Chew on This: Softer Diets of Preindustrial Dairy Farmers Influenced the Shape of Their Skull

Written by: Lauren Milligan Newmark, Ph.D. | Issue # 91 | 2019

The human family tree has an extinct genus that is remarkable for their massive jawbones, molars, and cranial crests (picture a bony mohawk). All of these anatomical features are proposed adaptations to the tough, fibrous diet of genus Paranthropus; hard and chewy diets require large chewing muscles, which in turn require larger jaw and cranial bones (and crests!) for points of attachment.

Although Paranthropus were an evolutionary side-branch and not directly related to modern humans, these same form-function principles should also apply to the skeleton of Homo sapiens. Over 40 years ago, the masticatory-functional hypothesis was proposed as a way of explaining variation in the shape and form of craniums and mandibles (which together are known as the skull) between early human farming populations and the foraging populations from which they descended. The hypothesis argues that as populations adopted agricultural practices and began to consume softer foods, such as grains and cheese, demands on mastication were reduced and skulls became less robust. Could softer diets bring about changes in the human skull just as tougher diets did in Paranthropus?

It seems like a straightforward hypothesis to test—take measurements on crania and jaws associated with agricultural and foraging populations and then compare the findings. But, as is usually the case with studying humans, it is much more complicated in practice. Diet is only one factor that influences skull size and shape; differences among human groups could also be the result of temperature or neutral evolutionary forces such as gene flow (migration) and genetic drift (small population size, isolation). To tease out these other factors requires a large sampling of skeletal material from across the globe; to date, studies have been small and mainly sampled from geographically local populations.

But a 2017 study [1] hoped to finally put the hypothesis to a rigorous test by assessing skull shape, form, and size on a global scale. Katz and colleagues selected 37 cranial and 23 mandibular landmarks and digitally recorded three-dimensional data from 559 crania and 534 mandibles, representing 25 groups of pre-industrial foragers and farmers from across the globe [1].

Here’s where things get a bit tricky—these landmark data represent the size and shape of an individual’s skull, which are phenotypes, or observable traits. Some phenotypes, like blood type, can be tied solely to genes (e.g., if you have O blood, you have two copies of the O gene). But many phenotypes, like those measured in this study, are the result of the interaction of genetic and environmental traits. To understand the degree of influence from diet, a non-genetic factor, Katz and colleagues applied a sophisticated quantitative genetics model to their 3D landmark data [1].

Each cranium and mandible was fit to one of three models, each with a different diet predictor: Milk (dairy), Mush (cereals such as rice or wheat), and Soft (all Milk and Mush populations). Dairy and cereals were selected because they represent the softest agricultural diets; if reduced mastication has an effect on the size, shape, or form of the cranium or jaw, it should be easiest to detect in populations with these diets. For each model, the team indicated whether the diet predictor was present or absent. Skeletal elements from foragers would have all three predictors listed as absent, whereas those of dairy farmers would have Milk and Soft listed as present. In addition to diet, each model also took into account the sex of the skeletal element and temperature; both factors have been demonstrated to influence human cranial and mandibular diversity [2].

Katz and colleagues found several specific differences in size, shape, and form between foragers and farmers, with the most noticeable changes in the Milk group: the ramus (which is the part of the mandible that projects upward, along the side of the face) was narrower, which suggests smaller masseter muscles; the point of attachment for the temporalis muscle (which covers the sides of the head by the temples) was lower down on the cranium, suggesting a smaller temporalis muscle; and mandibles were generally smaller in size and less robust. All signs pointed to a reduction in bite force, especially in dairy farmers, exactly as predicted by the masticatory-functional hypothesis [1].

When the results of this study were published in 2017, a string of popular science articles followed with titles implying that eating cheese and other dairy foods directly changed the shape of the human skull. The reality is that the study found a significant but small effect of diet, which is exactly what Katz and colleagues had predicted [1].

Because of migration (and the movement of genes), most human genetic variation is found within groups rather than between them. Variation in human skull shape and form is known to conform to this same pattern; this means that a cranium from a forager and a cranium from a farmer have the potential to be more similar to one another than crania from two farmers from the same population.

Add to that an evolutionary perspective on the transition to agriculture; even before the archaeological evidence for agriculture, human forager populations were cooking, cutting, grinding, and processing foods [1]. As such, the difference in chewing demands between foragers and farmers was probably not terribly drastic. Eating cheese and milk on its own didn’t change the shape of the human skull, but rather it is one category of human cultural practices that influenced the size, shape, and form of the human skull. That might not make for as catchy of a title, but it certainly makes for a more interesting story.

References

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