Another Icon of Evolution: The Darwinian Myth of Human "Tails"

Casey Luskin
Evolution News & Views
May 22, 2014
Print Article

[This article was adapted from a series of articles originally published on Evolution News & Views. For the original articles, please see: Part 1, Part 2, Part 3, Part 4, Part 5, Part 6, Part 7.]

Part 1: Introduction
Writing for The Daily Beast about his 2014 debate with Stephen Meyer, theistic evolutionist Karl Giberson commented:

I showed pictures of otherwise healthy humans who had been born with webbed feet and tails. I asked the challenging question: "Why does the human genome contain instructions for the production of features we don't use?" The scientific explanation is that we inherited these instructions from our tailed ancestors but the instructions for producing them have been shut off in our genomes, which is why Shallow Hal is the only person most people know who has a tail. Sometimes the "ignore these genes" message gets lost in fetal development, however, and babies are born with perfectly formed, even functional tails.

Dr. Giberson, a physicist, has done a fine job of stating the often-heard Darwinian evolutionary view of human "tails." But is any of it true?

  • Are humans ever born with "perfectly formed, even functional tails"?
  • Is the "scientific explanation" of the human tail that really they're simply an accidental turning on of the "complete instructions for the production" of tails retained from our "tailed ancestors," that is, the failure to "ignore" our vestigial "genes" for tails?
  • Are humans born with tails typically "otherwise healthy" -- i.e., are human "tails" simply benign structures that indicate nothing more than our descent from tailed ancestors?

The answer, unequivocally, is no, no and no. Because this point comes up periodically in the Darwin debate, achieving the status of what Jonathan Wells has called (in his book of the same name) an "icon of evolution," these questions are worth considering in detail. I plan to do just that in coming days here at ENV, examining the peer-reviewed scientific and medical literature on human tails.

In brief, what are called human "tails" lack bone (i.e., vertebrae normally found in tails), cartilage, notochord, and spinal cord. This makes them totally unlike any tail found anywhere among other mammals or for that matter, any other animals. As one paper found, there is "no zoological precedent" for the so-called "tails" found in humans. In fact, Giberson's statement notwithstanding, no human baby has ever been documented as possessing a "perfectly formed, even functional tail."

Moreover, the most current medical thinking about human tails views them as a type of developmental defect, not an evolutionary regression. They are an abnormality almost always associated with other neurodevelopmental defects. They are not the result of the accidental "turning on" of "vestigial genes." Specifically, they are the result of the failure of a developmental structure, one that grows during normal human development, to regress into the embryo. Though the causes of this are still not entirely clear, what is clear is that the failure to regress is caused by underlying developmental abnormalities that cause a variety of other birth defects as well. As a general rule, tails are not found in "otherwise healthy" babies.

Moreover, as this article will show, the classical evolutionary view of human tails has caused some doctors to wrongly view them as nothing more than a mere benign evolutionary relict -- a diagnosis that multiple experts have warned is potentially dangerous. Why? Because it could lead to treatments that ignore other defects commonly associated with tails. In other words, the evolutionary myth of human tails is not only mistaken. It is also medically harmful.

Part 2: Are Humans Ever Born with "Perfectly Formed" Tails?
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 2014 debate with Stephen Meyer. This myth 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 the next section I will assess Giberson's claim that tails result from the simple turning on of vestigial genes.

Part 3: Do Human "Tails" Represent the Simple "Turning On" of Genes Retained from Our Ancestors?
In reference to a debate he had with Stephen Meyer, physicist and Darwin advocate Karl Giberson explained: "We inherited these instructions [for tails] from our tailed ancestors but the instructions for producing them have been shut off in our genomes." He suggests that humans born with tails have switched on genes that are normally just sitting there in our genome, unused. He wants us to think that true tails are some kind of a regression to an earlier form.

This pseudo-recapitulationist position recalls Giberson's arguments in his book Saving Darwin, where he writes, "Two-month-old embryos of chicken, pigs, fish, and humans look similar. They all have gills, webbed hands and feet, and tails. In a few weeks these formations disappear from the human embryo." (p. 200) Except that human embryos in fact never have gills,13 and webbed feet ("syndactyly") in embryos aren't a holdover from our amphibian ancestors but are part of a normal (and quite logical) way that hands and feet develop.14 In any case, Giberson's attempt to describe human tails as a regression is undermined by the evidence of how so-called "true tails" develop.

During normal human embryogenesis, a "tail"-like structure appears around the fourth or fifth week of development. This isn't the result of vestigial genes that are normally "turned off" (as Giberson puts it) being accidentally turned back on when our development regresses to some primitive state. Rather, they are always and normally "turned on," and the formation of this tail-like structure is part of the normal process of the development of the human body plan and nervous system -- a point supported by the fact that the human embryonic notochord and neural tube are thought to extend through much if not all of the entire extent of the human embryonic tail.15 By the fifth or sixth week the "tail" reaches its full extent, but during the seventh and eighth week of development, it is reabsorbed into the embryo. By the end of the eighth week, the tail is usually completely gone.16

It's worth noting that the tail is not unusual or unique in being a structure that temporarily appears, and then disappears, during our development. A paper in Annals of Anatomy recognizes, "During normal human development a number of transient structures form and subsequently regress completely," and notes: "One of the most prominent structures that regress during development is the human tail."17 The paper goes on to explain that it is not unusual for cells or even macrostructures to die: "In the process of development and in adult life, large numbers of cells are known to die in many different tissues. In some cases, whole regions or entire organs are eliminated."18

Thus, by suggesting that the tail is the result of ancient genes that failed to be "turned off," Giberson is thus promoting something of a Darwinian urban legend. In his telling, the human tail is something special that grows abnormally when vestigial genes are suddenly turned on. In reality, it's a normal part of human development, and babies are only born with a tail when it fails to be reabsorbed back into the embryo.

Another article states that "the true tail can be explained as a failure of complete regression of the nonvertebrate part of the tail at 8 weeks of pregnancy."19

So does the fact that human babies are sometimes born with tails suggest we are related to animals with tails? No -- the actual causes of "true tails" have caused some doctors to suspect that it isn't an evolutionary regression, but rather a "disturbance" in development. This is widely recongized in the medical community. An oft-cited paper in Pediatric Neurology by Lu et al. (1998) states:

"During the seventh and eighth weeks, the vertebrated portion retracts into the soft tissue. The nonvertebrated part projects temporarily and then undergoes regression caused by phagocytosis, with the debris-laden macrophages migrating back to the body, and it disappears completely at the end of the eighth week. Thus, the presence of human tail can be considered a disturbance in the development of the embryo but not a regression in the evolutionary process."20

A strong piece of evidence that even "true tails" are a developmental glitch is the fact that they are strongly associated with other deformations and medical problems. We'll explore that further in subsequent sections.

Part 4: Are Human Tails Mere "Vestigial" or "Benign" Structures Born to "Otherwise Healthy" Babies?
For years, proponents of Darwin's notion that human tails are mere vestiges of our ancestry have also promoted the idea that these tails are benign structures that can be removed with little thought or concern about other potential problems. Dr. Giberson, a physicist, thus states without qualification that human tails are born to "otherwise healthy" humans. Not true. This false belief follows from the evolutionary paradigm, and it has led to some dangerous medical practices. Because human tails are so rare, and because techniques like magnetic resonance imaging (MRI) have only been around for a few decades, biologists have been slow to accumulate and interpret data about what so-called "tails" actually are, what's going on when they fail to regress, and what else is likely going wrong when a baby is born with a tail.

Prominent medical researchers on the "human tail" now see it as "a disturbance in the development of the embryo but not a regression in the evolutionary process."21 Why do they say this? Because, as a paper I noted from Pediatric Neurology by Lu et al. (1998) explains, the human tail is often associated with other deformations and developmental problems:

The high incidence of human tail combined with spinal dysraphism noted recently may throw light on the problem. A high association with skin lesions such as subcutaneous tumors, skin dimples or sinuses, hemangiomas, hypertrichosis, and hyperpigmentation was observed in the patients with spinal dysraphism. These associations give a hint to the relationship between human tail and spinal dysraphism.

Focal premature dysjunction of the neural tube may thus be an important factor in the disturbance of human tail development and regression. The unfused neural tube exposes the paraxial mesoderm to the dorsal aspect of the neural ectoderm and induces formation of fatty elements and lipoma. This event can be supported by the high incidence of lipoma (27.12%) in the authors' series. The formation of fatty elements and lipoma may prevent the fusion of the neural tube thus tethering the cord because of attachment of the fatty elements with neural structures, resulting in tethered spinal cord syndrome. The different extents of the fusing tube may develop different features of the lumbosacrococcygeal area that may result in human tail.22

The human tail isn't as an evolutionary regression yielding a benign structure left over from our ancestors. It is a developmental aberration, the result of improper forming of the neural tube. In support of their thesis, Lu et al. present data showing a high level of concurrence of the human tail with other "anomalies." In 59 examples of human tails examined between 1960 and 1997 they found:23

Associated Anomaly Incidence (%)
Spinal dysraphism 29/59 (49.15)
Meningocele 8/29 (27.59)
Spina bifida only 21/29 (72.41)
Lipoma 16/59 (27.12)
Tethered spinal cord 12/59 (20.34)
Coccygeal vertebrae 7/59 (11.86)
Syndactyly 2/59 (3.39)
Hemangioma 1/59 (1.69)
Cleft palate 1/59 (1.69)
Crouzon syndrome 1/59 (1.69)
Congenital tracheal stenosis 1/59 (1.69)
Clubfoot 1/59 (1.69)
Von Recklinghausen disease 1/59 (1.69)
Omphalocele 1/59 (1.69)
Hypoplasia of digits 1/59 (1.69)
Tetralogy of Fallot 1/59 (1.69)

This data strongly contradicts the view that the human tail is a "benign" structure found in "otherwise healthy" babies. In fact, Lu et al. probably significantly underestimated the incidence of disease and abnormalities associated with the human tail, because MRI technologies -- which in many cases are the only way to detect some of these defects -- were not available at the time many of these tails were examined. They write:

After 1980 the incidence of associated anomalies, such as spinal dysraphism, became higher because CT or MRI began to be performed as part of examination. The incidence is probably underestimated because some reports did not perform any detailed image studies.24

Indeed, a later paper commenting on the study observed, "Of the 16 cases imaged with MR or CT myelography, 81% had a tethered spinal cord"25 -- a defect also called an occult spinal dysraphism, which typically leads to serious neurological problems if untreated. This is an extremely significant finding. It means Karl Giberson is simply wrong to claim that tails are normally found in "otherwise healthy" humans.

Other papers concur. A paper in the Journal of Pediatric Surgery finds that human tails are "often associated with occult spinal dysraphism."26 A paper in British Journal of Plastic Surgery notes the tail "is potentially a site of peculiar tumours."27 A paper in Journal of Perinatology states: "Caudal appendages also occur as typical or frequent manifestations of certain syndromes."28 A paper in the Journal of the Indian Association of Pediatric Surgery states, "Tails are usually associated with occult spinal dysraphism."29 A paper in Pediatric Neurosurgery found:

Children born with a tail-like appendage have a rare malformation that is frequently associated with abnormalities of the spine and spinal cord. ... many [cases of tails] simply represented a form of spina bifida, rather than a "true" tail. ... Children born with a tail appendage have a high rate of associated spinal dysraphism and tethered cord.30

In fact, another paper in Pediatric Neurosurgery finds the reason the tail is "important" isn't because it reveals something about our ancestry, but because it can indicate something is wrong with the nervous system:

The human tail is a rare congenital anomaly and is important primarily because it sometimes accompanies central nervous system anomalies, such as spinal dysraphism, lipoma and spinal cord tethering.31

While considering the strong evidence that the human tail is associated with developmental defects, various papers have found, the view of the "tail" as a regression is just as wrong as old superstitions that it represented some supernatural omen:

Many authors saw this curious and rare condition to be evidence of man's descent from or relation to other animals, while others made it the subject of superstition. Advanced imaging technology in recent decades has allowed a more thorough investigation of these patients and better defined their association with spinal dysraphism and tethered spinal cord.32

Still another paper in Advances in Orthopedics finds that the "so-called human tail...is often considered to be a cutaneous marker of underlying occult dysraphism."33 Or, as a paper in Pediatric Neurosurgery states, "The human tail or caudal appendage is usually associated with occult spinal dysraphism."34 And on and on. In other words, the human tail is typically the sign of an underlying developmental abnormality, and according to Lu et al., "This fact plays an important role in understanding the disturbance of development and regression of human tails."35 It's unfortunate that the evolutionary viewpoint at first led doctors to think of the tail as a benign structure that, without further trouble or careful examination, could simply be surgically excised. In a subsequent section I'll examine the medical harms that have resulted from this now thoroughly outdated idea -- a legacy of Charles Darwin, still being advanced today by Darwin-advocates like Karl Giberson.

Part 5: How the Darwinian View of Human "Tails" Leads to Harmful Medical Practices
Darwin-defenders like Karl Giberson have used the rare phenomenon of babies born with supposed "tails" as evidence for common descent. This Darwinian thinking is not merely mistaken but, worse than that, it has led to medical harm. Under the influence of Darwinism, some doctors have viewed so-called "true tails" as benign growths that could simply and expeditiously be excised, calling for no additional examination of the patient. For example, a 1985 paper, "The human tail: a benign stigma," states that "the true human tail is a benign condition not associated with any underlying [spinal] cord malformation."36 Or, as a 1984 paper recommended, a "vestigial tail" can be "easily removed surgically, without residual effects."37 In his recent debate with Stephen Meyer, Dr. Giberson echoed this idea that human tails are found in "otherwise healthy" people.

The papers I just mentioned were written in the mid 1980s, before MRI analyses became widespread and revealed that human tails were commonly associated with other defects and abnormalities. Thus, a more recent paper in Clinical Pediatrics describes the older, "vestigial" view of "true tails":

Attempts have been made to classify tails in human beings as "true" (persistent vestigial) or as "pseudo tail." The true tail is thought to be a benign condition which earlier was thought to have no underlying malformation of the spinal cord or involvement of the neural elements.38

The result of this evolutionary view was that doctors were less likely to suspect, expect, or explore potential neurological problems that might be associated with a tail:

Most human tails have been reported to be located in the lumbar or sacrococcygeal region, and of varying length. As a rule, they were surgically removed, although in many cases the authors failed to discuss the depth of their surgical exploration and whether or not neuroimaging had been undertaken.39

Evolution-based medicine was dangerous in this case because tails are indeed often associated with other underlying problems in development. They are not merely a reversion to a benign ancestral state. The evolution-inspired view has been overturned by medical research finding high levels of defects associated with tails. We discussed this in the previous section, but a little more documentation won't hurt. A 2008 paper in Journal of Pediatric Surgery stated: "In contradiction to a previous report, true vestigial tails are not benign because they may be associated with underlying dysraphic state. About 50% of the cases were associated with either meningocele or spina bifida occulta."40 Such findings now lead doctors to recommend a careful search for other deformities whenever any kind of tail is present. As one paper in Clinical Pediatrics recommends:

[M]ost of the reports in the literature demonstrate such significant associated dysrhaphic states that appropriate neuroimaging should be performed in both ["true" and "pseudo" tail] groups of patients prior to surgical intervention.41

A paper in the journal Minimally Invasive Neurosurgery likewise noted, "The human tail may be related to spinal dysraphism and requires detailed neuroimaging investigation and microsurgery."42 Thus, when a tail is discovered, additional neuroimaging is necessary to determine whether further procedures are needed to remove other tail-associated defects that could lead to neurological problems, if left untreated.

Another paper recounted the dangers of leaving these defects untreated:

This [high incidence of defects associated with tails] is the most clinically significant finding, since removal of lipomas and spinal cord untethering can prevent the development of irreversible neurologic deficits of the lower extremities, bowel and bladder. ... Removal of the tail is indicated for cosmetic purposes only, but removal of a spinal lipoma and untethering of the spinal cord are necessary to prevent the development of irreversible neurologic deficits.43

A paper in Journal of Child Neurology likewise elaborates the dangers of failing to look for these other defects, since the human tail "can be associated with an underlying spinal lesion that, if not recognized early, can lead to permanent neurologic disabilities."44 If you treat the tail as a vestigial throwback in "otherwise healthy" babies, then you won't do the detailed examinations necessary to find these problems and properly treat them.

Unfortunately, some experts wanted so badly to see tails as benign that they misguidedly defined "true tails" and "pseudotails" in such a way that if an abnormality were found, then by definition the tail could not be a "true" tail. One paper in British Journal of Plastic Surgery explains:

To state that a caudal appendage is not a true tail simply because it is associated with spina bifida or meningocele, or with neoplastic or ectopic histological elements, is probably inaccurate.45

This Darwinian view also ignores the fact that both "true tails" and "pseudotails" can be associated with abnormalities, a fact that could blur or negate such distinctions between the two:

Although more than 10 years have passed, the 'human tail' is still ill-defined. It has been classified as either a true (persistent vestigial) tail or a pseudotail. A true tail is thought by some authors to be a benign condition not associated with any underlying spinal cord malformation. Other researchers have emphasized that the classification of each caudal appendage into true tail or pseudotail remains obscure and the distinction on clinical examination is dubious at best.46

Because true tails are associated with defects, including abnormalities of the spinal cord, these authors reject the idea that there is a clinically important distinction between the true tail and the pseudotail: both can be associated with problems and need to be removed with great care. Thus, one. A paper in the Journal of Pediatric Surgery agrees:

The distinction between the true tail and pseudotail on clinical examination is almost always dubious, and despite a normal neurological examination, normal plain x-ray of spine, and clinical classification of true tail, subfascial exploration may show tethered cord or lipomeningomyelocele as what occurred in our case. Hence, ignoring the type of tail, each individual case needs comprehensive preoperative investigation, and surgical exploration of the intraspinal content should be done microsurgically to avoid any damage or neurological deficit.47

Another paper likewise concludes that the "true tail" versus "pseudotail" distinction derives from evolutionary theory, and is clinically unhelpful:

The terms "true tail" and "pseudotail" have no clinical significance and should probably be abandoned, except possibly by embryologists. Lu et al. have proposed a more practical description using MR, and simply divide tailed patients into those with or those without associated tethering of the spinal cord. ... Conclusions regarding the evolutionary significance of the tail and distinctions between true tail and pseudotail are clinically unimportant and should be abandoned. 48

In sum, it was overcoming Darwinian thinking that led to better medicine. But this raises additional questions. If both pseudotails and true tails are associated with many of the same developmental defects, could that suggest they have similar causes? If so, where does that leave the view, articulated by Karl Giberson and others, of the tail as a vestigial evolutionary holdover?

Part 6: "Pseudotails" and "True Tails" Have Similar Causes, Suggesting Both Are Birth Defects, Not Vestigial Holdovers
There is still much debate over exactly why tails arise during development, and at least one paper has recognized that the tail's "etiology is poorly understood."49 That said, no one believes that pseudotails are an evolutionary relic. This makes it noteworthy that some experts suggest that true tails and pseudotails share similar developmental origins -- as birth defects, not as evolutionary relics. A paper in Journal of Perinatology argues that true tails and pseudotails may have similar origins as abnormalities:

Caudal appendages or human tails were divided by Dao and Netsky into true tails, which contain muscle and are movable, and pseudotails, which do not move. However, this is now considered arbitrary and without clinical significance as both kinds are derived from notocordal remnants and the etiology of both is probably similar. ... Their etiology is probably related to an abnormal sequence of caudal neurulation.50

Thus, some experts have rejected the "vestigial" view of tails, based on the existence of pseudotails -- structures that clearly cannot be an evolutionary relic, yet have the appearance and associated defects similar to true tails. A paper in Clinical Pediatrics explains that a shared etiology would seem to contradict an evolutionary view:

One of the earliest etiological hypotheses suggested that the human tail was a remnant of an embryological vestigial structure seen during early phases of gestation. Other authors have discussed various problems with this [vestigial structure] theory. One is that these tails may [be] present in other areas than the typical sacrococcygeal region. Accordingly, some authors prefer to use the term "neuroectodermal appendages." Such appendages may be the superficial extension of a dermal sinus tract that results from failure of the neuroectoderm to separate from the epithelial ectoderm of the primitive medullary plate, creating an epithelialized tract that extends into the spinal canal. If simultaneous extension of the epithelium moved outward as well as inward, an appendage could form. In support of that hypothesis these authors claim that pathologic specimens in these series are quite consistent. An important aspect of this pathology is that all of these entities involve neural elements to some degree.51

Thus, some experts now believe that the causes of the "true tail" may be similar to those of the "pseudotail," the latter being an obvious deformity. Here's one view of how it happens, from Pediatric Neurosurgery:

The current prevailing concept of the embryology of this malformation is that the tail structure is a form of congenital dermal sinus tract resulting from premature or incomplete dysjunction of the somatic and neuroectoderm during primary neurulation.52

Another paper in Pediatric Neurosurgery explains that the high incidence of abnormalities associated with all kinds of tails raises suspicions that we may not be dealing with genuine tails at all:

The human tail or caudal appendage is usually associated with occult spinal dysraphism. Lu et al. reviewed 59 cases of human tails reported in the literature from 1960 to 1997. The human tails were associated with occult spinal dysraphism and lipoma in 49.2 and 27.1% of the cases, respectively. However, the lumbosacral stigmata such as subcutaneous tumors, skin dimples or sinus, hemangiomas, hair tufts and hyperpigmentation may be associated with spina bifida occulta and spinal dysraphism. A review of 200 cases of occult spinal dysraphism showed the condition to be associated with cutaneous signs in more than 50% of the instances. For the above reason, we raise the suspicion as to whether the skin appendage presented with spinal dysraphism is a true human tail or only contingent on an ectodermal abnormality.53

They conclude that when associated abnormalities are present, such as spinal dysraphism, it's inappropriate to consider the tail a vestigial relic:

To our knowledge, the occurrence of such a caudal appendage has not been previously reported. The present case could support the above suspicion that the so-called human tail presented with spinal dysraphism is only an appendage, not a true human tail. Besides, the cartilaginous tissue in this caudal appendage also supports the hypothesis of abnormal development of the mesoderm induced by the dorsal neural tube...54

This view is scientifically parsimonious, in the sense that it explains the existence of tails that appear "normal" and those that are associated with defects:

An alternative hypothesis is that the principle cause is a disorder of secondary neurulation. Since secondary neurulation and formation of the tail bud begin prior to completion of primary neurulation (closure of the caudal neuropore), a disorder of the secondary neural tube or notochord could possibly affect closure of the neuropore and result in spinal cord lipoma and abnormal tail bud regression. Alternatively, tails with lipomas involving only the inferior conus medullaris or filum terminale may not involve primary neurulation at all. They may simply represent abnormal secondary neurulation without affecting the caudal neuropore, since these structures all arise from the caudal cell mass alone. The S4 hemivertebra in this case is further evidence of the abnormal development of the caudal cell mass. The spectrum of abnormalities seen with the human tail could possibly be explained then by the timing of the abnormality in caudal cell mass formation and regression. This would span the range from an early disorder resulting in spinal lipoma and tethering of the cord to the more infrequent later ones that cause only persistence of some of the tail bud tissue without spinal abnormalities.55

Without doubt, debate will continue over exactly why humans are occasionally born with tails. While the causes of this medical phenomenon are not fully understood, I have presented ample evidence from research literature that they are most readily and sensibly interpreted not as evolutionary relict, but as a developmental defect, often associated with other abnormalities. Yet another evolutionary icon has fallen to the data.

Part 7: Human Tails: Another Evolutionary Icon that Giberson, Coyne, and Other Darwin Defenders Would Be Wise to Abandon
In this article, I have taken a close look at an often-repeated claim that humans born with a "vestigial" tail, a rare medical condition, represent a relic of evolution, pointing to our descent from a tailed ancestor. Charles Darwin was the first to articulate this notion, but it survives, a true vestige, among his modern advocates.

I used Karl Giberson's statements in a recent debate with Stephen Meyer as my occasion for taking up the subject, but other Darwinian spokesman have said just the same. Jerry Coyne, for example, repeats the myth of the human tail on pages 65-66 of his book Why Evolution Is True where he writes "tails appear to be true atavisms" and wrongly states that these tails can appear like a normal "tail" with vertebrae.

To sum up: Dr. Giberson expressed the typical view, claiming:

  • Sometimes "babies are born with perfectly formed, even functional tails."
  • Babies with tails tend to be "otherwise healthy."
  • "The scientific explanation is that we inherited these instructions from our tailed ancestors but the instructions for producing them have been shut off in our genomes."

As we have seen, each of these claims is false:

  • No babies are born with perfectly formed, fully functional tails.
  • Babies that do have tails typically have serious associated neurological defects.
  • The exact causes of tails are debated, but because of their persistent association with neurological defects, the most plausible view is that they result from abnormalities and deviations in development.

Thankfully, most doctors today take their view on this not from Darwinian evolution, but from the evidence, the accumulated experience of clinical researchers as represented in the medical literature. Researchers warn that seeing tails as benign structures can lead doctors to miss serious developmental problems.

It's a good thing, not least for patients, that the Darwinian doctrine, little more than an urban legend, is fast on its way to being abandoned.

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).

[13.] Jonathan Wells, "Haeckel's Embryos and Evolution: Setting the Record Straight," The American Biology Teacher, Vol. 61(5):345-349 (May, 1999), at http://www.discovery.org/f/629

[14.] The "webbed hands and feet" Giberson cites are easily explained as a simple part of the way human hands and feet normally develop: digits grow within a mass of tissue, and later tissue between the digits recedes. This doesn't imply we're going through an "amphibian" stage -- it's just a logical way to grow a hand. Sometimes babies are born with webbed feet as the result of a birth defect, as one article states: "The exact cause of webbed toes is still unknown. Some used to speculate that it was inherited, since family members often share the condition, but it is also common for only one member of a family to have webbed toes. Some studies show a pregnant woman's nutritional intake during early gestation and smoking during pregnancy can also contribute to deformities such as webbed toes. Webbed toes are also associated with rare conditions such as: •Down syndrome •Apert's syndrome •Poland syndrome •Holt-Oram syndrome •Carpenter's syndrome •Edward's syndrome •Fetal hydantoin effect (using the medication hydantoin during pregnancy) •Miller syndrome •Pfeiffer syndrome •Amniotic Band syndrome, also known as constriction band syndrome •Smith-Lemli-Opitz syndrome •Aarskog-Scott syndrome •Bardet-Beidl syndrome •Cornelia de Lange syndrome •Familial syndactyly •Timothy syndrome Unfortunately, the list above is not complete. There are over 100 different syndromes that are associated with webbing of the digits." Steven Miller, "Webbed Toes," Footvitals.com, http://www.footvitals.com/toes/webbed-toes.html

[15.] Daniel J. Donovan Robert C. Pedersen, "Human Tail with Noncontiguous Intraspinal Lipoma and Spinal Cord Tethering: Case Report and Embryologic Discussion," Pediatric Neurosurgery, 41:35-40 (2005).

[16.] Good descriptions of this process can be found in: Chunquan Cai, Ouyan Shi, and Changhong Shen, "Surgical Treatment of a Patient with Human Tail and Multiple Abnormalities of the Spinal Cord and Column," Advances in Orthopedics, 2011: 153797; Frank L. Lu, Pen-Jung Wang, Ru-Jeng Teng, and Kuo-Inn Tsou Yau, "The Human Tail," Pediatric Neurology, 19 No. 3 (1998); Daniel J. Donovan Robert C. Pedersen, "Human Tail with Noncontiguous Intraspinal Lipoma and Spinal Cord Tethering: Case Report and Embryologic Discussion," Pediatric Neurosurgery, 41:35-40 (2005); Anh H. Dao, Martin G. Netsky, "Human Tails and Pseudotails," Human Pathology, 15(5): 449-453 [May 1984); D. Sapunar, K. Vilovic, M. England, and M. Saraga-Babic, "Morphological diversity of dying cells during regression of the human tail," Annals of Anatomy, 183: 217-222 (2001).

[17.] D. Sapunar, K. Vilovic, M. England, and M. Saraga-Babic, "Morphological diversity of dying cells during regression of the human tail," Annals of Anatomy, 183: 217-222 (2001) (emphases added).

[18.] D. Sapunar, K. Vilovic, M. England, and M. Saraga-Babic, "Morphological diversity of dying cells during regression of the human tail," Annals of Anatomy, 183: 217-222 (2001) (emphases added).

[19.] 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).

[20.] Frank L. Lu, Pen-Jung Wang, Ru-Jeng Teng, and Kuo-Inn Tsou Yau, "The Human Tail," Pediatric Neurology, 19 No. 3 (1998) (emphasis added). See also Chunquan Cai, Ouyan Shi, and Changhong Shen, "Surgical Treatment of a Patient with Human Tail and Multiple Abnormalities of the Spinal Cord and Column," Advances in Orthopedics, 2011: 153797.

[21.] Frank L. Lu, Pen-Jung Wang, Ru-Jeng Teng, and Kuo-Inn Tsou Yau, "The Human Tail," Pediatric Neurology, 19 No. 3 (1998) (emphases added). See also Chunquan Cai, Ouyan Shi, and Changhong Shen, "Surgical Treatment of a Patient with Human Tail and Multiple Abnormalities of the Spinal Cord and Column," Advances in Orthopedics, 2011: 153797.

[22.] Frank L. Lu, Pen-Jung Wang, Ru-Jeng Teng, and Kuo-Inn Tsou Yau, "The Human Tail," Pediatric Neurology, 19 No. 3 (1998) (emphasis added). See also Chunquan Cai, Ouyan Shi, and Changhong Shen, "Surgical Treatment of a Patient with Human Tail and Multiple Abnormalities of the Spinal Cord and Column," Advances in Orthopedics, 2011: 153797.

[23.] "Associated anomalies in human tail" table reprinted from Lu et al. (1998): Frank L. Lu, Pen-Jung Wang, Ru-Jeng Teng, and Kuo-Inn Tsou Yau, "The Human Tail," Pediatric Neurology, 19 No. 3 (1998).

[24.] Frank L. Lu, Pen-Jung Wang, Ru-Jeng Teng, and Kuo-Inn Tsou Yau, "The Human Tail," Pediatric Neurology, 19 No. 3 (1998) (emphasis added). See also Chunquan Cai, Ouyan Shi, and Changhong Shen, "Surgical Treatment of a Patient with Human Tail and Multiple Abnormalities of the Spinal Cord and Column," Advances in Orthopedics, 2011: 153797.

[25.] Daniel J. Donovan Robert C. Pedersen, "Human Tail with Noncontiguous Intraspinal Lipoma and Spinal Cord Tethering: Case Report and Embryologic Discussion," Pediatric Neurosurgery, 41:35-40 (2005) (emphasis added).

[26.] Deepak Kumar Singha, Basant Kumarb, V.D. Sinhaa, and H.R. Bagariaa, "The human tail: rare lesion with occult spinal dysraphism--a case report," Journal of Pediatric Surgery, 43: E41-E43 (2008).

[27.] Abraham M. Baruchin, Dan Mahler, Dan J. Hauben, and Lior Rosenberg, "The human caudal appendage (human tail)," British Journal of Plastic Surgery, 36: 120-123 (1983).

[28.] T.E. Herman and M.J. Siegel, "Human tail-caudal appendage: tethered cord," Journal of Perinatology, 28: 518-519 (2008).

[29.] 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).

[30.] Daniel J. Donovan Robert C. Pedersen, "Human Tail with Noncontiguous Intraspinal Lipoma and Spinal Cord Tethering: Case Report and Embryologic Discussion," Pediatric Neurosurgery, 41:35-40 (2005).

[31.] 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).

[32.] Daniel J. Donovan Robert C. Pedersen, "Human Tail with Noncontiguous Intraspinal Lipoma and Spinal Cord Tethering: Case Report and Embryologic Discussion," Pediatric Neurosurgery, 41:35-40 (2005); Chunquan Cai, Ouyan Shi, and Changhong Shen, "Human born with a tail: A case report," SA Journal of Child Health, 7(1): 38-39 (2013); Chunquan Cai, Ouyan Shi, and Changhong Shen, "Surgical Treatment of a Patient with Human Tail and Multiple Abnormalities of the Spinal Cord and Column," Advances in Orthopedics, 2011: 153797.

[33.] Chunquan Cai, Ouyan Shi, and Changhong Shen, "Surgical Treatment of a Patient with Human Tail and Multiple Abnormalities of the Spinal Cord and Column," Advances in Orthopedics, 2011: 153797 (emphasis added).

[34.] Pei-Jung Lin, Yu-Tang Chang, Hsing-I Tseng, Jan-You Lin, Yu-Sheng Huang," Human Tail and Myelomeningocele," Pediatric Neurosurgery, 43:334-337 (2007) (emphasis added).

[35.] Frank L. Lu, Pen-Jung Wang, Ru-Jeng Teng, and Kuo-Inn Tsou Yau, "The Human Tail," Pediatric Neurology, 19 No. 3 (1998).

[36.] Roberto Spiegelmann, Edgardo Schinder, Mordejai Mintz, and Alexander Blakstein, "The human tail: a benign stigma," Journal of Neurosurgery, 63: 461-462 (1985) (emphasis added).

[37.] Anh H. Dao, Martin G. Netsky, "Human Tails and Pseudotails," Human Pathology, 15(5): 449-453 [May 1984).

[38.] Hector E. James and Timothy G. Canty, "Human Tails and Associated Spinal Anomalies," Clinical Pediatrics, 34: 286-288 (1995) (emphases added).

[39.] Hector E. James and Timothy G. Canty, "Human Tails and Associated Spinal Anomalies," Clinical Pediatrics, 34: 286-288 (1995) (emphasis added).

[40.] Deepak Kumar Singha, Basant Kumarb, V.D. Sinhaa, and H.R. Bagariaa, "The human tail: rare lesion with occult spinal dysraphism--a case report," Journal of Pediatric Surgery, 43: E41-E43 (2008) (emphasis added).

[41.] Hector E. James and Timothy G. Canty, "Human Tails and Associated Spinal Anomalies," Clinical Pediatrics, 34: 286-288 (1995) (emphases added).

[42.] E. Gonul, Y Izci, O. Onguru, E. Timurkaynak, N. Seber, "The Human Tail Associated with Intraspinal Lipoma Case Report," Minimally Invasive Neurosurgery, 43: 215-218 (2000).

[43.] Daniel J. Donovan Robert C. Pedersen, "Human Tail with Noncontiguous Intraspinal Lipoma and Spinal Cord Tethering: Case Report and Embryologic Discussion," Pediatric Neurosurgery, 41:35-40 (2005) (emphases added).

[44.] Dipti Kumar and Akshay Kapoor, "Human Tail: Nature's Aberration," Journal of Child Neurology, 27: 924 (2012) (emphasis added).

[45.] Abraham M. Baruchin, Dan Mahler, Dan J. Hauben, and Lior Rosenberg, "The human caudal appendage (human tail)," British Journal of Plastic Surgery, 36: 120-123 (1983).

[46.] A.M. Baruchin, "Human tail," British Journal of Plastic Surgery, 48: 114-115 (1995).

[47.] Deepak Kumar Singha, Basant Kumarb, V.D. Sinhaa, and H.R. Bagariaa, "The human tail: rare lesion with occult spinal dysraphism--a case report," Journal of Pediatric Surgery, 43: E41-E43 (2008) (emphases added).

[48.] Daniel J. Donovan Robert C. Pedersen, "Human Tail with Noncontiguous Intraspinal Lipoma and Spinal Cord Tethering: Case Report and Embryologic Discussion," Pediatric Neurosurgery, 41:35-40 (2005).

[49.] Dipti Kumar and Akshay Kapoor, "Human Tail: Nature's Aberration," Journal of Child Neurology, 27: 924 (2012).

[50.] T.E. Herman and M.J. Siegel, "Human tail-caudal appendage: tethered cord," Journal of Perinatology, 28: 518-519 (2008) (emphasis added).

[51.] Hector E. James and Timothy G. Canty, "Human Tails and Associated Spinal Anomalies," Clinical Pediatrics, 34: 286-288 (1995) (emphasis added).

[52.] Daniel J. Donovan Robert C. Pedersen, "Human Tail with Noncontiguous Intraspinal Lipoma and Spinal Cord Tethering: Case Report and Embryologic Discussion," Pediatric Neurosurgery, 41:35-40 (2005).

[53.] Pei-Jung Lin, Yu-Tang Chang, Hsing-I Tseng, Jan-You Lin, Yu-Sheng Huang," Human Tail and Myelomeningocele," Pediatric Neurosurgery, 43:334-337 (2007) (emphasis added).

[54.] Pei-Jung Lin, Yu-Tang Chang, Hsing-I Tseng, Jan-You Lin, Yu-Sheng Huang," Human Tail and Myelomeningocele," Pediatric Neurosurgery, 43:334-337 (2007).

[55.] Daniel J. Donovan Robert C. Pedersen, "Human Tail with Noncontiguous Intraspinal Lipoma and Spinal Cord Tethering: Case Report and Embryologic Discussion," Pediatric Neurosurgery, 41:35-40 (2005).