I have been reviewing the evidence behind a frequently cited icon of evolution: the phenomenon of human “tails.” (See my previous posts here and here.)

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. I’ll have more to say on that in a subsequent post.

References Cited:
[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.

Photo credit: Tim Ellis/Flickr.