New paper in Evolution on hominin foraging behaviors

Fig_EnamelVol

Many hominin species are distinguished by their large teeth. Notably, the genus Paranthropus had enormous bunodont molars covered in thick enamel. The evolution of the suite of traits collectively referred to as ‘megadontia’ was doubtless in response to the foods that these animals were consuming. We have just published a paper in the journal Evolution [1, link] where we explore how such traits may have influenced the foraging behaviors of human ancestors using a process-based state-dependent foraging model. The primary assumption in our model is that foraging decisions depend on both the energetic state of the organism (is the organism hungry or not), as well as its enamel volume (what is the state of the organism’s teeth). The attached figure shows a cartoon depiction of the enamel layer on a hominin molar tooth, as well as the simple method we use to approximate enamel volume (which, in terms of total volume across all molar teeth is quite an accurate approximation!).  As we consume foods over our lives, our enamel slowly erodes away. Generally this enamel loss is unnoticeable, and sometimes it is disastrous (for example, if you chip or lose a tooth). If you are a hunter or gatherer living in the wild, such enamel loss will have an impact on what foods you can eat – particularly if the foods that are being sought after have different mechanical properties with different demands on enamel.

Interestingly, there has long been a ‘disquieting discrepancy’ between the morphology of hominin species and their molar microwear [2]. The patterns of scratches and pits on the surfaces of molar teeth can be used to reconstruct the types of foods the animal consumed prior to its death. Many hominins have a microwear fabric that suggest softer, pliable foods. However, their robust morphology suggests a diet based on harder foods, particularly for hominins with megadont traits. Moreover, analysis of stable isotope ratios among hominin species indicates a diet that is based on the consumption of C-4 photosynthetic foods [3]. These foods include tropical grasses and sedges, and tend to be more fracture-resistent, or ‘hard’.

We use our modeling framework to investigate these seemingly disparate conclusions. We find that the predicted fitness-maximizing foraging behaviors bring consilience to morphological, isotopic, and microwear-based lines of evidence. We also show that our model correctly predicts the observed range of isotopic values for hominin species such as Paranthropus robustus. Of course, it is just a model, and to quote a reviewer ‘hominins cannot eat theory’. We heartily agree, but suggest that such process-based models are well-suited for exploring the important physical and behavioral constraints that are expected to impact the actual foraging behaviors of organisms in the wild. We also explore how body size is expected to interact with energetic reserves and tooth wear, how this has an impact on when foods become preferred vs. fallback resources (eaten only in times of stress), and to what extent very tough but common foods such as tropical grasses are likely to be major components of diet. Despite much emphasis on grass leaves as likely hominin foods in recent literature [4,5], our model predictions suggest that grass leaves are unlikely to maximize fitness except in extreme circumstances.

Literature cited

1. Yeakel, J. D., Dominy, N. J., Koch, P. L., & Mangel, M. (2013). Functional morphology, stable isotopes, and human evolution: a model of consilience. Evolution. doi:10.1111/evo.12240      [link]

2. Ungar, P. S., Grine, F. E., & Teaford, M. F. (2008). Dental microwear and diet of the Plio-Pleistocene hominin Paranthropus boisei. PLoS ONE, 3(4), e2044.

3. Ungar, P. S., & Sponheimer, M. (2011). The diets of early hominins. Science, 334(6053), 190–193. doi:10.1126/science.1207701

4. Lee-Thorp, J. (2011). The demise of “Nutcracker Man”. Proceedings of the National Academy of Sciences of the USA, 108(23), 9319–9320. doi:10.1073/pnas.1105808108

5. Rabenold, D., & Pearson, O. M. (2011). Abrasive, silica phytoliths and the evolution of thick molar enamel in primates, with implications for the diet of Paranthropus boisei. PLoS ONE, 6(12), e28379. doi:10.1371/journal.pone.0028379.t002

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