The "Billowing Skirt" Phenomenon: A Molar Tooth from the Denisova Hominin

The quest for answers about our hominid ancestors took a new twist recently when researchers found two molar teeth and a small finger bone in the Denisova cave in Siberia that allowed the genome characterization of a new hominin that shares a common ancestor with the Neanderthal population.

Additional details of this find were reported in Nature in December of 2010. (Figure 1) This tooth shares no derived morphological features with Neanderthal or modern humans, further indicating that Denisova hominins have an evolutionary history distinct from Neanderthal and modern humans. (1,3)

 Figure 1

Figure 1

The characterization of the entire genome of this species was derived from a small piece of the distal phalanx of the pinkie finger and a molar tooth that is at least 30,000-years of age. This tooth is of unusually large proportions as reported by Reich et al. 1

The morphology of this tooth is distinctly different from both Neaderthal and modern humans. In fact the shape of these molar teeth more closely resembles ancient hominins such as Homo erectus or Homo habilis. These molar teeth have been labeled as Denisovan after the name of the Siberian cave in which they were discovered. Even though the exact sequence of the Denisova genome has been described, the anatomic features of these molars await a more complete review in the dental literature. A few digital photographs have been made available that disclose a large number of characteristics that are not seen in dental anatomy textbooks. The most striking feature is the enamel surface that lacks the smooth contours that are seen in modern man.

The enamel surface has a “billowing skirt” look with many rounded segments that do not mimic any modern human molar feature. This morphologic feature may be the result of multi-centric loci of enamel initiation. Since the cusp tip calcification is the initial site for hard tissue formation, numerous enamel calcification sites likely appear in near simultaneous fashion.  (Figure 2)

Enamel Surface

 Figure 2 - Distal View

Figure 2 - Distal View

The multi-lobulated nature of the enamel surface is unique to the Denisovan molar and this feature has not been sufficiently characterized. The division between each segment is distinct and presents a picture of independent lobes that are fused vertically. In many of these lobules the occlusal terminus of this segment appears sharpened or reminiscent of the “spiked” look of the buccal cusps of bicuspids and canines. The appearance of the molar in general is one of bulbous refinement characterized by “dagger-like” spikes at uneven intervals around the entire crown. This kind of anatomic feature most likely would improve the penetrating and tearing ability of the molar quadrants. The anatomy appears to favor the tough and often difficult oral management of foods that resist mechanical maceration. (Figure 2)

Root Shape & Angulation

The root shape of this Denisova hominin is somewhat stubby. The long axis of the lingual root is very straight and that root appears to substantially diverge lingually from the long axis of molar. The mesio and disto-buccal roots appear to fall more in-line with the long axis of the tooth. The overall view of the root structure depicts a robust structure with lingual flaring from the long axis.

Cervical Line

The junction of the enamel and the cemental surface creates the cemento-enamel junction (CEJ) that is irregular in its overall distribution. The horizontal axis of the CEJ taken between the mesial and distal points of the CEJ would not intersect the long axis of the tooth at 90 degrees. The description of the CEJ is less uniform than in present-day molar anatomy. (Figure 3)

Crown Anatomy

 Figure 3 - Lingual View

Figure 3 - Lingual View

The four cusps of a molar are named the Mesio-buccal (Paracone), the Disto-buccal (Metacone), the Mesio-lingual (Protocone), and the Disto-lingual (Hypocone). As viewed from the lingual aspect, the cusps do not occupy a similar amount of total volume of the crown. The disto-lingual cusp is larger in length and in mesio-distal dimension. That volume discrepancy skews the cemento-enamel junction (CEJ) toward the apex and adds emphasis to the value of that cusp in mastication. This cusp ordinarily occludes with the central fossa of the opposing mandibular molar in a Class I molar relation, thus improving the efficiency of the masticatory process. This emphasis on cuspal size and shape likely improves the quality of the gnathologic interdigitation. The anatomic form of this cusp is in direct relation to its primary role in reducing the fibrous nature of the bolus into a mass that is readily digested by the enzymatic action contained within the oral fluids. The proper introduction of salivary fluid into the bolus does improve the ability of the tissues to acquire adequate amounts of nutrients. Therefore the anatomic emphasis on cuspal form and function does provide a maximum level of nutritive intake. The unusual size and shape of the disto-lingual cusp is indicative of superior masticatory range of function and may provide the highest level of functional dispersion. (Figure 3)

Enamel Depth and Distribution

The enamel depth covering the entire crown is somewhat thinner than might be expected of an early hominid. The protective action of the enamel is not emphasized in this very old molar and likely represents a decreased incremental response to external stimuli that is not currently present in modern human oral structures. The enamel covering is also distributed somewhat differentially as compared to the modern human counterpart. (4)

 Figure 4 - Occlusal View

Figure 4 - Occlusal View

Occlusal Surface

The occlusal surface of the other (the second) molar found at the site appears consistent with the anatomic form observed in many modern humans. While its trapezoidal shape is somewhat distinctive, it generally has a form that would not be considered unusual by present-day anatomic standards.

The central fossa is well delineated, as is the distal fossa with a disto-lingual groove. Certainly the lingual cusp (protocone) is easily observed and appears similar is size and volume to current living humans. However, this tooth also displays some of the lobulated features that can be seen in Figure 2.

Pulp Chamber

Using unconventional techniques the pulp chamber of these teeth have been shown to be of very large size. The multifunctional characteristics of the well-vascularized pulp tissue indicate that the molar might react favorably to external stimuli.

For additional data regarding the morphology of the Denisova molar see Supplementary Information 12 contained in reference 1.

Readers may be interested in a 10- minute film produced by the Max Planck Society that features Dr. Bence Viola delivering an overview of the Denisova acquisition. (4)

Figures courtesy Max Planck Institute for Evolutionary Anthropology

 

For further reading

1. Genetic history of an archaic hominin group from Denisova Cave in Siberia.

Reich D, Green RE, Kircher M, Krause J, Patterson N, Durand EY, Viola B, Briggs AW, Stenzel U, Johnson PL, Maricic T, Good JM, Marques-Bonet T, Alkan C, Fu Q, Mallick S, Li H, Meyer M, Eichler EE, Stoneking M, Richards M, Talamo S, Shunkov MV, Derevianko AP, Hublin JJ, Kelso J, Slatkin M, Pääbo S.

Nature. 2010 Dec 23; 468(7327):1053-60. Department of Genetics, Harvard Medical School, Boston, Massachusetts 02115, USA. See: http://www.nature.com/nature/journal/v468/n7327/abs/nature09710.html

2. http://www.ncbi.nlm.nih.gov/pubmed/21179161 (Resourced in July2013)

3. http://en.wikipedia.org/wiki/Denisova_hominin (Resourced in July 2013)

4. http://www.youtube.com/watch?v=eweVB0XPC_8