Evolution of Nano- and Micro-Structure of Tooth Enamel in Response to Dietary Changes in the Last 18 Million Years

Enamel-like structure is common to all vertebrate animals, and has been evolving since it first appeared nearly 400 million years ago (Ma). Here, we focus on hominoids, including apes, hominins, and humans in the last 18 Ma, and observe variations of enamel nano- and micro-structure in response to dietary changes. Using PhotoEmission Electron Microscopy (PEEM)^1,2 and Polarization-dependent Imaging Contrast (PIC) mapping³, we examined enamel sections from 16 fossil, archaeological, and modern hominoid teeth representing 8 species. Our sample spans three major dietary shifts in human evolution: meat eating (2 Ma), advent of agriculture 10,000 years ago (ka), and the Industrial Revolution 200 years ago. The study includes specimens from modern and extinct apes, including nine Homo sapiens samples.
Key findings include:
A. At the microscale, we observe differences in rod/interrod size and packing pattern with a shift observed ~1.6 Ma.
B. At the nanoscale, we observe distinct crystal orientation structure intraspecies and interspecies and investigate whether co/misoriented enamel nanocrystals respond to masticatory demands.This research represents several firsts, including in-depth examination of crystal orientation evolution within a clade and evaluation of intra-rod crystal orientation in fossil hominins. Our findings provide crucial insights into the relationship between enamel nano- and micro-structure adaptations to dietary changes throughout hominoid evolution, offering a new perspective on the evolution of our lineage.

Funding: This project received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 101026776 and Turkana Basin Institute-Simons Foundation grant to DRG to MCOH; DOE grant DE-FG02-07ER15899 and NSF grant DMR-2220274 to PUPAG. PEEM and PIC mapping experiments were done at the Advanced Light Source, a U.S. DOE Office of Science User Facility under Contract no. DE-AC02-05CH11231.

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