Researchers explore the evolution of enamel

Geneticists and evolutionary anthropologists at Duke University have identified two segments of DNA where natural selection may have acted to give modern humans their thick tooth enamel - a feature which distinguishes the genus Homo from our primate relatives and forebears. Their study has been published in the Journal of Human Evolution.

Clear differences in enamel thickness among primates have been linked to diet. When the researchers compared humans with five other primate species, they found that fruit- and leaf-loving gorillas and chimpanzees have the thinnest enamel; omnivorous orangutans, gibbons and rhesus macaques have an intermediate thickness; and humans possess the thickest enamel, well suited to crushing tough foods.

The skulls of a human, a gorilla and a macaque - three of the species in which researchers looked at the genomics of enamel evolution. Photo Credit: Les Todd, Duke Photography.

The team set out to identify some of the genetic changes that contributed to humans acquiring thicker enamel. They chose four genes - enamelysin, amelogenin, ameloblastin and enamelin - each of which codes for a protein involved in tooth formation. The researchers then acquired the sequences for the four genes across the six species and fed them into a software program that pinpointed which base pairs had changed between the species and where changes had accumulated at an accelerated rate.

“That’s when we know a gene is under positive selection,” said first author Julie Horvath, director of the genomics and microbiology lab at the Nature Research Center and Research Associate Professor of Biology at North Carolina Central University. They used the concept of genetic drift to reach this conclusion, Horvath said - a phenomenon in which changes to the DNA sequence accumulate at an expected rate. When changes add up faster than expected, it suggests that the affected genes are under positive selection - that they give organisms some kind of advantage.

Previous research had shown positive selection on one of the genes, called MMP20, also known as enamelysin. The present analysis confirmed that MMP20 shows the distinct signature of natural selection acting on tooth enamel thickness in humans. The researchers also found another gene, called ENAM or enamelin, which is under positive selection.

Selection pressure did not affect ENAM and MMP20 in the protein-coding region, where even slight changes can dramatically alter or destroy a gene’s functionality. Instead, ENAM and MMP20 showed positive selection changes in their regulatory regions, a sequence slightly upstream or downstream in the DNA that controls how a gene is transcribed.

By connecting genes and fossils across species - and in the future, across different age groups - the team hopes to build a roadmap for untangling how the many pieces of natural selection are linked.

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