Cranial Modularity and Integration in Modern Humans: Evidence for Regional Variability and Evolutionary Flexibility

This study examines the modular organization of the human skull and variation in cranial integration across Eurasian populations.

The study analyzed 450 male crania from 10 Eurasian populations using 35 craniometric variables. Correlation matrices were constructed and evaluated using EMMLi analysis and the covariance ratio (CR) to test alternative modularity models, while two-block partial least squares (2B-PLS) analysis was used to estimate the strength of cranial integration.

EMMLi and CR analyses supported a highly modular organization of the human skull, with generally weak correlations within and between cranial modules. Viscerocranial modules showed stronger internal integration than neurocranial modules. Asian populations displayed weaker intermodular correlations, especially in the orbital and zygomatic regions, suggesting greater independence of these traits. The Khanty sample occupied an intermediate position between major macroregional groups. Increased cranial integration was observed in island and peripheral populations.

The findings indicate that weak integration among cranial modules may increase the potential for craniofacial variation in humans. Reduced integration of orbital and zygomatic modules in North Asian populations may reflect climatic adaptation and/or high population mobility. Lower modularity in island populations may result from stronger genetic drift or stabilizing selection, whereas increased modularity may reflect directional selection.

Cranial modularity varies not only among species but also among modern human populations, reflecting microevolutionary processes and environmental influences on morphological diversity.