Evolutionary Origin of Teeth

Abstract

Teeth and jaws have been widely perceived as key innovations underpinning the adaptive radiation and the evolutionary success of jawed vertebrates. However, the origin, evolution and developmental evolution of teeth are all the subject of controversy. There are three competing hypotheses that are more or less well supported by the available data: (1) the ‘outside‐in’ hypothesis which contends that odontogenic competence spread from the external dermis to the oropharynx – the traditional hypothesis based on the observation that teeth and scales exhibit common patterns of development that extend to the molecular genetic level, combined with evidence that scales appear before teeth within phylogeny; (2) the ‘inside‐out’ hypothesis that teeth and scales evolved independently, based on the observation that some jawless scale‐covered vertebrates also possess oro‐pharyngeal scales, and (3) the ‘inside and out’ hypothesis which is effectively agnostic on the question. The available evidence supports an origin of teeth through extension of odontogenic competence from the external dermis to the oropharynx.

Key Concepts

  • ‘Outside‐in’ hypothesis argues for an evolution of teeth after the evolution of jaws from ectodermal epithelium, based on the observation that teeth and scales are considerably similar in their development and morphology.
  • ‘Inside‐out’ hypothesis argues for an evolution of teeth before jaws from an endodermal epithelium, based on the assumption that teeth and scales are considerably different and have no common origin.
  • ‘Inside and out’ hypothesis argues for a deep homology and ancient gene regulatory network for nonmineralised (taste buds) and mineralised appendages (scales and teeth).
  • Odontodes are a descriptive observation defined as hard tissue structures that develop in a single papilla, consist of dentine or dentinous tissue and frequently a superficial enameloid layer.
  • Whole genome duplications as drivers of vertebrate evolution enabling mutations ultimately resulting in the evolution of novelties considered as key‐innovations, e.g. jaws and teeth.
  • Phylogenetic congruence is a test of an evolutionary scenario with cladistics and a well‐supported phylogeny.

Keywords: teeth; development; evolution; vertebrates; key innovations; whole genome duplication; homology; phylogenetic congruence

Figure 1. Phylogeny and distribution of teeth, oral denticles and scales redrawn after Donoghue and Rücklin changes based on Coates et al. (2018) and Murdock et al. .
Figure 2. Schematic illustration of hypotheses on dental evolution, redrawn after Fraser et al. . (a) Outside‐in hypothesis and (b) inside‐out hypothesis. ect – Ectoderm; end – Endoderm; opc – oro‐pharyngeal cavity.
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Rücklin, Martin, and Donoghue, Philip CJ(Jul 2019) Evolutionary Origin of Teeth. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0026408]