A simple rule governs the evolution and development of hominin tooth size

Abstract

The variation in molar tooth size in humans and our closest relatives (hominins) has strongly influenced our view of human evolution. The reduction in overall size and disproportionate decrease in third molar size have been noted for over a century, and have been attributed to reduced selection for large dentitions owing to changes in diet or the acquisition of cooking. The systematic pattern of size variation along the tooth row has been described as a 'morphogenetic gradient' in mammal, and more specifically hominin, teeth since Butler and Dahlberg. However, the underlying controls of tooth size have not been well understood, with hypotheses ranging from morphogenetic fields to the clone theory. In this study we address the following question: are there rules that govern how hominin tooth size evolves? Here we propose that the inhibitory cascade, an activator-inhibitor mechanism that affects relative tooth size in mammals, produces the default pattern of tooth sizes for all lower primary postcanine teeth (deciduous premolars and permanent molars) in hominins. This configuration is also equivalent to a morphogenetic gradient, finally pointing to a mechanism that can generate this gradient. The pattern of tooth size remains constant with absolute size in australopiths (including Ardipithecus, Australopithecus and Paranthropus). However, in species of Homo, including modern humans, there is a tight link between tooth proportions and absolute size such that a single developmental parameter can explain both the relative and absolute sizes of primary postcanine teeth. On the basis of the relationship of inhibitory cascade patterning with size, we can use the size at one tooth position to predict the sizes of the remaining four primary postcanine teeth in the row for hominins. Our study provides a development-based expectation to examine the evolution of the unique proportions of human teeth.

Is the deciduous/permanent molar enamel thickness ratio a taxon-specific indicator in extant and extinct hominids?

“…The results collected from the present exploratory test using a newly developed analytical tool -the "lateral enamel thickness diphyodontic index" (LETDI) -did not, however, provide an unambiguous and immediately readable picture, as might otherwise have been predictable on the basis of some ontogenetic and morphological studies using sequential teeth (e.g., Bailey et al, 2014Bailey et al, , 2016Evans et al, 2016). Rather, our results suggest complex patterns that likely result from the influence of a number of interactive factors.…”

“…in Bailey et al, 2014Bailey et al, , 2016; see also Evans et al, 2016), i.e., they are meristic elements with a similar and serially repeated structure within the same organism (Butler, 1956(Butler, , 1967Kraus and Jordan, 1965). In this study on some extant and fossil hominids, we thus investigate the degree of covariation in enamel proportions between the dm2 and the M1 (for the extant human condition, see Gantt et al, 2001;Grine, 2005;Huszár, 1972;Mahoney, 2010;Rossi et al, 1999).…”

Brain enlargement and dental reduction were not linked in hominin evolution

“…That study, however, used the area of the second molar (M2) as a proxy for molar size without considering variation in molar proportions across the molar row. Those proportions are known to change in the genus Homo in concert with absolute molar size, thus making M2s and M3s disproportionately small in species with overall small dental size (22,23). Reduction in the dentition was not…”

The integration of quantitative genetics, paleontology, and neontology reveals genetic underpinnings of primate dental evolution

“…Despite empirical evidence for how primate dental patterning differs from dental patterning of the mouse (19,20), proponents of the mouse model continue to adhere tightly to it. Most recently, Evans et al (21) report that the IC model from mice applied uncritically to hominids reveals that the genus Homo is distinct from other hominid genera in having smaller third molars because of a "simple rule. "…”