Diet and Dentition- A Chemical Analysis

With an anthropology and biochemistry double major, I have an interest in the application of chemical analysis for forensic anthropology. For my senior project in anthropology I will be using chemical analysis of bone and dental remains to try to determine the diet of the humans in past cultures. Through this chemical analysis I should be able to determine the main components of the diet of the human I will be analyzing.

The different diets that can be determined include primarily marine, meat, or plant diet. The diets that consist primarily of vegetation can further be separated into two types according to the photosynthetic pathway followed- plants which follow the Calvin-Benson, or C3, cycle, and plants that follow the Hatch-Slack, or C4, cycle. C3 plants are called such because they produce an intermediate molecule with 3 carbons during photosynthesis. These plants include trees, shrubs, flowering plants and temperate zone grasses. C4 plants produce an intermediate molecule with 4 carbons. There are about 10 families, these include tropical grasses, maize, cane sugar, and some types of millet, sorghums, amaranths and chenopods. The carbon plants use in photosynthesis is taken in from atmospheric carbon dioxide. It is therefore important to note whether the remains are from a time before the 1850’s because the atmospheric carbon dioxide content was relatively stable until the industrial revolution, in the 1850’s, when it started to steadily increase.

In order to analyze the diets of the past, we must first know what to look for. When using bone and dental samples we tend to look at the stable isotope ratios of carbon and nitrogen. This is especially useful when working with collagen from bones. Collagen is an organic component of the bone which is generally well preserved in sufficient quantities. Collagen extract is generally known as gelatin. The one possible downfall of using collagen for analysis is that in life it takes up to ten years to replace. This means any short term or seasonal changes in diet will not show up, however this can also prove to be useful if the study is aimed at an average diet and not a seasonal diet.

Atmospheric carbon dioxide has a stable ratio of carbon-12 and carbon-13, therefore plants have a stable ratio, which is incorporated into plant tissue. These plants are then digested by animals, human or otherwise, a percent of this is then used by the animal for tissue. If an animal, such as a human, eats an herbivore they will then incorporate some of the C-12 and C-13 into their own tissue, giving another ratio. Carbon ratios for samples are obtained through the use of a mass spectrometer. This is a mechanical chemical analyzer. When using the mass spectrometer the sample material must be tuned into its gaseous form. In order to do this the sample usually collagen, is heated at very high temperatures- often 900 degrees Celsius. The sample gas is then injected into the mass spectrometer where the elements that make up the gas are separated according to density. They are analyzed in the mass spectrometer by nucleic mass, therefore isotopes can be separated and the number of each one can be compared, giving a ratio.

The ratios for the samples are then compared to a standard using this mathematical equation:

The results are expressed in parts per thousand. The more negative a result indicates a lower amount of carbon- 13 in the sample. As this graph shows, through experimentation we have found average ratios for diets of C3 plants only, meat from herbivores on C3 plant diets, C4 plants only, meat from C4 herbivores, marine plankton only, meat from marine herbivores and, finally, meat from marine carnivores.

Using such information we can determine if the diet of a human was more likely one of these or if it was a combination of these. For instance, if the carbon ratio for a human was found to be in between C3 diet and the diet of meat from C3 herbivores, the most logical would be that the human was an omnivore.

Nitrogen isotopes are used in much the same way as carbon isotopes. Nitrogen is taken up by plants through the nitrates and nitrites in the soil, as opposed to the atmospheric source of carbon. The ratio of nitrogen- 15 and nitrogen- 14 of the sample is measured using a mass spectrometer. This is then compared to the standard using the same mathematical equation as carbon isotopes. One of the first applications of nitrogen isotopes was in identifying legumes in terrestrial diets. Nitrogen was useful because legumes have a lower nitrogen-14 to nitrogen-15 ratio due to nitrogen fixation. Nitrogen fixation by legumes do not have to rely on the soil for nitrogen, therefore they will receive a lower ratio of isotopes. The ratio of nitrogen isotopes is also used to determine terrestrial or marine protein. In studies done on historic and prehistoric populations there was a significantly higher nitrogen ratio in people with a primarily marine diet. Not only can nitrogen isotopes help to determine marine or terrestrial diets, it can help distinguish terrestrial carnivorous or herbivore diets. Terrestrial carnivores average a higher nitrogen ratio than terrestrial herbivores. Studying nitrogen isotope ratios can help identify some major aspects of diet. However, one aspect of diet this method cannot distinguish between as readily is terrestrial and freshwater diet.

Unfortunately, there are still some questions and problems with determining diet using stable isotopes. Because this is a relatively new approach, there are no set standards on the technique, such as heating time or temperature of the sample before mass spectrography, or storage of samples. Long term storage of non-gaseous samples is also important, improper storage could introduce contaminants such as bacteria or mold. There has only been limited work on standards of carbon from collagen, this means that individual labs may be using their own standards. This can cause problems in that each lab, which tests the same sample, can obtain a different ratio due to different standards. When working with nitrogen isotopes, one aspect that researchers have to remember is that there can be different levels of nitrates and nitrites in soil in different areas. If the soil level is not tested, the data may be skewed.

Scientists are also looking at using tooth remains by identifying phytoliths, silica impressions of plant cells which can be found on teeth. However, this technique is still in its infancy.

Although chemical analysis of remains to determine diet is a relatively new technique, it is quickly becoming useful in the field of anthropology. Since it’s beginning in the 1970’s it has helped in proposing a tentative timeline for the development of certain types of agriculture, specifically maize, and proposing diets of past cultures. Due to some of the problems in the young field, it would be safest to use archaeological evidence, such as food remains, in conjunction with the chemical analysis. It would also be best to use carbon and nitrogen isotopes in conjunction with each other- this could limit uncertainties because the analysis would overlap in some areas. Once this technique is perfected, we may be able to discern much knowledge from a single piece of bone.

Bibliography

Bower, B. Tooth analysis May Decipher Prehistoric Diets

Ed. Price, T. D. The chemistry of prehistoric Human Bone University Press, Cambridge 1989.

Schoeninger, M.J., De Niro, M.J. Stable Nitrogen Isotope Ratios of bone Collagen Reflect Marine and Terrestrial Components of Prehistoric Human Diet Science Vol 220, No 4640, June 24, 1983.

Sillen, A., Sealy, J.C., van der Merwe, N. Chemistry and Paleodietary Research: No More Easy Answers American Antiquity Vol 54, No 3, 1989.

Note: Subjects for the project have been changed from humans to animals.

Special thanks to :

Matthew T. Millard

Tisha Strachan

Author: Dena Connors-Millard