Over nearly 18 million years of evolution, the tooth enamel of primates, including humans, has changed at the level of the crystalline molecules that make up this outer layer of teeth. The addition of meat and agricultural products to human diets led to changes in how individual nanocrystals in enamel orient to one another (Nature 2006, DOI: 10.1038/s41586-026-10583-8).
Enamel, the hardest tissue in vertebrates, is formed of tightly packed rods made up of tiny hydroxyapatite crystals, and interrods, the spaces between them.
Pupa Gilbert, a biophysicist at the University of Wisconsin–Madison, has previously shown how misorientation of individual crystals in these rods correlates with hardness in tooth enamels. She and colleagues have now explored how the nanoscale arrangement of these individual crystals changed in the teeth of humans and other primates over time.
To do this, the researchers compared 12 molar or premolar teeth from 9 species across different periods of time. The team found that the orientations of the crystals, or the angles between adjacent ones, changed with increasing food hardness. In humans, there were two main changes: the first was about 1.5 million–2.0 million years ago, corresponding to the transition to meat eating, and the other about 12,000 years ago, when agriculture introduced stone-ground grains in the diet. In nonhuman primates, those with diets consisting primarily of fruit have the least amount of misorientation, while species that consume seeds have the highest.
The enamel crystal nanostructure from six species, as human diet’s changed, so too did the nanostructure of our tooth enamel. Credit:
Emma Lagan and Pupa Gilbert
The researchers also found that a third major dietary shift that took place during the Industrial Revolution didn’t correspond with any nanolevel alteration in enamel structure. Mackie O’Hara, a coauthor of the new paper and an anthropologist at Ball State University, says one possible reason is that the Industrial Revolution is quite recent—“only about 200 years old.” With evolution in general, just because something can evolve doesn’t mean that it has to, she says. “Especially in the case of enamel, there are so many different hierarchical levels of the structure that any one of those levels could be affected due to any environmental condition.”
The researchers say their findings on the crystal misorientation in enamel’s nanostructure could be of interest to materials scientists developing bioinspired materials.
Nicholas Kotov, a materials engineer at the University of Michigan who wasn’t involved in this project, agrees. “This study is a great help for researchers designing complex multifunctional materials for virtually any technology,” he says. “The work highlights the need to balance order and disorder in order to satisfy multiple property requirements.”
CORRECTION:
This story was updated on June 10, 2026, to correct the description of the teeth the researchers compared. They compared 12 molar or premolar teeth from 9 species, not premolar teeth in 12 species.
