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Saturday, February 28, 2009

Painted skulls, genetics and craniofacial phenotypic variation

Believe it or not, there are still people out there who read up on early 20th century anthropology and then wonder what "sub-race" or "type" they are based on the shape of their melons. They're all over the net, on various fora and blogs, discussing the finer points of Carleton Coon's 1939 typological bible, The Races of Europe (aka. TRoE). It's a depressing sight, but hopefully one that won't last much longer, because we're finally seeing detailed studies on the evolution, phenotypic variation and genetics of the human skull.

Some of the best work is being done at the University of Barcelona, which recently published a massive
presentation on its "Hallstatt Project" and a couple of resulting scientific studies. The thing that really jumps out at me from this extremely detailed PDF, apart from the bizarre way in which the Hallstatt folk painted the skulls of their dead, is the conclusion that "the classification between brachy- and dolicephalic skulls is not grounded on the genetic level". This of course means that looking at the basic shape of the head from above, which used to be one of the favorite pastimes of physical anthropologists, won't reveal anything about genetic ancestry. The skull is indeed an integrated phenotypic complex, as these studies confirm, but to learn anything of value, one must focus on the right traits, how they relate to each other, and also be aware to what extent they're influenced by the environment.

Abstract: Quantitative craniometrical traits have been successfully incorporated into population genetic methods to provide insight into human population structure. However, little is known about the degree of genetic and non-genetic influences on the phenotypic expression of functionally based traits. Many studies have assessed the heritability of craniofacial traits, but complex patterns of correlation among traits have been disregarded. This is a pitfall as the human skull is strongly integrated. Here we reconsider the evolutionary potential of craniometric traits by assessing their heritability values as well as their patterns of genetic and phenotypic correlation using a large pedigree-structured skull series from Hallstatt (Austria). The sample includes 355 complete adult skulls that have been analysed using 3D geometric morphometric techniques. Heritability estimates for 58 cranial linear distances were computed using maximum likelihood methods. These distances were assigned to the main functional and developmental regions of the skull. Results showed that the human skull has substantial amounts of genetic variation, and a t-test showed that there are no statistically significant differences among the heritabilities of facial, neurocranial and basal dimensions. However, skull evolvability is limited by complex patterns of genetic correlation. Phenotypic and genetic patterns of correlation are consistent but do not support traditional hypotheses of integration of the human shape, showing that the classification between brachy- and dolicephalic skulls is not grounded on the genetic level. Here we support previous findings in the mouse cranium and provide empirical evidence that covariation between the maximum widths of the main developmental regions of the skull is the dominant factor of integration in the human skull.

Neus Martínez-Abadías et al, Heritability of human cranial dimensions: comparing the evolvability of different cranial regions. Unitat d'Anthropologia, Departament de Biologia Animal, Universitat de Barcelona.

Full PDF link...

Evolutionary patterns of the human skull. A quantitative genetic analysis of craniofacial phenotypic variation.