Secular Changes in Cranial Morphology and Pattern of Sexual Dimorphism in Modern Japanese: A Geometric Morphometric
Shiori Usui1,2, Hideki Amano1, Hideyuki Hayakawa3
1Laboratory of Human Evolutionary Biomechanics, Department of Biological Sciences, Graduate School of Science, the University of Tokyo, Bunkyo-ku, Tokyo, Japan.
Objectives:
Although human cranial morphology is thought to have undergone changes in various populations over the past century, the details of this three-dimensional transformation remain largely unknown. To address this problem, we conducted a geometric morphometric analysis of cranial shapes in the historical Japanese group who lived approximately 100 years ago and the present-day Japanese group who died in recent years.
Materials And Methods:
We used a total of 112 computed tomography (CT) scan data: CT scans of 56 historical Japanese dry crania from individuals who lived in the late 19th to early 20th century and postmortem CT data of 56 present-day Japanese who died in the 2020s. Each cranium was represented by 161 landmarks, and their coordinates were normalized by centroid size and registered using generalized Procrustes analysis to remove variation due to uniform scaling, translation, and orientation. Principal component analysis was then conducted to explore shape variation among the specimens.
Results:
Comparison between these two populations revealed clear secular changes, including brachycephalization and enlargement of the mastoid processes. Furthermore, our results indicate that certain forms of sexual dimorphism, such as larger mastoid processes and more prominent external occipital protuberances in males, have become more pronounced over time.
Discussion:
These findings demonstrate that the human cranium has undergone substantial changes over the last century, likely influenced by shifts in environmental, nutritional, and lifestyle factors. They can also contribute to improving the accuracy of sex estimation for unidentified skeletal remains in both forensic and archeological contexts.
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