Climate and the evolution of brachycephalization

Abstract

Significant associations of cranial shape for 82 ethnic groups and seven climate variables are described. Variation among current populations is partially attributed to cold adaptation throughout the Pleistocene. Application of data files tabulated by the authors is described for a number of problems. Temporal distribution of 115 specimens indicates a geometric trend (CI = 76.7-1.96 log time X 10(3) ). Cranial indices are summarized within alternative taxonomic models and between climatic ecotypes. Evidence supports the hypothesis of cold adaptation among "Classic" Neandertals. Limitations of the thermodynamic model are discussed. It is probable that a decrease of the cranial index occurs from the Middle to Upper Paleolithic. During the Holocene, the index increases under all climatic conditions.

Climate Signatures in the Morphological Differentiation of Worldwide Modern Human Populations

“…Most of the high Fst variables showing a correlation with climate were located on the face, with a few also reflecting neurocranial breadth, again agreeing with previous work (Coon et al, 1950;Carey and Steegmann, 1981;Beals et al, 1983;Franciscus and Long, 1991;Roseman, 2004;Roseman and Weaver, 2004;Harvati and Weaver, 2006a,b). They comprise measurements of facial breadth and height, as well as the nasal dimensions and simotic subtense.…”

“…These differences can be interpreted again as distinct pathways toward wider braincases, which would be advantageous for populations in extremely cold climates under the assumption of Bergman's and Allen's rules (Beals et al, 1983(Beals et al, , 1984: By increasing neurocranium breadth, globularity of the braincase is increased and the ratio surface/volume reduced, thus reducing heat loss through the surface of the skull. These changes toward a wider skull could be related to changes in basicranial morphology, which seems to be influencing vault morphology (Lieberman et al, 2000), as well as to the increase of general brain size (Beals et al, 1984).…”

“…The relevance of cranial morphology to phylogenetic reconstruction has been questioned in the past (see e.g., Collard and Wood, 2000), and convergence, parallelism, reversals, and environmen-tal plasticity are commonly cited as obscuring any phylogenetic signal that it may preserve. The effects of mastication and climatic adaptation on the cranium are thought to be particularly extensive, with some anatomical regions or measurements believed to be affected differentially by these processes (e.g., Hylander, 1977;Carey and Steegmann, 1981;Olson, 1981;Beals et al, 1983;Skelton and McHenry, 1992;Larsen, 1999;Wood and Lieberman, 2001;Lieberman et al, 2004;Roseman, 2004;Roseman and Weaver, 2004;Gonzá lez-José et al, 2005;Harvati and Weaver, 2006a,b;Sardi et al, 2006).…”

Human cranial anatomy and the differential preservation of population history and climate signatures

“…Cranial size has been previously linked to climate, with larger crania found in cold-climate human groups (Beals et al, 1983;see also Roseman, 2004). This size effect has been related to the larger body and brain sizes found in cold climates (Beals et al, 1983).…”

“…In humans, facial form, and particularly the shape of the nose, has been linked to climatic adaptation (Coon et al, 1950;Carey and Steegmann, 1981;Franciscus and Long, 1991;Roseman, 2004;Roseman and Weaver, 2004;Nicholson and Harvati, 2006) and to dietary or masticatory practices (Hylander, 1977;Skelton and McHenry, 1992;Lieberman et al, 2004;Sardi et al, 2006), probably through a combination of developmental responses to the environment and genetic adaptation. The size and shape of the neurocranium have also been related to climate differences (Beals et al, 1983;Roseman, 2004). On the other hand, the shape of the basicranium has been proposed to be the most genetically determined and evolutionarily conservative aspect of the cranium and is thought to be minimally influenced by environmental factors (Olson, 1981;Wood and Lieberman 2001).…”