How has the human skull evolved?

The study is based on the analysis of 390 skulls, decorated according to local tradition, from the ossuary in Hallstatt (Austria), which houses an exceptionally valuable collection for anthropological research. The more than 700 items of skeletal remains are famous for their painted decoration, depicting flowers, leaves and crosses, with the name of the deceased printed on the forehead of most of the skulls. By cross-referencing with local registers of births, deaths and marriages, experts have been able to use the collection to reconstruct the genealogical relationships of the population from as far back as the 17^th century and make informed estimates of the influence of genes on skull shape.

A map with the coordinates of 29 anatomical landmarks

According to the results of the study, the evolutionary potential of the human skull is formally constrained by the relatively high morphological integration of the different skull structures. "In this type of evolutionary scenario, it would be different to change or alter one element without also altering the others," explains Miquel Hernàndez. "Traditionally, experts have studied how selection acts on a specific trait. In practice, however, the various traits are all inter-related. 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 researcher Neus Martínez-Abadías, first author of the study, explains: "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."

The experts applied geometric morphometric and quantitative genetic methods to examine human skull shape, using the three-dimensional coordinates of 29 anatomical 'landmarks' to create morphological maps and simulating a range of scenarios in which different key traits are selected during the evolutionary process of modern humans: the forward shift of the foramen magnum, flexion of the cranial base, retraction of the face and enlargement of the neurocranium. Although it was long believed that these traits evolve separately -- through selection for bipedalism, dietary changes and encephalization -- the results of this new study suggest that they are strongly integrated and that the developed of each trait may have favoured the evolution of the others.

Individual traits do not evolve independently

The study calls for a reinterpretation of modern human evolutionary scenarios. As the lecturer Mireia Esparza explains, "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."

In human genealogies, each trait evolves gradually but within a pattern of strong morphological integration built on powerful genetic foundations. "This functional, operational morphological pattern should not be considered a negative restriction. If we think about disorders affecting cranial development, for example, the pattern of integration could constrain the variation to keep it within the limits of a functional framework," concludes the researcher Neus Martínez-Abadías.

The study was conducted in association with the Catholic church of Hallstatt (Austria), the Wenner Gren Foundation for Anthropological Research (USA) and the Spanish Ministry of Education and Science.