The protoconid: a key cusp in lower molars. Evidence from a recent modern human population

Abstract

Background

The molar (M) size sequence in the genus Homo is decreasing and the general pattern in Homo sapiens is M1> M2 > M3.

Aim

To gain a better understanding of the reduction patterns of M components (cusps), we aim to assess the area of the protoconid (Prd), the phylogenetically oldest cusp of the lower Ms.

Subject and methods

We measured the Prd and the total crown area in the scaled photographs of a recent modern human sample of lower Ms (76 males and 39 females). The values were statistically analysed.

Results

The absolute size of the Prd increases significantly between M1 and M2/M3, whereas the relative size of this cusp increases significantly from M1 to M3. In the latter, reduction or disappearance of the cusps of the talonid is common.

Conclusions

The results can be explained in the framework of the patterning cascade model. As the first cusp to appear developmentally, the Prd forms in response to signals from the primary enamel knot, likely contributing to its stability. Inhibitory signals emitted during the Prd formation may lead to the reduction or disappearance of the talonid cusps, if these do not have enough time to form before the end of the M morphogenetic process.

Keywords:

• Teeth morphogenesis
• protoconid size
• modern humans

Acknowledgements

We also acknowledge The Leakey Foundation through the personal support of Gordon Getty and Dub Crook to one of the authors (M.M.-T.). We thank the University of Coimbra for access to the collections of the Institute of Anthropology. In particular, our thanks to Eugénia Cunha.

Author contributions

José María Bermúdez de Castro: Conceptualisation; data collection; formal analysis; original draft; project management; fund raising. Cecilia García-Campos: Formal analysis; methodology; revision of original draft. Susana Sarmiento: Data collection; methodology. María Martinón-Torres: project management; fund raising, revision of original draft.

Disclosure statement

The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the article.

Data availability statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Additional information

Funding

This work has been mainly supported by the Dirección General de Investigación of the Spanish Ministry of “Ciencia, Innovación y Universidades”, grant number PGC2018-093925-B.C31, as well as the Fundación Atapuerca through a posdoctoral grant to CGC and The Leakey Foundation through the personal support of Gordon Getty and Dub Crook to one of the authors (M.M.-T.).