Getting to the root of enamel evolution

May 05, 2014
This shows the skulls of a human, a gorilla and a macaque -- three of the species in which researchers looked at the genomics of enamel evolution. Credit: Les Todd

Along with our big brains and upright posture, thick tooth enamel is one of the features that distinguishes our genus, Homo, from our primate relatives and forebears. A new study, published May 5 in the Journal of Human Evolution, offers insight into how evolution shaped our teeth, one gene at a time.

By comparing the human genome with those of five other primate species, a team of geneticists and evolutionary anthropologists at Duke University has identified two segments of DNA where natural selection may have acted to give modern humans their thick .

Teeth have been an invaluable resource for scientists who study evolution, the authors said.

"The is always the most complete for teeth," said coauthor Christine Wall, associate research professor of evolutionary anthropology at Duke. "And enamel thickness has long been a key trait used to diagnose fossil hominins and reconstruct their diets and life histories."

Clear differences in enamel thickness among primates have been linked to diet. Of the six species included in this study, 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.

"Teeth also preserve their growth bands," Wall said, referring to the way enamel is deposited in layers, like concentric tree rings. "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."

Teeth are common in the fossil record and full of information, making them an excellent candidate for combining fossil and genomic studies. Credit: Les Todd

All of this makes tooth enamel one of the few traits that's both found in the fossil record and amenable to genomic analyses, Wall said.

The team set out to identify some of the genetic changes that contributed to humans acquiring thicker enamel. The work is part of a large-scale investigation of the links between genes, physical characteristics and diet during .

"We decided to look just at genes that have a known role in tooth development," said Greg Wray, professor of biology at Duke. The team chose four genes, each of which codes for a protein involved in tooth formation (enamelysin, amelogenin, ameloblastin and enamelin), making the genes good candidates for seeing evidence of positive selection, but not necessarily the only ones involved in tooth evolution, Wray said.

Publicly available data provided the sequences for the four genes across six species—except in the case of the gorilla and orangutan, whose DNA the team isolated themselves.

The researchers then fed the sequences to 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 in Raleigh, NC and research associate professor of biology at North Carolina Central University.

They used the concept of genetic drift to reach this conclusion. Drift is a phenomenon in which changes to the DNA sequence accumulate at an expected rate, Horvath said. When changes add up faster than expected, it suggests to scientists 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 thickness in humans. They 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.

"This study provides the important bridges between morphology, developmental processes, and their underlying genetic regulating mechanisms," said Timothy Bromage, professor of biomaterials and biomimetics at New York University, who was not involved with the study. "Already the results of the reported work are whittling away the many layers of regulation and evolution of enamel structure."

By connecting 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 are linked.

Explore further: Stiffness and hardness of sheep molar enamel is lower than that of humans

More information: "Genetic comparisons yield insights into the evolution of enamel thickness during human evolution." Julie Horvath, Gowri Ramachandran, et al. Journal of Human Evolution, Online May 5, 2014. DOI: 10.1016/j.jhevol.2014.01.005

add to favorites email to friend print save as pdf

Related Stories

New study explains evolution of duplicate genes

Apr 07, 2014

From time to time, living cells will accidently make an extra copy of a gene during the normal replication process. Throughout the history of life, evolution has molded some of these seemingly superfluous ...

Recommended for you

New feather findings get scientists in a flap

3 hours ago

Scientists from the University of Southampton have revealed that feather shafts are made of a multi-layered fibrous composite material, much like carbon fibre, which allows the feather to bend and twist to ...

User comments : 1

Adjust slider to filter visible comments by rank

Display comments: newest first

JVK
1 / 5 (2) May 06, 2014
Excerpt: "They used the concept of genetic drift to reach this conclusion."

My comment: Had they used experimental evidence of nutrient-dependent pheromone-controlled ecological adaptations to differences in nutrient availability, they might have concluded that a single nutrient-dependent base pair change led to differences in one-carbon metabolism, DNA methylation and amino acid substitutions that resulted in differences in teeth, hair, sweat, and mammary tissue. For example, that's what happened in a population of modern humans that appears to have arisen in what is now central China during the past ~30,000 years. See for review: Nutrient-dependent/pheromone-controlled adaptive evolution: a model http://www.socioa...53/27989

Apparently, the popularity of evolutionary theory (e.g., genetic drift) has again triumphed over experimental evidence. Natural selection somehow occurred for something else besides food.