DURHAM, NORTH CAROLINA—Tooth enamel is found in the fossil record and it can yield genetic material, making it possible to study changes in genes and physical characteristics in the process of human evolution. Scientists at Duke University have identified two segments of DNA where natural selection may have acted to give modern humans their thick tooth enamel. They examined four genes that code for a protein involved in tooth formation of gorillas and chimpanzees, which have the thinnest enamel and eat fruit and leaves; omnivorous orangutans, gibbons, and rhesus macaques, whose teeth have an intermediate thickness of enamel; and modern humans, which can eat tough foods with their thick enamel. The team of geneticists and evolutionary anthropologists used computer software to compare how the sequences for the genes changed across the six primate species, and where those changes accumulated at an accelerated rate. “That’s when we know a gene is under positive selection,” project leader Julie Horvath of the Nature Research Center in Raleigh and North Carolina Central University told Science Daily. One gene, known as enamelysin, was confirmed to act on tooth enamel thickness in humans.