Winter 2007–2008

Congenital Human Baculum Deficiency

Scott F. Gilbert and Ziony Zevit

The letter below, written by a professor of biology and a professor of biblical literature, first appeared in the American Journal of Medical Genetics in 2001. The letter and its references have been reproduced exactly as they appeared in the journal.

To the Editor:
There are certain genetic diseases that affect 100% of the human population. One of these is gulonolactone oxidase deficiency [OMIM 420400], caused by a deletion on chromosome 8p21 [Nishikimi et al., 1994]. The lack of this enzyme causes severe connective tissue disease and makes us dependent upon dietary supplements of ascorbic acid. Another genetic condition, extending to 100% of human males, is the congenital lack of a baculum (os priapi; os penis). Whereas most mammals (including common species such as dogs and mice) and most other primates (excepting spider monkeys) have a penile bone, human males lack this bone and must rely on fluid hydraulics to maintain erections. This is not an insignificant bone. The baculum of a large dog can be 10 cm long x 1.3 cm wide x 1 cm thick [Sisson and Grossman, 1953]. In rodents, the proximal segment of the os penis is formed by intramembranous ossification, while the distal region appears to be formed by endochondral ossification. The size of the rodent baculum is regulated by the posterior members of the HoxD set of transcription factors [Williams-Ashman and Reddi, 1991; Zakany et al., 1997] and appears to be induced by members of the TGF-ß and BMP families [Origuchi et al., 1998]. It has not been determined if the deficiency in human males is due to lack of paracrine factor expression in the genital mesoderm. Human bacula have been reported, usually in association with other congenital diseases or penile abnormalities [see Hoeg, 1986, Gelbard, 1988; Sarma and Weilbaecher, 1990; Vahlensieck et al., 1995].

One of the creation stories in Genesis may be an explanatory myth wherein the Bible attempts to find a cause for why human males lack this particular bone. Our opinion is that Adam did not lose a rib in the creation of Eve. Any ancient Israelite (or for that matter, any American child) would be expected to know that there is an equal (and even) number of ribs in both men and women. Moreover, ribs lack any intrinsic generative capacity. We think it is far more probable that it was Adam’s baculum that was removed in order to make Eve. That would explain why human males, of all the primates and most other mammals, did not have one. The Hebrew noun translated as “rib,” tzela (tzade, lamed, ayin), can indeed mean a costal rib. It can also mean the rib of a hill (2 Samuel 16:13), the side chambers (enclosing the temple like ribs, as in 1 Kings 6:5,6), or the supporting columns of trees, like cedars or firs, or the planks in buildings and doors (1 Kings 6:15,16). So the word could be used to indicate a structural support beam. Interestingly, Biblical Hebrew, unlike later rabbinic Hebrew, had no technical term for the penis and referred to it through many circumlocutions. When rendered into Greek, sometime in the second century BCE, the translators used the word pleura, which means “side,” and would connote a body rib (as the medical term pleura still does). This translation, enshrined in the Septuagint, the Greek Bible of the early church, fixed the meaning for most of western civilization, even though the Hebrew was not so specific.

In addition, Genesis 2:21 contains another etiological detail: “The Lord God closed up the flesh.” This detail would explain the peculiar visible sign on the penis and scrotum of human males—the raphé . In the human penis and scrotum, the edges of the urogenital folds come together over the urogenital sinus (urethral groove) to form a seam, the raphé. If this seam does not form, hypospadias of the glans, penis, and scrotum can result. The origin of this seam on the external genitalia was “explained” by the story of the closing of Adam’s flesh. Again, the wound associated with the generation of Eve is connected to Adam’s penis and not his rib.

A rib has no particular potency nor is it associated mythologically or symbolically with any human generative act. Needless to say, the penis has always been associated with generation, in practice, in mythology, and in the popular imagination. Therefore, the literal, metaphorical, and euphemistic use of the word tzela make the baculum a good candidate for the singular bone taken from Adam to generate Eve.

Baculum of a gray seal. Photo Ryo Manabe. Collection Joshua Foer.

Geldbard MK. 1988. “Dystrophic penile calcification in Peyronie’s disease.” J Urol 139:738—740.
Hoeg OM. 1986. “Human penile ossification.” Scand. J Urol Nephrol 20:231—232.
Nishikimi M, Fukuyama R, Minoshima S, Shimizu N, Yagi K. 1994. “Cloning and chromosomal mapping of the human nonfunctional gene for L-gulono-gamma-lactone oxidase, the enzyme for L-ascorbic acid biosynthesis missing in man.” J Biol Chem 269:13685—13688.
Origuchi N, Ishidou Y, Nagamine T, Onishi T, Matsunaga S, Yoshida H, Sakou T. 1998. “The spatial and temporal immunolocalization of TGFß1 and bone morphogenesis protein-2/4 in phallic bone formation in inbred Sprague Dawley male rats.” In vivo 12:473—480.
Sarma DP, Weilbaecher TG. 1990. “Human os penis.” Urology 35:349–350.
Sisson S, Grossman JD. 1953. The Anatomy of Domestic Animals. WB Saunders, Philadelphia.
Vahlensieck WK Jr., Schaefer HE, Westenfelder M. 1995. “Penile ossification and acquired penile deviation.” Eur Urol 27:252—256.
Williams-Ashman HG, Reddi AH. 1991. “Differentiation of mesenchymal tissues during phallic morphogenesis with emphasis on the os penis: roles of androgens and other regulatory agents.” J Steroid Biochem Mol Biol 39:873—881.
Zakany J, Fromental-Ramain C, Warot X, Duboule D. 1997. “Regulation of number and size of digits by posterior Hox genes: a dose-dependent mechanism with potential evolutionary implications.” Proc Natl Acad Sci USA 94:13695—13700.

Scott F. Gilbert, a professor at Swarthmore College, teaches developmental biology, developmental genetics, and the history of biology.

Ziony Zevit is a professor at American Jewish University in Los Angeles.