Variation in Molar Size and Proportions in the Hominid Lineage: An Inter- and Intraspecific Approach

Area of Science:

• Evolutionary Biology
• Paleontology
• Anthropology

Background:

• The inhibitory cascade (IC) model is widely used to explain dental diversification at higher taxonomic levels.
• Limited research exists on the IC model's implications for organisms with recent evolutionary divergence or at interpopulational scales.

Purpose of the Study:

• To assess the impact of molar size changes and the balance of local activators and inhibitors on molar proportions.
• To analyze allometric changes in molar size within hominids and modern human populations.

Main Methods:

• Compilation of crown diameter data for first, second, and third lower and upper molars from extinct and extant hominids and modern human populations.
• Analysis of allometric changes between individual tooth size and overall molar row size.
• Comparison of molar proportions with the morphospace defined by the IC model.

Main Results:

• Negative allometry observed in first molars (M1), isometric changes in second molars (M2), and positive allometry in third molars (M3) across hominin phylogeny and modern humans.
• Most modern human populations align with the IC model's prediction (M1 > M2 > M3), while several hominid species fall outside this morphospace.
• Phylogenetic structuring of molar size, particularly in the maxilla, shows a trend of mesial-to-distal reduction with associated allometric changes.

Conclusions:

• The IC model has limitations in explaining primate molar proportions, especially at the interspecific scale within the hominid lineage.
• Phylogenetic factors significantly influence molar size and proportions, with distinct patterns observed in hominin evolution and modern human populations.