.P.S. Chauhan, N.N. Gopal, Mohit Jain, and Anurag Gupta, “Human tail with spina bifida,” British Journal of Neurosurgery, 23(6): 634-635 (December 2009) (emphasis added).
[6.] Fred Ledley, “Evolution and the Human Tail,” The New England Journal of Medicine, 306 (20): 1212-1215 (May 20, 1982) (emphases added).
[7.] Anh H. Dao, Martin G. Netsky, “Human Tails and Pseudotails,” Human Pathology, 15(5): 449-453 [May 1984) (emphasis added).
[8.] Surasak Puvabanditsin, Eugene Garrow, Sharada Gowda, Meera Joshi-Kale, and Rajeev Mehta, “A Gelatinous Human Tail With Lipomyelocele: Case Report,” Journal of Child Neurology, 28(1) 124-127 (2013).
[9.] Se-Hyuck Park, Jee Soon Huh, Ki Hong Cho, Yong Sam Shin, Se Hyck Kim, Young Hwan Ahn, Kyung Gi Cho, Soo Han Yoon, “Teratoma in Human Tail Lipoma,” Pediatric Neurosurgery, 41:158-161 (2005).
[10.] Frank L. Lu, Pen-Jung Wang, Ru-Jeng Teng, and Kuo-Inn Tsou Yau, “The Human Tail,” Pediatric Neurology, 19 No. 3 (1998).
[11.] Terry J. Dubrow, Phillip Ashley Wackym, Malcolm A. Lesavoy, “Detailing the Human Tail,” Annals of Plastic Surgery, 20: 340-344 (April, 1988).
[12.] Allan Joel Belzberg, Stanley Terence Myles, and Cynthia Lucy Trevenen, “The Human Tail and Spinal Dysraphism,” Journal of Pediatric Surgery, 26: 1243-1245 (October, 1991).

Photo source: Tambako the Jaguar/Flickr.