Beyond thick versus thin: mapping cranial vault thickness patterns in recent Homo sapiens

Cranial vault thickness (CVT) has been reported at many different osteometric landmarks and features on the vault. Historically, only a few landmarks are used, often bregma, lambda, vertex, and right and left euryon, and frequently comparisons are based only on “thick” versus “thin” to describe the vault overall. What is inherent in this strategy is the use of a few locations to characterize the entire vault. The problem remains that there is little information concerning CVT variation throughout an individual's vault, and the causes of variation within recent Homo sapiens important to investigating thickness variation between species in Homo. This work describes thickness variation over the entire superior vault and compares the sexes, age groups and populations in recent H. sapiens. A proportional grid is applied to the superior vault to measure thickness at 219 sampling points in a geographically diverse sample of recent H. sapiens. Thickness values are analyzed in their two-dimensional spatial relationships to determine patterns of vault thickness.

Males were identified to be thicker than females at more lateral locations and along the midsagittal plane, although this finding is not statistically significant. Individuals over the age of 45 years are found to be statistically significantly thicker than individuals younger than 31 years at more lateral locations of the vault. Aboriginal Australians are statistically significantly thicker at more lateral locations of the vault than any other populations, whereas Northern Canada/Greenland individuals were thinner than other populations at these locations. The trend of thicker vaults in the older age group and the Australians is identified across the vault, although is not statistically significant at more locations.

Several thickness patterns are identified. The boss thickening pattern is the most common pattern, followed by a midsagittal pattern, a posterior pattern, and an anterior pattern. Some specimens do not demonstrate thickness variation and are coded as undifferentiated. Each pattern is observed alone and in combination with others, signifying that pattern causes are not mutually exclusive. Boss thickening is interpreted as the result of passive bone thickening during normal bone and brain growth during fetal and adolescent development. The midsagittal thickness pattern coincides with inferred strain along the sagittal suture from nuchal muscle engagement during mastication.

Previous researchers have proposed adaptive explanations for thickness variation, such as protection from interpersonal violence; the patterns of cranial vault thickness reported here point to normal growth and development of the brain as a driving force, a relationship that could drive thickness variation in other Homo species. Comparing thickness at bregma, and the frontal and parietal eminences for recent H. sapiens and H. erectus, there is no statistical difference between African and Asian H. erectus, and between the on average thicker H. sapiens populations and H. erectus, based on published data. Future work will investigate the presence or absence of thickness patterns in these fossil species.