The emergence of agriculture as the predominant means by which people obtain food resources is one of the most significant economic shifts in human evolutionary history, and its biological consequences are a frequent subject in evolutionary anthropology. The history of this subsistence transition is complex, with agriculture emerging by independent invention in some parts of the world, and by the spread of cultures and/or people in others. As a result, studies that test whether the biological consequences of agriculture are better explained by unique local ecologies and histories, or instead by factors common to agricultural subsistence, wherever adopted, are of particular interest.
This study by doctoral student David Katz (University of California, Davis), under the supervision of Dr. Timothy Weaver, provides a global analysis of human skull shape in order to test two hypotheses regarding the extent of agriculture's common consequences. First, the investigators test the claim that because the diets of early farmers were universally softer and more heavily processed than those of their hunter-gatherer predecessors, the shift from hunting and gathering to agriculture produced common shape changes - mostly related to reductions in size - in the chewing architecture of early agriculturalists. Second, by studying shape changes in the mandible during growth and development, the investigators test whether differences in skull shape between hunter-gatherers and agriculturalists are more likely due to natural selection or skeletal plasticity - bone's ability, particularly during development, to change shape in response to the demands of lifetime behaviors.
The results of the study will directly inform public understanding of the biological consequences of the agricultural transition, and also shed light on the much longer-term relationship between technology, diet, and anatomy over the course of human evolutionary history. In addition, data from this study will be freely available on the internet for use as an educational resource.
This research assesses the extent to which modern human cranial and mandibular form evolved in response to dietary changes associated with the agricultural revolution, as well as whether information about the diets of human populations improves our understanding of global human cranial variation. The emergence of agriculture as the predominant means by which people obtain food resources is one of the most significant economic shifts in human evolutionary history. Physical anthropologists have long hypothesized that the shift from hunting and gathering to agriculture produced common shape changes in the chewing architecture of early agriculturalists because the diets of early farmers were softer and more heavily processed than those of their hunter-gatherer predecessors. At the same time, it is well-established that global human cranial variation reflects a signature of population history, consistent with patterns of neutral genetic variation among modern human groups. With rare exception, these research questions have been addressed separately: comparative analyses of hunter-gatherer and agriculturalist cranial form focus on populations from a particular geographic region, while studies of worldwide cranial variation do not model subsistence effects. Our study unites these lines of inquiry, situating subsistence-driven craniofacial evolution within the pattern of human cranial variation observed on a worldwide scale. We posed three research questions: (1) do models of global cranial variation that incorporate both subsistence and geography effects do a better job of explaining patterns of cranial variation than models which consider geographic relationships only, (2) do patterns of integration between cranial and mandibular shape vary with subsistence strategy, and (3) do mandibular growth trajectories vary with subsistence strategy in a manner which suggests whether subsistence effects are more likely the product of selection or phenotypic plasticity. Data collection commenced September 2012 and concluded December 2013. To model geographic and subsistence effects, we collected 3-dimensional shape data for more than 550 adult crania and mandibles from 25 hunter-gatherer and agriculturalist groups from 6 continents. Approximately 50% of these adult specimens are paired (crania and mandible derive from the same individual), and we address integration of the crania and mandible with these paired elements. Finally, in order to assess developmental trajectories, we collected landmark data on approximately 200 subadult mandibles from a subset of the populations studied. Data analysis is ongoing. Our visual inspection of shape change along major axes of variation (principal components analysis) indicates a small but relatively consistent difference between hunter-gatherers and agriculturalists from the same geographic region. Similarly, we observe a subsistence effect in patterns of integration between the cranium and mandible. However, our scientific conclusions await implementation of a model that successfully quantifies effect sizes and partitions shape variation among geographic effects, subsistence effects, and other factors of interest. We are in the process of developing such a model, grounded in quantitative genetic theory, based on phylogenetic extensions to the animal model. This project represents the dissertation training and work of the co-PI. Our results will directly inform public understanding of the biological consequences of the agricultural transition, and also shed light on the much longer-term relationship between technology, diet, and anatomy over the course of human evolutionary history. In addition, while phylogenetic applications of the animal model have become more common in recent years, the extension of the animal model to intraspecific comparative analysis is still in relatively early stages of development. We therefore anticipate that our analytic approach will be of substantial interest to evolutionary biology community in general. Finally, during data collection, the co-PI conducted comparisons of established 3D data collection methods (surface scans) with a relatively new, low-cost approach known as 3D photogrammetry. The results were promising, and produced a collaboration with Building a Community of Natural History Collections, an NSF-funded RCN, to develop best-practices protocols for the use of 3D photogrammetry as a means to create digital archives of the skeletal material curated in natural history collections for use in research, education, and outreach.
