The Coccyx

Darwin, Charles.  The Descent of Man, and Selection in Relation to Sex.  Volume 1.  John Murray, London.  1871.  The first volume of the 1871 first edition of Darwin's book on the descent of humans from a lower form of animal.  Its first chapter gathers the evidence, including rudimentary structures, which are organs reduced in size or use, and the coccyx passage falls within that chapter.  The article uses the passage to show that Darwin wrote functionless as a tail, not simply functionless.

1. 1a.

   Page 16:  “the os coccyx projects like a true tail, "extending considerably beyond the rudimentary legs."” ↩

2. 1b.

   Page 29:  “The os coccyx in man, though functionless as a tail, plainly represents this part in other vertebrate animals.” ↩ ↩ ↩ ↩

3. 1c.

   Page 29:  “The os coccyx is short, usually including only four vertebræ: and these are in a rudimental condition, for they consist, with the exception of the basal one, of the centrum alone.” ↩

4. 1d.

   Page 29:  “They are furnished with some small muscles; one of which, as I am informed by Prof. Turner, has been expressly described by Theile as a rudimentary repetition of the extensor of the tail, which is so largely developed in many mammals.” ↩

5. 1e.

   Page 30:  “Luschka has recently discovered at the extremity of the coccygeal bones a very peculiar convoluted body, which is continuous with the middle sacral artery; and this discovery led Krause and Meyer to examine the tail of a monkey (Macacus) and of a cat, in both of which they found, though not at the extremity, a similarly convoluted body.” ↩

6. 1f.

   Pages 17–18:  “The former are either absolutely useless, such as the mammæ of male quadrupeds, or the incisor teeth of ruminants which never cut through the gums; or they are of such slight service to their present possessors, that we cannot suppose that they were developed under the conditions which now exist.”  Page 18:  “Rudimentary organs are eminently variable; and this is partly intelligible, as they are useless or nearly useless, and consequently are no longer subjected to natural selection.” ↩ ↩

Naylor, Bruce G.  “Vestigial organs are evidence of evolution.”  Evolutionary Theory 6:91–96.  Sep 1982.  A journal reply to a 1981 paper by Scadding, which argued that vestigial organs cannot be evidence for evolution because a vestige must be non-functional.  Naylor argues that a vestige can keep some function, answers Scadding's examples one by one, and adds a long list of further examples.  The article adopts his restatement as its live definition of a vestige, under which the coccyx is still vestigial although it does useful work.

1. 2a.

   Page 92:  “Certainly it is functional (Scadding, 1981), but just as certainly it is not functioning as an external tail for balance or grasping. It is a functional, vestigial organ (contra Scadding, 1981).” ↩ ↩ ↩

2. 2b.

   Page 92:  “Nevertheless, it is also rudimentary in comparison to its homolog in other mammals and is most reasonably explained on the theory of descent with modification.” ↩

Gray, Henry.  Anatomy of the Human Body.  Lea & Febiger, Philadelphia.  20th edition.  Edited by Warren H. Lewis.  1918.  An anatomy text on the human body, in its twentieth edition.  The article uses it for how the coccyx's segments are built, for the muscles and ligaments attached to the bone, and for the joint above it.

1. 3a.

   Page 111:  “The coccyx (Fig. 100) is usually formed of four rudimentary vertebrae; the number may however be increased to five or diminished to three.” ↩

2. 3b.

   Page 111:  “In each of the first three segments may be traced a rudimentary body and articular and transverse processes; the last piece (sometimes the third) is a mere nodule of bone. All the segments are destitute of pedicles, laminae, and spinous processes.” ↩ ↩

3. 3c.

   Page 111:  “It gives attachment to the anterior sacrococcygeal ligament and the Levatores ani, and supports part of the rectum.” ↩

4. 3d.

   Page 111:  “The borders of the coccyx are narrow, and give attachment on either side to the sacrotuberous and sacrospinous ligaments, to the Coccygeus in front of the ligaments, and to the Glutaeus maximus behind them.” ↩ ↩

5. 3e.

   Page 111:  “The apex is rounded, and has attached to it the tendon of the Sphincter ani externus.” ↩

6. 3f.

   Page 309:  “This articulation is an amphiarthrodial joint, formed between the oval surface at the apex of the sacrum, and the base of the coccyx. It is homologous with the joints between the bodies of the vertebrae, and is connected by similar ligaments.” ↩ ↩ ↩

7. 3g.

   Page 401:  “The Extensor coccygis is a slender muscular fasciculus, which is not always present; it extends over the lower part of the posterior surface of the sacrum and coccyx. ... It is a rudiment of the Extensor muscle of the caudal vertebrae of the lower animals.” ↩ ↩ ↩ ↩ ↩

8. 3h.

   Page 424:  “The Coccygeus (Fig. 404) is situated behind the preceding. It is a triangular plane of muscular and tendinous fibers, arising by its apex from the spine of the ischium and sacrospinous ligament, and inserted by its base into the margin of the coccyx and into the side of the lowest piece of the sacrum.” ↩

9. 3i.

   Page 424:  “The Coccygei pull forward and support the coccyx, after it has been pressed backward during defecation or parturition. The Levatores ani and Coccygei together form a muscular diaphragm which supports the pelvic viscera.” ↩ ↩

10. 3j.

    Page 1281:  “The glomus coccygeum is placed in front of, or immediately below, the tip of the coccyx. It is about 2.5 mm. in diameter and is irregularly oval in shape.” ↩

Lirette, Lesley Smallwood et al.  “Coccydynia: An Overview of the Anatomy, Etiology, and Treatment of Coccyx Pain.”  Ochsner Journal 14(1 (Spring)):84–87.  2014.  A review article by four pain-management doctors at the Ochsner Clinic Foundation in New Orleans, summarizing the anatomy, causes and treatment of coccydynia, which is pain in the coccyx.  The review reports that conservative treatment succeeds in 90 percent of cases and that surgical removal of the coccyx is generally not recommended.  The article uses it for the coccyx's role as one leg of the tripod that carries weight in sitting, and for the claim that women are five times as often affected.

1. 4a.

   Section Anatomy and Function:  “Moving anteriorly to posteriorly, the lateral edges serve as insertion sites for the coccygeal muscles, the sacrospinous ligament, the sacrotuberous ligament, and fibers of the gluteus maximus muscle. Inferiorly, the iliococcygeus muscle tendon inserts onto the tip of the coccyx. These ligaments and muscles help support the pelvic floor and also contribute voluntary bowel control.” ↩

2. 4b.

   Section Anatomy and Function:  “Despite its small size, the coccyx has several important functions. Along with being the insertion site for multiple muscles, ligaments, and tendons, it also serves as one leg of the tripod—along with the ischial tuberosities—that provides weight-bearing support to a person in the seated position. Leaning back while in a seated position leads to increased pressure on the coccyx.” ↩ ↩

3. 4c.

   Section Incidence and Etiology:  “Women are 5 times more likely to develop coccydynia than men.” ↩

4. 4d.

   Section Incidence and Etiology:  “The location of the coccyx makes it particularly susceptible to internal injury during childbirth, especially during a difficult or instrumented delivery.” ↩

5. 4e.

   Section Treatment:  “However, this procedure can be associated with a high complication rate and failure to relieve the pain. Consequently, based on current available information, this procedure generally is not recommended.” ↩

Maigne, Jean-Yves et al.  “Postpartum coccydynia: a case series study of 57 women.”  European Journal of Physical and Rehabilitation Medicine 48(3):387–392.  Sep 2012.  A case series of 57 women from one specialist coccydynia clinic in Paris, whose tailbone pain began after childbirth.  The authors compared them with 192 women whose pain had other causes, and they used dynamic X-rays to compare how far the coccyx moved in the two groups.  The article uses it for the point that the mobility of the coccyx leaves it open to injury in childbirth.

1. 5a.

   Section Abstract:  “7.3% of the cases of coccydynia in female patients seen in our clinic were related to childbirth. The pain appeared as soon as the patient adopted the sitting position after delivery. The deliveries had often been performed with instruments (forceps deliveries: 50.8%; vacuum-assisted deliveries: 7.0%) or were spontaneous but described as "difficult" (12.3%). Luxation of the coccyx was observed in 43.9% of the cases and 17.0% of the controls. Fracture of the coccyx was involved in 5.3% of the cases.” ↩ ↩

Coyne, Jerry A.  Why Evolution Is True.  Viking, New York.  2009.  A 2009 book written for general readers, which the article uses for Coyne's concession that the coccyx still has a function.  The article uses it to show that the vestige argument never said the coccyx was useless.

1. 6a.

   Page 62:  “It still has a function (some useful muscles attach to it), but remember that its vestigiality is diagnosed not by its usefulness but because it no longer has the function for which it originally evolved. Tellingly, some humans have a rudimentary tail muscle (the "extensor coccygis"), identical to the one that moves the tails of monkeys and other mammals. It still attaches to our coccyx, but since the bones can't move, the muscle is useless.” ↩ ↩

Karadimas, Efthimios J. et al.  “Surgical treatment of coccygodynia: an analytic review of the literature.”  European Spine Journal 20(5):698–705.  May 2011.  A systematic review of 24 English-language papers on coccygectomy (surgical removal of the coccyx) for tailbone pain, with literature searched from 1980 to 2010, covering 671 patients.  Only 2 of the papers were prospective studies and the other 22 were retrospective case series, and 84.6 per cent of the patients had an excellent or good result, with a complication rate of 10.9 per cent, mostly wound infection.  The article uses it for those outcomes and for the limits the authors admit: the scores were subjective and most series were uncontrolled.

1. 7a.

   Section Abstract:  “In total, 671 patients with coccygodynia underwent coccygectomy following failed conservative management. The sex ratio, male/female was 1:4.4. The most popular aetiology for coccygodynia was direct trauma in 270 patients.” ↩

2. 7b.

   Section Results:  “This systematic analysis revealed that 504 over 596 patients treated with coccygectomy had excellent or good outcome (84.6%), 46 patients fair (7.7%) and 46 poor outcome (7.7%)”. ↩ ↩

3. 7c.

   Section Results:  “There were 56 (8.34%) reported wound infections (9 deep and 47 superficial)”.  “The overall complication rate was 10.9%.”  Section Abstract:  “Other complications included two haematomas, six delayed wound healings and nine wound dehiscence.” ↩ ↩

4. 7d.

   Section Results:  “It was advisable to preserve the ligamentous and muscular attachments together with the periosteum.” ↩ ↩ ↩

5. 7e.

   Section Discussion:  “The identified series lacked of strength as to their level of evidence. The majority but two [4, 18] were retrospective uncontrolled series”.  “the coccygectomy outcomes were based on subjective evaluation systems such as the VAS and the ODI”. ↩ ↩

6. 7f.

   Section Discussion:  “according to Wray, only 23 (19.2%) of the 120 cases with coccygodynia finally underwent operative treatment”. ↩

Williams, Scott A., and David Pilbeam.  “Homeotic change in segment identity derives the human vertebral formula from a chimpanzee-like one.”  American Journal of Physical Anthropology 176(2):283–294.  Oct 2021.  A count of the vertebrae in a sample of 893 hominoids, which are apes and humans, using measures borrowed from population genetics.  The authors find that humans most resemble the African apes in their total number of vertebrae, and that the chimpanzee and bonobo pattern is the only proposed ancestral pattern consistent with their results.  The article uses it for the three coccygeal vertebrae that chimpanzees and bonobos carry.

1. 8a.

   Section Abstract:  “the modal formula demonstrated by chimpanzees and bonobos (7 cervical-13 thoracic-4 lumbar-6 sacral-3 coccygeal).” ↩ ↩

Xia, Bo et al.  “On the genetic basis of tail-loss evolution in humans and apes.”  Nature 626:1042–1048.  Feb 29, 2024.  A laboratory study in Nature that combined genome comparisons with engineered mice.  The team found an Alu element, a repeated sequence that can jump around a genome, inserted into a non-coding part (an intron) of the gene TBXT, a repeat found only in apes and humans, and showed in mice that it lets the gene skip one of its coding exons.  Mice carrying the skipped form lost or shortened their tails in 21 of 63 heterozygous cases, and the authors list the limits of that result, which is why the article uses the study as the strongest genetic evidence on tail loss.

1. 9a.

   Section Main:  “The loss of the tail is inferred to have occurred around 25 million years ago when the hominoid lineage diverged from the ancient Old World monkeys (Fig. ), leaving only 3–5 caudal vertebrae to form the coccyx, or tailbone, in modern humans.” ↩ ↩

2. 9b.

   Section Main:  “we initially screened 31 human genes—and their primate orthologues—for which mutations are associated with the absence of an external tail”.  “Together, we detected 85,064 single nucleotide variants (SNVs), 5,533 deletions and 13,820 insertions that are hominoid-specific”. ↩

3. 9c.

   Section A hominoid-specific intronic AluY in TBXT:  “Together, the AluY and AluSx1 elements form an exon-flanking inverted repeat pair (Fig. ). We therefore posited that during transcription, the hominoid-specific AluY element pairs with the simian-shared AluSx1 element to form a stem–loop structure in TBXT pre-mRNA and traps exon 6 in the loop”. ↩

4. 9d.

   Section TbxtΔexon6 expression induces tail loss:  “Specifically, 21 out of the 63 heterozygous mice showed tail phenotypes, whereas none of their 35 wild-type littermates showed phenotypes (Table ). The incomplete penetrance of phenotypes among the heterozygotes was stable across generations and founder lines”. ↩ ↩ ↩

5. 9e.

   Figure Extended Data Fig. 8 legend:  “The presented embryos were the only two cases found in this study that died after birth with neural tube closure defects.” ↩

6. 9f.

   Section Discussion:  “the selective advantage must have been strong because the loss of the tail may have included an evolutionary trade-off of neural tube defects, as demonstrated by the presence of neural-tube-closure defects in mice expressing the TbxtΔexon6 transcript”. ↩

7. 9g.

   Section Discussion:  “We presented evidence for a plausible evolutionary scenario for tail-loss evolution in hominoids, which involves the insertion of an AluY element into an intron of TBXT.” ↩

8. 9h.

   Section Discussion:  “However, even if the AluY insertion substantially influenced tail-loss evolution in hominoids, additional genetic changes may have acted to stabilize the no-tail phenotype”.  “Such a possible set of genetic events suggest that a change to the AluY element in modern hominoids would be unlikely to result in the reappearance of the tail.” ↩ ↩ ↩ ↩

9. 9i.

   Section Discussion:  “Moreover, tail loss or reduction occurred independently multiple times throughout primate evolution, including in loris (Lorisidae), mandrill (Mandrillus) and some species of macaques (Macaca).” ↩

10. 9j.

    Section A hominoid-specific intronic AluY in TBXT:  “Heterozygous mutations in the coding regions of TBXT orthologues in tailed animals such as mouse … Manx cat, dog and zebrafish lead to the absence or reduced forms of the tail, and homozygous mutants are typically non-viable.” ↩

11. 9k.

    Section Abstract:  “we generated multiple mouse models that express both full-length and exon-skipped isoforms of Tbxt, mimicking the expression pattern of its hominoid orthologue TBXT.” ↩

12. 9l.

    Section Inserting intronic sequences in mouse Tbxt:  “We confirmed that both TbxtinsASAY/insASAY and TbxtinsRCS/insRCS ES cells expressed the TbxtΔexon6 splicing isoform after differentiation”.  Section Abundance of Tbxt isoforms explains tail phenotypes:  “Neither heterozygous nor homozygous TbxtinsASAY mice showed obvious tail phenotypes in adulthood”.  “TbxtinsASAY homozygous embryos expressed low levels of TbxtΔexon6 transcript relative to the full-length transcript”.  “Consequently, the fact that our Alu-pair insertion model did not express high levels of the TbxtΔexon6 transcript in the embryonic tailbud renders this particular mouse model as inconclusive, beyond the insight that small amounts of this isoform are insufficient to lead to tail loss.”  TbxtinsASAY mice carry the human AluY and AluSx1 elements inserted around mouse exon 6 in the human arrangement. ↩ ↩ ↩ ↩ ↩

13. 9m.

    Section Abundance of Tbxt isoforms explains tail phenotypes:  “However, homozygous TbxtinsRCS2 mice (TbxtinsRCS2/insRCS2) consistently had around 10% shorter tails relative to wild-type or heterozygous mice”.  “Notably, all 19 compound heterozygous mice (TbxtinsRCS2/Δexon6) presented a complete absence of an external tail”.  TbxtinsRCS2 carries a 220 base-pair piece of the mouse's own intron 6 inserted in reverse into intron 5, forming an inverted pair like the Alu pair. ↩

14. 9n.

    Section Methods, Comparative genomics analyses of tail-development-related genes:  “We selected all six hominoid species (hg38, gorGor5, panTro5, panPan2, ponAbe2 and nomLeu3) to calculate a hominoid-consensus sequence, and used two non-hominoid species (pig-tailed macaque, macNem1, and marmoset, calJac3) as the outgroups.”  “DNA sequences present in all six hominoid species but absent in either of the two outgroup monkey species were identified as hominoid-specific insertions”.  nomLeu3 is the genome assembly of the northern white-cheeked gibbon, Nomascus leucogenys. ↩ ↩ ↩

DeWitt, David A.  “Setting the Record Straight on Vestigial Organs.”  Answers in Genesis.  May 28, 2008.  An online reply by David DeWitt, of the creationist ministry Answers in Genesis, to Laura Spinney's New Scientist list of "Five Things Humans No Longer Need."  He argues that the coccyx anchors the pelvic diaphragm, the sheet of muscle that floors the pelvis, so calling it vestigial depends on the assumption that humans descended from tailed animals.  The article uses it as the creationist reply that moves the dispute to ancestry rather than function.

1. 10a.

   Section The Tailbone:  “Evidence that is cited includes the variable number of bony segments humans can have (usually 4 but can be 3 or 5) as well as "babies born with tails." But these so called tails are not really tails at all and instead are a type of fatty tumor. There are no bones or muscles in them at all”. ↩

2. 10b.

   Section The Tailbone:  “In fact, the coccyx is the anchor point for the muscles that form the entire pelvic diaphragm. Therefore, while the coccyx has a clear function in humans today, the only reason to claim that the function has been modified is because of evolutionary assumptions. If you believe that humans descended from animals that possessed tails, then there must have been a modification of the tailbone. In contrast, if our ancestor Adam was created by God then there was no modification, and our tailbone is just as it always was. Without the evolutionary presupposition, the evidence that the tailbone is vestigial evaporates.” ↩ ↩

Mitchell, Elizabeth.  “Vestigial Organ Arguments: Useful or Unnecessary?”  Answers in Genesis.  Oct 22, 2011.  A short news reply by Dr. Elizabeth Mitchell for Answers in Genesis, a creationist ministry, to a Fox News list of nine "useless" body parts.  It argues that the coccyx has a vital function, because the muscles attached to it form slings that support the pelvic organs.  The article uses it for two creationist claims that it tests: that humans have never had tails, and that the coccyx is essential to our vertical lifestyle.

1. 11a.

   “Humans have never had tails and do not share common ancestry with creatures that do (or did).” ↩

2. 11b.

   “The muscles of the pelvis attach to the coccyx to form slings supporting all the pelvic organs in their proper geometric orientation. When those muscles weaken, pelvic organs can herniate downward causing discomfort and loss of control. The coccyx, essential to our vertical lifestyle, is not a marginal evolutionary remnant on its way out.” ↩ ↩

Bergman, Jerry.  “Tale of the Tail: Why the Human Coccyx Is Not a Vestigial Organ.”  Creation-Evolution Headlines.  Apr 3, 2024.  A 2024 reply by Jerry Bergman, PhD, a creationist writer who co-wrote a 1990 book on vestigial organs, published on the creationist news site Creation-Evolution Headlines.  It argues that the coccyx has important functions and that the Xia paper found only a genetic difference between tailed and tailless primates.  The article uses it as the second creationist reply to the TBXT paper.

1. 12a.

   “For example, I published a book on this topic 34 years ago titled, "Vestigial Organs" are Fully Functional: A History and Evaluation of the Vestigial Organ Origins Concept (with George Franklin Howe, Ph.D.;”. ↩

2. 12b.

   “In short, when comparing the DNA of tailless apes and humans to that of tailed monkeys, Xia and his team found a DNA insertion shared by tailless apes and humans, but missing in tailed monkeys. They found no evidence for evolution, only a genetic difference between tailed and tailless primates.” ↩ ↩

3. 12c.

   “If humans lost their tail about 25 million years ago, then we have possessed a useless coccyx for about 25 million years.” ↩

Kloster, Kaia.  “Does New Mouse Experiment Explain How Humans Lost Their Tails?”  Answers in Depth 18.  Answers in Genesis.  Jul 7, 2023.  A creationist magazine article answering the 2021 preprint of the Xia study, before the published version added the mouse carrying the human Alu pair.  It accepts that the AluY is there and that it affects splicing, reads it as designed regulation, and argues from the mouse breeding results that a variant leaving most carriers tailed and killing homozygotes could not have spread.

1. 13a.

   Section But Wait a Minute:  “A far more likely scenario is that the AluY is strategically positioned in both humans and apes—regulated by complex genetic controls—to provide the structure and function most suited for these unique creations and their God-intended habitats and lifestyles.” ↩

2. 13b.

   Section But Wait a Minute:  “It is also important to note that in the mouse model, they did not insert an AluY into the mouse genome, which would have more closely mimicked what is observed in the human genome.” ↩

3. 13c.

   Section But Wait a Minute:  “Combined, that would imply that 96% of the offspring that survived would have had some type of tail! It is difficult to envisage how that 4% could have somehow become the prevailing—indeed, exclusive—phenotype in humans.”  The 96 percent comes from a Punnett square in which a quarter of offspring die as homozygotes and only 6 percent of heterozygous mice lacked a tail entirely. ↩ ↩

Woon, Jason T. et al.  “CT morphology and morphometry of the normal adult coccyx.”  European Spine Journal 22(4):863–870.  Apr 2013.  A retrospective study of 112 CT scans of adults with no known sacral or coccygeal disease, taken for non-orthopedic conditions, which counted the segments in each coccyx and checked its joints for fusion.  It found 4 segments in 76 percent of scans, 3 in 13 percent and 5 in 11 percent, and that the joint to the sacrum was fused in 57 percent.  The article uses it as the modern count of coccygeal segments in living adults.

1. 14a.

   Section Results:  “Coccyges had either 3 (13 %), 4 (76 %), or 5 segments (11 %); there was no evidence for an association with gender (P = 0.85). The sacrococcygeal joint was fused in 57 % of coccyges, the first intercoccygeal joint in 17 %, the second in 61 %, the third in 89 % of coccyges with 4 segments, and the fourth in all coccyges with 5 segments.” ↩ ↩ ↩

2. 14b.

   Section Abstract:  “Mean coccygeal curved length was 4.4 ± 0.8 cm in men and 4.0 ± 0.8 cm in women (P < 0.01).” ↩

3. 14c.

   Section Discussion:  “The normal adult coccyx comprises 3–5 segments, 4 being present in most individuals”. ↩

4. 14d.

   Section Discussion:  “Our findings show that the coccyx is shorter and tends to be straighter in women but other radiologic features are not significantly different between the sexes.” ↩

Wiedersheim, Robert.  The Structure of Man: An Index to His Past History.  Macmillan, London.  Translated by H. Bernard and M. Bernard.  1895.  A comparative anatomy book, the 1895 English translation of a German work by Robert Wiedersheim, a professor at Freiburg.  It sets out the parts of the human body that link humans with lower animals, and its preface warns that similar structures may have arisen independently in unrelated groups.  The article uses it for an older count of the segments in the tail and for the list of organs Wiedersheim called vestigial.

1. 15a.

   Page 26:  “The vertebral column of an adult human being consists normally of thirty-three to thirty-four vertebrae, numerical variation being due to the inconstancy of those of the coccygeal or caudal series.” ↩

2. 15b.

   Page 29:  In footnote 1 on that page:  “in the latter the degeneration of the os coccygis, which consists as a rule of only three vertebra, has gone still further than in Man.” ↩ ↩

3. 15c.

   Page 29:  In footnote 1 on that page:  “the opposite extreme for the mammalian series is reached by a small insectivore from Madagascar (Microgale longicaudala) and the long-tailed Pangolin (Manis macrura) of the old world, in which the caudal vertebrae may be close upon fifty in number.” ↩

4. 15d.

   Page 31:  “In the adult man the regular number of caudal vertebrae is five, whereas the number may be either four or five in the adult woman.” ↩

5. 15e.

   Page 32:  “the peculiar permanent internal tail of the adult is a very degenerate organ.” ↩ ↩

6. 15f.

   Page 200:  In the conspectus, section B:  “Os coccygis. Cauda humana.” ↩

7. 15g.

   Page 201:  In the conspectus, section B:  “The caudal muscles.” ↩

Woon, Jason T., and Mark D. Stringer.  “Clinical anatomy of the coccyx: A systematic review.”  Clinical Anatomy 25(2):158–167.  Mar 2012.  A systematic review, which gathers and weighs the published studies on one question, of what is known about the anatomy of the coccyx.  The authors searched five electronic databases and found 61 primary and 7 secondary English-language sources, and they report that several parts of the anatomy are poorly documented.  The article uses it to show that the function of the coccygeal body is still unknown.

1. 16a.

   Section Abstract:  “Although the musculoskeletal aspects of the coccyx are reasonably well described, the precise anatomy of the coccygeal plexus and its distribution, the function of the coccygeal body, and the anatomy of the sacrococcygeal zygapophyseal joints are poorly documented.” ↩ ↩

Maigne, Jean-Yves et al.  “Idiopathic coccygodynia. Lateral roentgenograms in the sitting position and coccygeal discography.”  Spine 19(8):930–934.  Apr 1994.  A study of 51 patients with tailbone pain and 51 people without it, who had X-rays of the coccyx taken sitting and lying on one side, and then the patients had a disc test (discography, which injects the disc to see whether it reproduces the pain).  The authors tested whether the pain came from a disc in the coccyx, and they found that in 25 patients the coccyx moved abnormally when they sat, with no such finding in the controls.  The article uses it for that abnormal movement in sitting.

1. 17a.

   Section Abstract:  “An abnormal motion (luxation or hypermobility) of the coccyx that occurred in the sitting position and spontaneously was reducible when placed in the lateral decubitus position was found in 25 patients.” ↩

Maigne, Jean-Yves, and B. Tamalet.  “Standardized radiologic protocol for the study of common coccygodynia and characteristics of the lesions observed in the sitting position.”  Spine 21(22):2588–2593.  Nov 1996.  A study that set a standard X-ray method for the coccyx, comparing standing films with sitting films in ninety-one patients with common coccygodynia and 47 control subjects who were healthy volunteers.  It defined hypermobility as a bend of more than 25 degrees and luxation as a displacement of more than 25% of the coccyx, and it found luxation or hypermobility in 48.4 per cent of the patients.  The article uses it for those thresholds, which the group set in 1996.

1. 18a.

   Section Abstract:  “Hypermobility was defined as a flexion of more than 25 degrees, luxation by displacement of more than 25% of the coccyx. The base angle is a good predictor of the direction in which the coccyx moves when sitting.” ↩ ↩ ↩

Maigne, Jean-Yves et al.  “Causes and mechanisms of common coccydynia: role of body mass index and coccygeal trauma.”  Spine 25(23):3072–3079.  Dec 2000.  A study of 208 consecutive patients with common coccydynia, all examined with the same X-ray protocol, that compared body mass index (a measure of weight for height) with the kind of coccyx lesion each patient had.  It found that posterior luxation (the coccyx slipping backward) was far more common in obese patients than in thin ones, 51 per cent against 3.7 per cent, and it concluded that body mass index determines the causative lesion.  The article uses it for the shift to body weight as the factor that sorts the lesions.

1. 19a.

   Section Abstract:  “The body mass index determines the way a subject sits down, and lesion patterns were different in obese, normal-weight, and thin patients (posterior luxation: 51%, 15.2%, 3.7%; hypermobility: 26.5%, 30.3%, 14.8%; spicules: 2%, 15.9%, 29.6%; normal: 16.3%, 32.6%, 48.1%, respectively; P < 0.0001).” ↩

Wray, C. C. et al.  “Coccydynia. Aetiology and treatment.”  Journal of Bone and Joint Surgery (British) 73(2):335–338.  Mar 1991.  A five-year prospective trial of 120 patients with coccydynia (tailbone pain), run by the orthopaedic surgery department at Leicester Royal Infirmary.  The authors found that the cause lies in a localised musculoskeletal abnormality around the coccyx, that a slipped disc in the lower back is not a significant factor, and that physiotherapy helped little while injections and removal of the coccyx helped more.  The article uses it to show the pain is real and to set out the treatments, from injections up to removal of the coccyx.

1. 20a.

   Section Abstract:  “The condition is genuine and distressing and we found no evidence of neurosis in our patients.” ↩

2. 20b.

   Section Abstract:  “60% of patients responded to local injections of corticosteroid and local anaesthesia. Manipulation and injection was even more successful and cured about 85%. Coccygectomy was required in almost 20% and had a success rate of over 90%.” ↩

Tague, Robert G.  “Fusion of coccyx to sacrum in humans: prevalence, correlates, and effect on pelvic size, with obstetrical and evolutionary implications.”  American Journal of Physical Anthropology 145(3):426–437.  Jul 2011.  A skeletal study of 2,354 American skeletons aged 20 or older, counting how often the coccyx is fused to the sacrum.  It found that fusion becomes more common with age in women, rising from 24 to 47 percent between the third and eighth decades, and that in 132 women fusion was linked to a shorter back-to-front diameter of the birth outlet.  The article uses it for the count of four rudimentary coccygeal vertebrae and for the link between fusion and childbirth.

1. 21a.

   Section Abstract:  “Humans do not have a tail, but we have four rudimentary coccygeal vertebrae.” ↩

2. 21b.

   Section Abstract:  “prevalence does increase with advancing age-from 24 to 47% from the third to eighth decades of life in females.” ↩

3. 21c.

   Section Abstract:  “Pelvimetric analysis of 132 females shows that those with sacral-coccygeal fusion have a shorter posterior sagittal diameter of the outlet compared to those without fusion; more than half of those with sacral-coccygeal fusion have an obstetrically contracted posterior sagittal diameter.” ↩ ↩

Postacchini, Franco, and Marco Massobrio.  “Idiopathic coccygodynia. Analysis of fifty-one operative cases and a radiographic study of the normal coccyx.”  Journal of Bone and Joint Surgery (American) 65(8):1116–1124.  Oct 1983.  A retrospective review of 51 patients who had part or all of the coccyx removed for idiopathic coccygodynia, which is tailbone pain with no known cause, over twenty years, together with an X-ray study of the normal coccyx in 120 people without pain.  It sorted the coccyx into four shapes and found that 68 per cent of the pain-free people had the first shape, a coccyx curved slightly forward; of the thirty-six operated patients followed for at least two years, thirty-two (88 per cent) had an excellent or good result.  The article uses it for the shapes and for the surgical results.

1. 22a.

   Section Abstract:  “In Type I the coccyx was curved slightly forward ... in Type IV it was subluxated at the sacrococcygeal or the intercoccygeal joint. Most subjects (68 per cent) had a Type-I configuration.” ↩

2. 22b.

   Section Abstract:  “The results of surgery were excellent or good in thirty-two (88 per cent) of the thirty-six patients who were followed for at least two years.” ↩

Sagoo, Navraj S. et al.  “Coccygectomy for refractory coccygodynia: a systematic review and meta-analysis.”  European Spine Journal 31(1):176–189.  Jan 2022.  A systematic review and meta-analysis, which pools the results of published studies: 21 studies with 826 patients who had their coccyx removed for long-lasting pain (coccygodynia) that other treatment had not relieved.  The authors found that pain fell by about 5 points on a 0 to 10 scale six to twelve months after surgery, with complications in 8 percent of patients and reoperations in 3 percent.  The article uses it as the modern pooled record of what happened to patients after the coccyx was removed.

1. 23a.

   Section Abstract:  “A total of 826 patients (females = 75%) received coccygectomy (720 total and 106 partial) for refractory coccygodynia. ... The pooled mean difference (MD) in pain scores from baseline on a 0-10 scale was 5.03 (95% confidence interval [CI]: 4.35 to 6.86) at a 6-12 month follow-up (FU).” ↩

2. 23b.

   Section Abstract:  “The pooled incidence of complications following coccygectomy was 8% (95% CI: 5% to 12%), the most frequent of which were surgical site infections and wound dehiscence. The pooled incidence of reoperations was 3% (95% CI: 1% to 5%).” ↩

Kumar, A., and J. R. Reynolds.  “Mesh repair of a coccygeal hernia via an abdominal approach.”  Annals of the Royal College of Surgeons of England 82(2):113–115.  Mar 2000.  A single-patient case report from a Derby hospital surgical department.  The patient's rectum herniated through the pelvic floor where the coccyx had been removed, and the surgeons closed the gap with mesh through the abdomen.  It shows the gap that removal can leave in the pelvic floor;  one case gives no rate.

1. 24a.

   Section Abstract:  “We report on the presentation and management of a patient with herniation of the rectum following a coccygectomy. We used an abdominal approach and careful pelvic dissection to define the defect in the pelvic floor at the site where coccyx used to be.” ↩

Halley, Keaton.  “Tailbone "serves no purpose"?”  Creation Ministries International.  Apr 27, 2017.  A 2017 reply by Keaton Halley for Creation Ministries International to a sign at the American Museum of Natural History in New York that said the coccyx "serves no purpose."  Halley argues that the coccyx has several functions, including helping to bear weight in sitting, anchoring the pelvic floor and the filum terminale, and helping childbirth, and that surgical removal shows only that the bone is not essential, not that it has no purpose.  The article uses it for the creationist concession that the coccyx has a purpose without being essential.

1. 25a.

   Section To what end?:  “For example, the coccyx and the two ischium bones of the pelvis together form a tripod that helps to bear the weight of the body and provide balance when a person is seated. As a person leans back, more weight is transferred to the coccyx.” ↩

2. 25b.

   Section Life without the coccyx:  “Some evolutionists have pointed out that there are cases in which a damaged or diseased coccyx has been surgically removed, proving that the coccyx is not essential. But so what? Having a purpose and being essential are not the same thing.” ↩ ↩

3. 25c.

   Section Life without the coccyx:  “But personal testimonies are also available online that indicate recovery from a coccygectomy can be difficult—including pain when sitting or moving, not to mention bathroom complications.”  Halley's note 2 cites coccyx.org/personal, which it titles "Personal experiences of coccyx pain." ↩

4. 25d.

   Section Tall tales:  “Yes, the coccyx occupies the same position in the human body where a tail begins in other creatures, but since God gave humans a physical design that is similar to other vertebrates, our spine (a finite, linear structure, after all) must come to an end somewhere in our lower back.” ↩

Russo, Gabrielle A.  “Comparative sacral morphology and the reconstructed tail lengths of five extinct primates.”  Journal of Human Evolution 90:135–162.  Jan 2016.  A comparison of the sacrum, the bone that links the tail to the rest of the body, across 472 living mammals from six orders, used to estimate tail length in five extinct primates from their sacra.  It found that sacral shape differs with tail length, and it reconstructed Proconsul heseloni as tailless.  The article uses it as the newest evidence that the human coccyx descends from an ancestor that already had no tail.

1. 26a.

   Section Abstract:  “the sacrum-the sole bony link between the tail or coccyx and the rest of the body”.  “Proconsul heseloni was 'tailless.'” ↩

Nakatsukasa, Masato et al.  “Tail loss in Proconsul heseloni.”  Journal of Human Evolution 46(6):777–784.  Jun 2004.  A 2004 paper by six authors on the loss of the tail in Proconsul heseloni, a fossil species.  The article cites it together with a 1991 paper by Carol V. Ward, Alan Walker and Mark F. Teaford, "Proconsul did not have a tail," Journal of Human Evolution 21(3):215–220, doi:10.1016/0047-2484(91)90062-Z.  The article uses the two as the earlier and the later studies that both concluded Proconsul had no tail, which it relies on for when the tail was lost. ↩

Menton, David.  “The Human Tail, and Other Tales of Evolution.”  Answers in Genesis.  Jul 29, 2017.  A short popular essay by Dr. David Menton, first published in the St. Louis MetroVoice 4(1) in January 1994 and reposted by Answers in Genesis in 2017.  It argues that the coccyx does real work and that wherever the spine ends, evolutionists would call that end a vestigial tail.  The article uses it as the best-known statement of the first layer of the creationist case, a list of functions for the coccyx.

1. 28a.

   “Several muscles converge from the ring-like arrangement of the pelvic (hip) bones to anchor on the coccyx, forming a bowl-shaped muscular floor of the pelvis called the pelvic diaphragm. The incurved coccyx with its attached pelvic diaphragm keeps the many organs in our abdominal cavity from literally falling through between our legs.” ↩

2. 28b.

   “The coccyx does occupy the same relative position at the end of our vertebral column as does the tail in tailed primates, but then, where else would it be? The vertebral column is a linear row of bones that supports the head at its beginning and it must end somewhere. Wherever it ends, evolutionists will be sure to call it a vestigial tail.” ↩

Bergman, Jerry.  “Do any vestigial structures exist in humans?”  CEN Technical Journal 14(2):95–98.  2000.  A creationist article arguing that the newer definition of a vestigial organ, one with "reduced function", is so vague that almost any human structure could be called vestigial.  It notes that the list of vestigial organs in humans fell from 180 in 1890 to 0 in 1999.  The article uses it to attack that reduced-function definition and to open with Asimov's example of the coccyx.

1. 29a.

   Page 95:  The phrase in single quotation marks is Asimov's, quoted by Bergman.  “Asimov provides two examples of a vestigial organ: (1) the tiny bones posterior to the sacrum called the coccyx (which Asimov claims were 'once meant for a tail')”. ↩

Sherwin, Frank.  “The Tail of Man's Supposed Ancestors.”  Institute for Creation Research.  Apr 11, 2024.  A short article by Frank Sherwin, the science news writer for the Institute for Creation Research, replying to a LiveScience article about the Xia paper.  It argues that the genetic findings cannot show that such features drove evolution for millions of years, and calls that belief an assumption taken on faith.  The article uses it as one of the two creationist replies to the TBXT paper, and for its claim about what the coccyx does.

1. 30a.

   “The coccyx, or tailbone, is an important part of the spinal column. It not only aids in upright sitting posture (ask anyone who has bruised or broken their coccyx!), but it is also the site of muscle attachment for the defecation process.” ↩

2. 30b.

   “Yes, transposable elements of different types and functions are found in nearly all plants and animals, but there's no way one can assert these genetic features have been influencing evolution for millions of years. That is assumed and must be taken by faith.” ↩

Doyle, Shaun.  “Vestigial arguments: remnants of evolution.”  creation.com.  Jun 11, 2008.  A creationist reply, by Shaun Doyle of Creation Ministries International, to Laura Spinney's New Scientist article; the page was updated on 14 December 2024.  It says the coccyx is an important anchor for the muscles that hold the anus in place, and that it is vestigial only under the "revisionist" definition Spinney uses.  The article uses it beside DeWitt's reply as the second creationist answer on the coccyx.

1. 31a.

   Section Coccyx:  “Basically, it is only a vestigial organ under the revisionist definition that Spinney gives. (I.e. not a reduced function, but a modified function—obviously from an evolutionary perspective.) Once again, this provides no problem for design.” ↩