Make no bones about it - scientists discover how human skulls really evolved
Anthropologists have long believed that any changes to human skull shape over time occurred independently of each other, but new research from an international team of researchers shows that this might not in fact be the case.
After examining a unique collection of 390 skulls housed in the famous Hallstatt Catholic Church Ossuary in Austria, the team, made up of scientists from Spain, Sweden, the United Kingdom and the United States, discovered that changes previously thought to have occurred after separate evolutionary events might actually be linked. This means that the skull is actually highly integrated, and that variation in one part is connected to variation in several parts of the skull.
'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, one of the study authors from Manchester University in the United Kingdom.
The skulls, which were all decorated according to local tradition, are part of a collection of over 700 items of skeletal remains. The remains are all decorated with painted flowers, leaves and crosses; on most of the skulls, the name of the deceased has been drawn on the forehead. The scientists cross-referenced the names on the skulls with local registers of births, deaths and marriages so that they could reconstruct the genealogical relationships of the Hallstatt population from as far back as the 17th century. This meant they could make estimations about how much genes influence skull shape.
Writing in the journal Evolution, the team explain how it is genetically determined morphological integration that directs the evolution of skull shape in humans.
'In this type of evolutionary scenario, it would be different to change or alter one element without also altering the others,' explains one of the study authors, Miquel Hernàndez from the University of Barcelona in Spain. He elaborates: 'Traditionally, experts have studied how selection acts on a specific trait. In practice, however, the various traits are all interrelated. The key concept is morphological integration: if we change one of the elements in the shape of the skull, the overall structure also changes, and only those changes that follow the morphological pattern are favoured.'
The team used geometric morphometric and quantitative genetic methods to examine human skull shape, using the 3-dimensional coordinates of 29 anatomical 'landmarks' to create morphological maps and simulate a range of scenarios in which different key traits are selected during the evolutionary process of modern humans. Another study author, Neus Martínez-Abadías, also from the University of Barcelona, comments: 'One of the most innovative aspects of the study is the use of a methodology with which we can analyse the skull structure as a whole and quantify the impact of morphological integration. This means we are not obliged to study each trait separately as if evolution were a distributed process.'
Study author Mireia Esparza, also from the University of Barcelona, explains how their study supports arguments for thinking differently about modern human evolutionary scenarios: 'Evolution acts as an integrated process and specific traits never evolve independently. In the case of the skull, evolutionary changes have converged to this morphological pattern. Therefore, we cannot simplify things and study the selection response of single trait in isolation, since although it is likely to have been affected by the selective factor in question, it is also constrained by the factors affecting other parts of the skull.'
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Data Source Provider: Manchester University
Document Reference: Martínez-Abadías, N. et al. (2011) Pervasive genetic integration directs the evolution of human skull shape. Evolution DOI: 10.1111/j.1558-5646.2011.01496.x
Subject Index: Coordination, Cooperation; Scientific Research; Social Aspects