What's so interesting about this high level of variation? For one thing, it means that enamel thickness may evolve easily. If a trait does not vary within a population, natural selection is powerless to act on that trait: the population is stuck with whatever trait it's got even when faced with a changing environment (e.g., being stuck with a low heat tolerance in an environment with rising temperatures). However, when a trait varies a lot within a population, it can evolve quickly, since natural selection can easily weed out the variants that are less successful in the new circumstances (e.g., if a population had a lot of variation in temperature sensitivity and heat tolerance, it would be in a much better position to survive and evolve when faced with rising temperatures).
The speed with which enamel thickness can evolve in primates helps validate a method that paleontologists have been relying on for hundreds of years: using tooth shape and enamel thickness as indicators of diet and lifestyle. For example, we generally assume that a fossil mammal with thick enamel and rounded cusps used its teeth to grind hard objects, like seeds. But one could also imagine a situation in which a species' diet changed rapidly (e.g., a key food source went extinct and the species wound up relying on a new food source) but the evolution required to shape the species' teeth for the new food lagged behind in that case, there would be a mismatch between the fossil remains of the animal and its actual lifestyle. However, Leslea's findings downplay that possibility: natural selection can quickly mold enamel thickness to reflect diet and lifestyle changes, so there should be a tight match between the two.
This means that enamel thickness is a better indicator of current lifestyle than of evolutionary history. Traits that evolve slowly record evolutionary history and can help us figure out how different organisms are related to one another but traits that evolve rapidly often overwrite evidence of a species' evolutionary history and instead reflect local ecological pressures.
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Baboon tooth cross-section scan provided by Leslea Hlusko
Understanding Evolution © 2016 by The University of California Museum of Paleontology, Berkeley, and the Regents of the University of California