Do Human Embryos Have Tails

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I accept been reviewing the show backside a often cited icon of evolution: the phenomenon of human "tails." (See my previous posts here and hither.)

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 merely the instructions for producing them have been close off in our genomes." He suggests that humans born with tails have switched on genes that are ordinarily simply sitting there in our genome, unused. He wants us to remember that true tails are some kind of a regression to an earlier class.

This pseudo-recapitulationist position recalls Giberson's arguments in his book Saving Darwin, where he writes, "Ii-calendar month-old embryos of chicken, pigs, fish, and humans expect similar. They all take gills, webbed hands and feet, and tails. In a few weeks these formations disappear from the homo embryo." (p. 200) Except that human embryos in fact never take gills,^thirteen and webbed feet ("syndactyly") in embryos aren't a holdover from our amphibian ancestors but are office of a normal (and quite logical) fashion that hands and feet develop.¹⁴ In any example, Giberson'due south endeavour to describe human tails as a regression is undermined past the evidence of how and then-chosen "truthful tails" develop.

During normal human embryogenesis, a "tail"-similar structure appears around the 4th or fifth week of development. This isn't the result of vestigial genes that are commonly "turned off" (as Giberson puts it) being accidentally turned back on when our evolution regresses to some archaic state. Rather, they are always and commonly "turned on," and the formation of this tail-like structure is office of the normal process of the development of the man torso plan and nervous organisation — a point supported by the fact that the human being embryonic notochord and neural tube are idea to extend through much if not all of the entire extent of the man embryonic tail.^fifteen By the fifth or sixth week the "tail" reaches its full extent, only during the seventh and eighth week of development, it is reabsorbed into the embryo. Past the cease of the 8th week, the tail is ordinarily completely gone.^xvi

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 Beefcake recognizes, "During normal human being development a number of transient structures course and subsequently regress completely," and notes: "One of the near prominent structures that backslide during evolution is the man tail."¹⁷ The paper goes on to explicate that it is non unusual for cells or even macrostructures to dice: "In the procedure of development and in adult life, large numbers of cells are known to die in many unlike tissues. In some cases, whole regions or entire organs are eliminated."¹⁸

Thus, past suggesting that the tail is the result of aboriginal 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 man development, and babies are only born with a tail when information technology 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 function of the tail at eight weeks of pregnancy."¹⁹

So does the fact that man babies are sometimes born with tails propose we are related to animals with tails? No — the actual causes of "truthful tails" have caused some doctors to doubtable 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 acquired by phagocytosis, with the debris-laden macrophages migrating back to the torso, and it disappears completely at the end of the 8th week. Thus, the presence of human tail tin 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 bug. I'll have more to say on that in a subsequent mail.

References Cited:
[13.] Jonathan Wells, "Haeckel'southward Embryos and Development: Setting the Record Straight," The American Biology Teacher, Vol. 61(5):345-349 (May, 1999), at http://www.discovery.org/f/629
[fourteen.] The "webbed hands and feet" Giberson cites are easily explained as a unproblematic part of the mode man hands and anxiety 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'due south simply a logical way to grow a manus. Sometimes babies are born with webbed feet as the result of a birth defect, as 1 article states: "The exact cause of webbed toes is still unknown. Some used to speculate that information technology was inherited, since family members often share the condition, but information technology is too mutual for only i member of a family unit to have webbed toes. Some studies prove a meaning woman's nutritional intake during early on 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'south 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 non 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
[fifteen.] Daniel J. Donovan Robert C. Pedersen, "Homo Tail with Noncontiguous Intraspinal Lipoma and Spinal String Tethering: Case Written report and Embryologic Discussion," Pediatric Neurosurgery, 41:35-twoscore (2005).
[16.] Good descriptions of this process tin can be constitute 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 Homo Tail," Pediatric Neurology, nineteen No. 3 (1998); Daniel J. Donovan Robert C. Pedersen, "Human Tail with Noncontiguous Intraspinal Lipoma and Spinal String Tethering: Case Report and Embryologic Word," 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, G. Vilovic, One thousand. England, and Thousand. Saraga-Babic, "Morphological diverseness of dying cells during regression of the human tail," Annals of Beefcake, 183: 217-222 (2001).
[17.] D. Sapunar, K. Vilovic, Yard. England, and Thousand. Saraga-Babic, "Morphological diversity of dying cells during regression of the homo tail," Annals of Anatomy, 183: 217-222 (2001) (emphases added).
[18.] D. Sapunar, K. Vilovic, K. England, and Chiliad. Saraga-Babic, "Morphological variety of dying cells during regression of the human being tail," Register 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 Man 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) (accent 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.