Dental Landscapes Reveal Clues To Dietary Evolution

FAYETTEVILLE, Ark. — Pioneering a new technique that uses Geographic Information Systems to examine the features of teeth, University of Arkansas researchers have brought a technological leap to the study of dietary evolution. Their method, which uses GIS to measure and model dental surfaces as if they were landscapes, offers new insight into how teeth function as they’re worn down and how that wear affects the ability to process foods.

Peter Ungar, associate professor of anthropology, and graduate student Francis Kirera describe their technique and findings in an article titled, "A Solution to the Worn Tooth Conundrum in Primate Functional Anatomy." The article will appear in the April 1 issue of Proceedings of the National Academy of Sciences.

Because of their durability and their direct physical interaction with foods, fossil teeth are the best clue scientists have to the diets of human ancestors. Analysis of dental features such as shape and angularity can indicate the types of foods that organisms were best able to process.

"Unfortunately, traditional methods of dental analysis rely on measurements taken from unworn tooth specimens, and you just don’t find those very often in the fossil record," Ungar said.

This latest article is the culmination of five years of research, in which Ungar and his students have attempted to determine whether teeth maintain their mechanical efficiency despite wear. If so, researchers could begin using worn teeth from the fossil record to infer the diets of early primates and human ancestors. Ungar and Kirera proved that, indeed, teeth maintain their mechanical function even as their physical properties change with wear. In addition, they developed a systematic, quantifiable method for analyzing the features of teeth in varying stages of wear.

Using resin casts of chimpanzee and gorilla molars, the researchers scanned each tooth with a laser that recorded point by point elevation data across the tooth surface. They then used GIS technology to compile the data into 3-D models of the tooth.

Just as GIS can be used to measure topographical features on a landscape, such as slope, elevation and aspect, Ungar and Kirera found it could analyze dental features that are important to chewing and processing foods. In particular, the researchers used GIS to calculate slope and angularity values for each tooth. As explained in their article, slope refers to the steepness of a tooth cusp, whereas angularity is a measure of overall jaggedness.

For their study, Ungar and Kirera chose particular subspecies of gorilla and chimpanzee that maintain similar diets, with a high intake of soft fruits. However, differences in tooth morphology enable each species to process slightly different foods at times when soft fruits become scarce. Unworn gorilla teeth show steeper peaks and more angular cusps, which allow them to chew fibrous leaves when necessary. Unworn chimpanzee teeth show more gradual slope and less angularity, suitable for mashing fruits and nuts.

Ungar and Kirera expected these morphological differences to converge as tooth wear softened peaks and wore away cusps throughout the lifetime of the animals. But measurements from the GIS analysis showed that, even while cusps wear down, teeth maintain their original values of slope and angularity.

Teeth are coated in relatively thin layer of hard enamel. As wear occurs, the softer, underlying dentin is exposed, Ungar explained. "What we saw through the wear process was that the dentin eroded at a faster rate so that cusps on the teeth rapidly became pits. The sharp angles and steep slopes that had originally existed on the outside of the cusps recreated themselves on the inside of those pits," he said. "Effectively, the teeth changed shape but kept their functional properties and morphological differences."

That means that subtle variations in the diets of different species can be maintained throughout a lifetime, regardless of tooth wear — an important discovery for a field that attempts to infer the eating habits of long-extinct species.

In addition, by developing an objective method for analyzing dental properties that uses widely available technology, Ungar and Kirera have introduced a valuable new tool for the study of physical and dietary changes throughout evolution. In an invited commentary that will run alongside the Ungar and Kirera article, Mark Teaford of the Center for Functional Anatomy and Evolution at the Johns Hopkins School of Medicine states that the UA researchers have "changed tooth analyses forever," essentially catapulting the field into the 21st century.

Ungar points out that the laser and GIS modeling technique can provide a variety of measurements in addition to slope and angularity: surface area, relief, topographic aspect, basin volume and drainage patterns — all of which can be useful in correlating dental morphology to diet. Further, Ungar and Kirera’s research has proven that such correlations can be made even on worn tooth samples.

"With this method, we can look at an entire tooth surface, not just specific points and measurements, and that gives us a whole new perspective," Ungar said. "It's not just about specific features anymore but how all the qualities of the tooth function in the processing of food and the determination of diet."