title: 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.
authors.affiliation: School of Biological Sciences, Monash University, Victoria 3800, Australia.|Geosciences, Museum Victoria, Victoria 3001, Australia.
authors.forename: Alistair R
authors.initials: AR
authors.lastname: Evans
authors.affiliation: Institute of Human Origins, Arizona State University, Tempe, Arizona 85287, USA.|School of Human Evolution and Social Change, Arizona State University, Tempe, Arizona 85287, USA.
authors.forename: E Susanne
authors.initials: ES
authors.lastname: Daly
authors.affiliation: Institute of Human Origins, Arizona State University, Tempe, Arizona 85287, USA.|School of Human Evolution and Social Change, Arizona State University, Tempe, Arizona 85287, USA.
authors.forename: Kierstin K
authors.initials: KK
authors.lastname: Catlett
authors.affiliation: School of Human Evolution and Social Change, Arizona State University, Tempe, Arizona 85287, USA.|Center for Bioarchaeological Research, Arizona State University, Tempe, Arizona 85287, USA.
authors.forename: Kathleen S
authors.initials: KS
authors.lastname: Paul
authors.affiliation: Department of Anthropology, University of Massachusetts Amherst, Amherst, Massachusetts 01003, USA.
authors.forename: Stephen J
authors.initials: SJ
authors.lastname: King
authors.affiliation: School of Anthropology and Conservation, University of Kent, Canterbury CT2 7NR, UK.|Department of Human Evolution, Max Planck Institute for Evolutionary Anthropology, Leipzig 04103, Germany.
authors.forename: Matthew M
authors.initials: MM
authors.lastname: Skinner
authors.affiliation: School of Human Evolution and Social Change, Arizona State University, Tempe, Arizona 85287, USA.
authors.forename: Hans P
authors.initials: HP
authors.lastname: Nesse
authors.affiliation: Department of Human Evolution, Max Planck Institute for Evolutionary Anthropology, Leipzig 04103, Germany.
authors.forename: Jean-Jacques
authors.initials: JJ
authors.lastname: Hublin
authors.affiliation: School of Dentistry, The University of Adelaide, South Australia 5005, Australia.
authors.forename: Grant C
authors.initials: GC
authors.lastname: Townsend
authors.affiliation: Institute of Human Origins, Arizona State University, Tempe, Arizona 85287, USA.|School of Human Evolution and Social Change, Arizona State University, Tempe, Arizona 85287, USA.
authors.forename: Gary T
authors.initials: GT
authors.lastname: Schwartz
authors.affiliation: Institute of Biotechnology, University of Helsinki 00014, Finland.
authors.forename: Jukka
authors.initials: J
authors.lastname: Jernvall
mesh_terms: D000818:Animals; D005075:Biological Evolution; D005260:Female; D005580:Fossils; D015186:Hominidae / Q000033:anatomy & histology* / Q000145:classification; D006801:Humans; D008297:Male; D008963:Molar / Q000033:anatomy & histology; D009929:Organ Size; D010802:Phylogeny; D013045:Species Specificity; D014070:Tooth / Q000033:anatomy & histology*