Darwin’s tubercle has been studied in various populations and has been estimated to be present in about 10.5% of the Spanish adult population [11], 40% of Indian adults [12], and 58% of Swedish school children [13]. Inheritance of this trait was once thought to follow an autosomal dominant pattern, but some studies have called this theory into question [2, 14, 15].
Quelprud et al. [14] studied the presence of Darwin’s tubercle in German families and found that, in 52 families in which neither parent possessed the auricular prominence, 45% (n = 22) of the children possessed the tubercle. In addition, Beckman et al. [15] performed a similar study, in which he found that 24% (n = 14 of 58) of individuals whose parents did not possess Darwin’s tubercle, had the atavistic prominence. The results of these two studies are inconsistent with an autosomal dominant pattern of inheritance.
However, despite attempts to characterize the inheritance pattern of Darwin’s tubercle, it remains unclear what genetic influences, if any, control the expression of this trait. Quelprud performed additional studies on identical twins and found 58 pairs in which both individuals had Darwin’s tubercle and 32 pairs in which neither individual possessed it [14]. He also found 26 pairs of twins in which one individual possessed the trait and the other did not. In addition, although some studies have found no differences in the prevalence of Darwin’s tubercle with sex or age, [16, 17], others have observed associations with both. For example, Vollmer et al. [18] found that greater degrees of expression were associated with older males. Thus, the extent of genetic and environmental influences on the expression of Darwin’s tubercle remains unclear.
Clinical Presentation
Darwin’s tubercle is most commonly described as a swelling on the posterior superior portion of the helix [24]. However, variations may occur in the location and degree of prominence (Fig. 2), and various classifications of Darwin’s tubercle have been proposed; informed consent was obtained from the patients for being included in this review. Bertillon [19] was the first to suggest categorization of the tubercle into four groups: nodosity, enlargement, projection, and tubercle. Subsequently, Gurbuz [17] proposed five categories (undeveloped, semi-developed, fully developed, very significant, and multiple), and Singh and Purkait et al. [12] characterized three (nodosity, enlargement, and projection). To date, no consensus has been established regarding the classifications of Darwin’s tubercle.
Additionally, Darwin’s tubercle may be present on both ears or just on one ear. Dharap and Than [20] observed in a study of 1435 Malaysian subjects that, of 498 individuals who possessed Darwin’s tubercle, 50% (n = 249) had the prominence present on both ears, 26.5% (n = 132) only on the right ear, and 23.5% (n = 117) only on the left ear. Studies from Singh et al. [21] revealed similar findings, portions of the study population displaying the trait asymmetrically, although the majority of individuals with Darwin’s tubercle did possess the trait on both ears.
Of note, various studies on patterns of the external ears have documented the individuality of ears, observing that even the right and left ears of the same individual are not identical [3]. Thus, the asymmetric presentation of Darwin’s tubercle may further contribute to the uniqueness of human ears.
Darwin’s tubercle has also been linked to a number of associated conditions, such as congenital absence of the helix, accessory tragus, and weathering nodules [22] (Fig. 3). Although Darwin’s tubercle and its associated conditions appear to be benign with no significant clinical sequelae, surgical treatment may be an option in order to address cosmetic concerns [7]. This may be accomplished through full-thickness excision of the skin and the prominent cartilage underneath [25, 26].
Animal Analogues of Darwin’s Tubercle
As an atavistic feature linking humans and primates to a common ancestor, an exploration of the presence of this feature in the animal kingdom is warranted. Among primates, two genera of the Cercopithecidae, the Macaca and Papio, have been found to have a pointed upper margin of the ear, similar to Darwin’s tubercle in humans [23]. Interestingly, although pointed ears are found in many lower mammals, no other anthropoids except for the Macaca and Papio have a pointed stage during the development of their ears [23]. It is unclear whether the presence of a pointed ear provides any functional advantage in these primates, or whether, like in humans, they are merely a vestigial remnant of the past.