Laser Ablation Technique Reveals Ancient Dietary Behaviors

Lauren I. Rugani

Paranthropus robustus, a hominin that existed ~1.8 million years ago, has been thought to have followed a fixed diet. Recently, researchers from the University of Colorado at Boulder, the University of Utah at Salt Lake City, Texas A&M University in College Station, Ohio State University at Columbus and the University of Bradford in the UK, have suggested that P. robustus did not adhere to a rigid eating regimen but, in fact, had both seasonally and annually varying diets. Using laser ablation stable isotope analysis of four P. robustus teeth, the team revealed these variations in eating habits.

Several laser ablation tracks can be seen on the enamel of a permanent tooth belonging to Paranthropus robustus. Perikymata (tooth growth increments) are visible as faint horizontal lines parallel to the ablation tracks.

Individual diets varied among foods such as fruits and leaves, which use the C₃ photosynthetic pathway, and savanna-based foods, such as grasses, which use the C₄ pathway. Because C₃ foods are known to have less ¹³C than C₄ foods, the researchers could use the ratio of isotopic carbon that was found in the tooth enamel to determine which food types dominated an individual’s diet during a given period. Seasonal and annual periods were estimated by counting perikymata, or tooth growth increments on the enamel, which are assumed to represent a period of seven days.

Laser ablation samples of the teeth were taken in 0.48-mm intervals along the growth axis (representing an average of 54 days) by 8.5-ms pulses from a 10.6-mm CO₂ laser. Between three and eight laser ablation events generated 10 to 30 nmol of cryofocused CO₂, which was then analyzed with continuous-flow gas chromatography/isotope-ratio mass spectrometry. Penetrating less than 1 mm of the enamel surface, laser ablation is less destructive than previous techniques, which required drilling wide, deep grooves along the growth axis.

The mean range of carbon isotopic variation was found to be extremely strong within the individual teeth, with two of the teeth demonstrating diets in which the amount of C₄ foods varied by about 40 percent or more. The researchers suggest that the large seasonal and annual variations could reveal information about climate-related consumption as well as migratory patterns between wooded areas and savannas.

Control analyses of other primates and of antelope allowed the researchers to compare dietary similarities among species that may have lived in similar habitats. Carbon isotope analysis showed that antelope at the same site as the hominin did not display the same dietary variability, thus demonstrating that fossilization was not a factor in the observed variability.

The technique also was performed on hair samples of chimpanzees that live in environments believed to be similar to those inhabited by early hominins. This study, too, showed no similarity in dietary variability, revealing differences between the ancient hominin and today’s primates.

Science, Nov. 10, 2006, pp. 980-981.