Variation in the Facial Skeleton

Forensic anthropologists use shape and size of the human skull to estimate ancestry of unidentified, deceased individuals. Ancestry is one of the major clues for identifying human remains because it is one of the first ways people describe missing persons. Forensic anthropologists will build a biological profile for a set of skeletal remains to try to identify the individual. The biological profile will include age, sex, ancestry, stature, and any individualizing features, such as evidence of fractures that occurred during life or any dental work, such as fillings. The biological profile is a direct reflection of what would be incorporated into a missing person report created by law enforcement. The anthropological data can then be matched with missing persons reports to identify the skeleton as a missing person.

Ancestry can be determined using measurements and morphological features of the skull and teeth. These data are compared to worldwide reference datasets to determine the individual’s geographic origins. The problem then arises when anthropologists report ancestry and the legal system translates ancestry into social race classifications for identification. The anthropologist is bound to conform to social structures of race rather than using their more refined system that would increase the likelihood of identifying the individual. This is because geographic origins are more accurate information rather than classifying the individual as white, black, or Asian, the common 3-group framework used in the legal system.

The 3-group system is problematic because it is combines populations across large geographic space with diverse population histories into a single group. For example, the Asian population would represent Native American, Hispanic, and mainland Asian groups. These groups have lived in very different environments and have had little to no genetic exchange between them for long periods of time and have little to no cultural affiliations. Therefore, placing these groups under a single umbrella is essentially meaningless. Furthermore, many studies have found even using the groupings Hispanics or mainland Asians comprehensively is flawed as there is great regional variation. In the climate and genetics modules of this page, you learned that language and geographic barriers impact the rate of gene flow between regions and can increase phenotypic variation within these groups.

In the past, ancestry of a skull was determined by examining a list of traits and determining whether each trait was present or absent. A list of traits for were provided for each or the 3 ancestral groups (European, African, or Asian) which assumed the traits as discrete, meaning that each trait is represented for only one group and did not overlap between the groups. We know now that cranial morphological traits are exhibited across the groups at varying rates. The frequency of these traits can be recorded for geographic origins to develop a reference database for comparing a set of unidentified skeletal remains to estimate ancestry based on the combination of traits present.

In 2009, Dr. Joseph Hefner developed a method for scoring the degree of expression of 17 morphological features in the facial skeleton. These traits are known as macromorphoscopic (MMS) traits (see below). Since the development of this method, Dr. Hefner has been working to build a worldwide reference dataset of MMS trait data to capture to range of human facial variation across the world.

Below you will find 3 maps depicting how three facial skeletal features vary across space. These three features, known as Nasal Aperture Width (NAW), Interorbital Breadth (IOB), and Postbregamtic Depression (PBD), were selected because they are traditionally viewed as discrete African traits. The maps demonstrate how these traits vary in expression within a population that has traditionally been grouped as a single entity.

There are many language and geographic barriers in Africa that control gene flow between these regions. Additionally, there are some regions of Africa that are lush, humid, jungle environments while some regions a dry, desert environments. The variation in environments requires different phenotypic traits to increase the ability to survive.

Nasal Aperture Width

The first map depicts the frequency of wide nasal aperture width between countries. NAW is scored in the MMS method as narrow, intermediate, or wide and is scored relative to the width of the individual’s facial width. While African descendants were traditionally believed to all have wide nasal openings, only 7 of the 17 countries had at least 75% of the population with wide nasal apertures, 6 countries had between 50 and 75%, 2 had between 25 and 50%, and 2 had between 0 and 25%. Interestingly, the countries with higher frequencies are primarily adjacent to large bodies of water while those with lower frequencies are all inland. This may be a result of humidity controlling width of the nose.

Interorbital Breadth

The second map shows variation of wide interorbital breadths. IOB is scored in the MMS method as narrow, intermediate, or wide and is scored relative to the width of the individual’s facial width. Similar to NAW, interorbital breadth has traditionally been described as wide in the African population. Of the 17 countries observed, 10 have at least 75% of the population with wide interorbital breadths, 3 have between 50 and 75%, 3 have between 25 and 50%, and 1 has 25% or less. An interesting pattern to note here for future research is that those with lower frequencies cluster near the equator, meaning narrower interorbital breadths may be an adaptive trait for hot, humid climates. An alternative answer is that the overall facial skeleton is wider to increase surface area in order to release heat from the brain, causing underscoring of the interorbital breadth.

Postbregmatic Depression

The final map depicts the presence of postbregmatic depression across space. PBD is scored as either present or absent in the MMS method and is a dip in the top of the skull just behind the frontal bone, or the forehead. The cause of this depression is still somewhat debated but has traditionally been viewed as a discrete African trait. This map is particularly interesting because only 5 of the 17 countries have frequencies of at least 75% presence of a PBD, 8 have between 50 and 75%, and 4 have between 0 and 25% presence rates. This means that not only is the trait not as indicative of African ancestry as traditionally believed, but it also appears to be fairly variable within the population.

Overall, patterns of NAW, IOB, and PBD trait scores show that cranial morphological features cannot be assumed as discrete traits and the presence or absence of a trait does not equal a definitive ancestry determination. We must take into consideration that genetics and climate across space will influence phenotypic expression and that all human phenotypes are on a continuum. Therefore, arbitrary circles cannot be drawn around groups.

Hefner JT. 2018. The Macromorphoscopic Databank. American Journal of Physical Anthropology 166(4): 994–1004.