Geometric morphometrics in anthropology
Geometric morphometrics is used to observe variation in numerous formats, especially those pertaining to evolutionary and biological processes, which can be used to help explore the answers to a lot of questions in physical anthropology.
[4][5] In the 1940s, D’Arcy Wentworth Thompson (biologist and mathematician, 1860-1948) looked at ways to quantify that could be attached to biological shape based on developmental and evolutionary theories.
[8] In the late 1970s and early 1980s, Fred Bookstein (currently a professor of Anthropology at the University of Vienna) began using Cartesian transformations and David George Kendall (statistician, 1918-2007) showed that figures that hold the same shape can be treated as separate points in a geometric space.
[8][9] Finally, in 1996, Leslie Marcus (paleontologist, 1930-2002) convinced colleagues to use morphometrics on the famous Ötzi skeleton, which helped expose the importance of the applications of these methods.
There is a basic structure to successfully performing and completing every geometric morphometric study: The first step is to define your landmark set.
[4][5] Semilandmarks were created in order to take landmark based geometric morphometrics to the next step by capturing the shape of difficult areas such as smooth curves and surfaces.
[5] Bookstein proposed the use of a thin-plate spline (TPS) interpolation, which is a computed deformation grid that calculates a mapping function between two individuals that measures point differences.
The translation, rotation, and scaling bring the landmark configurations for all specimens into a common coordinate system so that the only differing variables are based on shape alone.
This is helpful in geometric morphometrics where a large set of landmarks can create correlated relationships that might be difficult to differentiate without reducing them in order to look at the overall variability in the data.
[6] Principal component scores are computed through an eigendecomposition of a sample’s covariance matrix and rotates the data to preserve procrustes distances.
It is important to explore what shape variables are being observed to make sure the principal components being analyzed are pertinent to the questions being asked.
PLS has been used a lot in studies that look at things such as sexual dimorphism, or other general morphological differences found at the population, subspecies, and species level.
Geometric morphometrics has been used to explore some of these questions using virtual endocasts (casts of the inside of the cranium) to gather information since brain tissue does not preserve in the fossil record.
They found that there is an early “globularization phase” in human brain development that shows expansion of the parietal and cerebellar areas, which does not occur in chimpanzees.
Studies on cranial morphology for these specimens have created arguments that Eurasian fossils from the Middle Pleistocene are a transition between Homo erectus and later hominins like Neanderthals and modern humans.
Harvati and colleagues decided to attempt to quantify the craniofacial features of Neanderthals and European Middle Pleistocene fossils using 3D landmarks to try to add to the debate.
[10][14] Freidline and colleagues further added to the debate by looking at both adult and subadult crania of modern and Pleistocene hominins using 3D landmarks and semilandmarks.
The study also found that some characteristics separating Neanderthals from Middle Pleistocene hominins, like the size of the nasal aperture and degree of midfacial prognathism, might be due to allometric differences[10][15] Crania can be used to classify ancestry and sex to aid in forensic contexts such as crime scenes and mass fatalities.
In 2010, Ross and colleagues were provided federal funds by the U.S. Department of Justice to compile data for population specific classification criteria using geometric morphometrics.
Bierry and colleagues used 3D CT reconstructions of modern adult pelvic bones for 104 individuals to look at the shape of the obturator foramen.
They placed landmarks and semilandmarks on 2D photographic images of 121 left pelvic bones from a collection of undocumented skeletons at the Museu Anthropológico de Coimbra in Portugal.
The results had a classification accuracy for the greater sciatic notch of 90.9% and the ischiopubic ramus at 93.4 to 90.1%[18] In archaeology, Geometric morphometrics are used to examine the shape variations or standardization of artifacts to answer questions about typological and technological changes.
[19][20][21][22][23] Some applications to pottery shape is to identify the level of standardization to explore ceramic production and its implication about social organization.
