They were what they ate

We are what we eat, so the saying goes, and that was as true thousands of years ago as it is today. Fortunately for bioarchaeologists, the type of diet eaten by an individual leaves an identifiable chemical ‘signature’ in skeletal and other remains. At the University of Oxford, Tamsin O’Connell is helping to decipher these signatures and add more to their meaning. Such work will shed important light on the lives and lifestyles of our distant ancestors.

An understanding of ancient diet can provide an archaeologist with a host of information about food, demography, economy and environment. Dietary analysis can add to information gained from techniques such as carbon dating of archaeological artifacts. For example, an excavated spear found near bones at an archaeological dig is evidence of a hunter/gatherer lifestyle but says little about what they hunted or gathered.

For that, bioarchaeologists such as Dr O’Connell focus their attention on bone, which can reveal a surprisingly large amount of information about ancient diet. The key technique is that of isotope analysis. Common elements such as carbon and nitrogen exist in different isotopic forms, which are present at stable ratios within the environment. Within living organisms, however, the isotopes are processed and integrated into body tissues in subtly different ways. As a result, the isotope ratios within different plants and animals tend to deviate from environmental norms in distinctive ways. Analysis of these ratios in tissues can thus provide information about diet, position in the food chain and other factors. In archaeological specimens, therefore, isotope analysis can provide clues to ancient dietary habits.

At the Research Laboratories for Archaeology in Oxford, Dr O’Connell is refining and extending these techniques. On a Wellcome Trust Bioarchaeology Fellowship, she has been exploring further the relationship between diet and isotope ratios in living human tissue, using isotope ratio mass spectrometry. A better understanding of the links between diet and tissue isotope ratios in modern peoples will provide key baseline data for work on historical specimens.

Elementary research

Analysis of carbon isotope values can tell us something about which plants an animal has eaten – specifically whether they consumed C3 or C4 plants, which differ in their internal biochemical pathways. Nitrogen analysis can also be enlightening: interestingly, the nitrogen isotope ratio changes in discrete steps up the food chain. In fact, the nitrogen enrichment factor appears to be the same regardless of the transition within the food chain, be it from grass to cow or cow to human or whatever. "The increased nitrogen isotopic value occurs as a result of the body retaining the heavier nitrogen-15 isotope over the lighter nitrogen-14," explains Dr O’Connell. "But we don’t know where it happens in the body." One possibility is linked to nitrogen excretion. "Urea in mammals is isotopically ‘light’, which implies that either the lighter nitrogen-14 isotope becomes used for urea production preferentially or maybe the heavier nitrogen-15 is used to make body tissues preferentially." These possibilities are currently being explored.

To gather information about nitrogen and carbon isotope ratios in archaeological bone samples, researchers generally extract bone collagen. But, points out Dr O’Connell, this provides only a partial picture. "Analysis of bone collagen alone does not represent the whole diet – only how dietary protein becomes body protein. Fat and carbohydrate intake for energy is unaccounted for." To get a handle on these constituents, explains Dr O’Connell, bone carbonate also needs to be measured. "Carbonate molecules within bone mineral, which is the interwoven substance within the protein matrix of bone, are derived from waste carbon dioxide produced when the body oxidises fats and carbohydrates for energy."

But such measurements have to be interpreted with caution: "Bone mineral samples are prone to contamination by water, which dissolves away bone carbonate to deposit atmospheric carbonate to leave an inaccurate result that bears no relation to the intended dietary analysis." Nevertheless, considered analysis of collagen and carbonate in archaeological bone can together help researchers piece together an ancient diet.

Global cuisine

To aid interpretation of ancient material, Dr O’Connell is collecting information about modern diets and tissue isotope ratios. However, work on living people presents certain difficulties: "Understandably people are somewhat reluctant to surrender a chunk of their femur," she points out. The alternative is to use what she describes as an ‘isotopic proxy’ – a protein made by the body that is similar to collagen, is easily sampled, yet still has a measurable isotope value.

"The simple answer is hair. Samples are easy to collect and they don’t degrade or need freezing." Moreover, hair has an added advantage: "Because hair grows linearly without resorption, analysis from scalp to tip details the changes in diet over time, unlike bone, which represents an average of the diet over its ten-year turnover." And while hair is an excellent proxy for collagen, exhaled carbon dioxide can be used in place of bone carbonate.

Dr O’Connell’s first task was to validate the use of keratin – hair protein – as an isotopic proxy for bone collagen. "I obtained bone samples from living humans with the help of Professor Hamish Simpson at the Nuffield Orthopaedic Centre in Oxford. He was undertaking research using bone samples from his patients undergoing hip and knee replacement surgery. I also had patient approval to take hair samples and their dietary details." In fact, the isotope values for hair keratin and bone collagen consistently differed, and even varied between hair keratin and nail keratin from the same individual. Nevertheless, says Dr O’Connell, there is a sufficiently strong relationship between the two for keratin to be used as a proxy for bone collagen.

But are differences in isotope values for bone collagen and bone carbonate actually caused by differences in diet? Could digestion have a significant role, as in the so-called ‘flatulence hypothesis’ which suggests that preferential isotope use in methane production will distort the observed ratios. To investigate this issue, Dr O’Connell set up a trial with hens, feeding them a diet of either corned beef, wheat, or an equal mixture of both, before measuring carbon isotope ratios in their egg shells (equivalent to bone carbonate) and egg whites (equivalent to bone collagen). The results suggest that diet does strongly influence composition, although other factors are also involved.

As well as hens, Dr O’Connell is studying the diets of different human populations, concentrating on indigenous non-westernised populations that still tend to eat traditional diets cultivated under traditional methods. "So far samples have been collected from Fiji, where a Polynesian diet includes lots of fish, some meat and sugar cane; Vietnam, where they eat plenty of fish and some rice; Argentina; Sweden, where the Lapps eat lots of meat; and The Gambia, where they eat some meat and fish but mostly rice and cereals."

In all the cases, Dr O’Connell typically travels to remote parts of the world armed with a device to collect exhaled breath, forms to gather information about diet, and a pair of scissors for hair samples. Though simple in practice, unexpected difficulties can arise. Having travelled to the heart of Africa, she discovered to her dismay that the local fashion was for very short haircuts. And because of the dangers of decompression in the cargo hold, she has to stuff all her breath samples in her hand luggage – no doubt an odd sight for custom officers unused to seeing large collections of Vietnamese breath samples.

See also

• Bioarchaeology Programme: Scheme details
• Artifacts and isotopes: Article describing modern technology in bioarchaeology
• Bone idol: Article describing bioarchaeology research on ‘fossil’ cells embedded in living animals

Further reading

O’Connell T C, Hedges R E M (1999). Investigations into the effect of diet on modern human hair isotopic values. American Journal of Physical Anthropology 108: 409–425.

O’Connell T C, Hedges R E M (1999). Isotopic comparison of hair and bone: archaeological analyses. Journal of Archaeological Science 26: 661–665.

Macko S A, Engel M H, Andrusevich A, Lubec G, O’Connell T C, Hedges R E M (1999). Documenting the diet in ancient human populations through isotope analysis of hair. Philosophical Transactions of the Royal Society, London B354 (1379): 65–76.

O’Connell T C, Levine M A, Hedges R E M (in press 2000). ‘The importance of fish in the diet of central Eurasian peoples from the Mesolithic to the Early Iron Age’ in Late Prehistoric Exploitation of the Eurasian Steppe, (Levine M A, Renfrew C, eds), Cambridge: McDonald Institute Monograph.