Thick Tooth Enamel Sheds Light on Human Evolution

Thick tooth enamel is part of what makes us human. Along with our big brains and upright posture, it's part of what distinguishes us from our primate relatives. But why did our teeth evolve like they did? A new study sheds light on how evolution shaped our teeth, one gene at a time.

Teeth are an invaluable resource for scientists who study evolution. They show what species ate at the time, and remain within the fossil record.

"The fossil record is always the most complete for teeth," said Christine Wall, one of the researchers, in a news release. "And enamel thickness has long been a key trait used to diagnose fossil hominins and reconstruct their diets and life histories."

In fact, differences in enamel thickness among primates have been linked to diet. Fruit and leaf-loving gorillas and chimps have the thinnest enamel while omnivorous orangutans, gibbons and rhesus macaques have an intermediate thickness.

"Teeth also preserve their growth bands," said Wall in a news release. "So in terms of understanding fossils, teeth can tell you how old a juvenile was when it died, or how long it takes for teeth to develop-so you can compare between living and extinct species."

In this case, the researchers identified some of the genetic changes that contributed to humans acquiring thicker enamel. More specifically, they examined four genes that have a known role in tooth development, called enamelysin, amelogenin, ameloblastin and enamelin. The scientists then looked at the sequences of the four genes across six species.

Previous research showed positive selection on one of the genes, called MMP20, also known as enamelysin. This latest research confirmed that MMP20 shows the distinct signature of natural selection acting on tooth enamel thickness in humans.

"This study provides the important bridges between morphology, developmental processes, and their underlying genetic regulating mechanisms," said Timothy Bromage, one of the researchers, in a news release. "Already the results of the reported work are whittling away the many layers of regulation and evolution of enamel structure."

The findings are published in the Journal of Human Evolution.