Skull shape changes are not independent

18/01/2012 by admin.

ScienceDaily had an item (here) on research (Martínez-Abadías and others, Pervasive genetic integration directs the evolution of human skull shape. Evolution, 2011) showing that human history may have intertwined evolutionary pressures rather than sequential ones.

They worked with a unique set of skulls that could be matched with good genealogical data to test various theories of skull shape determinants. In particular they looked at changes that were important in human evolution: the shift of the foramen magnum where the spinal cord enters the skull associated with upright walking, the retraction of the face associated with chewing soft food (cooked using of fire?) and vocalizing, and the rounding expansion of the top of the skull associated from increased neocortex mass leading to tool making, language, general smarts. These have been thought to have evolved independently.

(The researchers) found that, rather than being separate evolutionary events, changes in one part of the brain would facilitate and even drive changes in the other parts. “We found that genetic variation in the skull is highly integrated, so if selection were to favour a shape change in a particular part of the skull, there would be a response involving changes throughout the skull,” said Dr Chris Klingenberg.

Previously, environmental pressure were postulated separately for each of these three changes. This research points to a more fluid evolution where specific pressures might not be needed for some changes or where several less powerful pressures might work together.

Here is their abstract:

It has long been unclear whether the different derived cranial traits of modern humans evolved independently in response to separate selection pressures or whether they resulted from the inherent morphological integration throughout the skull. In a novel approach to this issue, we combine evolutionary quantitative genetics and geometric morphometrics to analyze genetic and phenotypic integration in human skull shape. We measured human skulls in the ossuary of Hallstatt (Austria), which offer a unique opportunity because they are associated with genealogical data. Our results indicate pronounced covariation of traits throughout the skull. Separate simulations of selection for localized shape changes corresponding to some of the principal derived characters of modern human skulls produced outcomes that were similar to each other and involved a joint response in all of these traits. The data for both genetic and phenotypic shape variation were not consistent with the hypothesis that the face, cranial base, and cranial vault are completely independent modules but relatively strongly integrated structures. These results indicate pervasive integration in the human skull and suggest a reinterpretation of the selective scenario for human evolution where the origin of any one of the derived characters may have facilitated the evolution of the others.

What does this have to do with consciousness? Maybe it has nothing to do with it, but it does remove the need for specific causes for every difference between human and other great ape brain proportions.

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