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

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

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.

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.

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.

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.