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Reserve this space Contribution of Stable Isotope Analysis to Understanding Dietary Variation among the Maya Robert H. Tykot Department of Anthropology, Univ. of South Florida, Tampa, FL 33620 Stable carbon and nitrogen isotope ratios in skeletal tissues are widely used as indicators of prehistoric human diets. This technique distinguishes between the consumption of C3 and C4 plants and assesses the contribution of aquatic resources to otherwise terrestrial diets. Isotopic ratios in bone collagen emphasize dietary protein; those in bone apatite and tooth enamel reflect the whole diet. Bone collagen and apatite represent average diet over the last several years of life, while tooth enamel represents diet during the age of crown formation. The isotopic analysis of all three tissues in individuals at Maya sites in Belize, Guatemala, Honduras and Mexico reveals variation in the importance of maize, a C4 plant, based on age, sex, status, and local ecological factors, as well as dramatic changes in subsis- tence patterns from the Preclassic to Postclassic periods. These results enable a tentative synthesis of the dynamic relationship between subsistence and sociopolitical developments in ancient Mesoamerica. Corn (maize) was the single most important New World crop at the time of European contact, and is widely considered to have been fundamentally important Reserve this space ACS Maya isotopes paper pdf format.wpd January 30, 2002 1 to the rise of indigenous civilizations including the Inca, Aztec, and Maya. Of the triumvirate of maize, beans and squash, there is evidence that at least squash (Cucurbita spp.) was domesticated by 10,000 years ago (1-2), although domesticated beans (Phaseolus spp.) appear a few thousand years ago at most (3), and maize was first domesticated somewhere in between. Other plants were also probably cultivated several thousand years prior to the introduction of ceramics and village settlements in several regions of Central and South America (4-5). It is the origins and subsequent importance of maize to these early civilizations that remains the focus of much archaeological and biological research. The focus of this paper will be on the Maya, the cultural group who occupied Belize, Guatemala, and the Yucatan peninsula of Mexico from about 1500 BC to 1500 AD. Using isotopic and other evidence of human diets, Maya subsistence practices will be examined in the context of developments and adaptations in other cultural and environmental areas of Mesoamerica, from its frontier in the highlands of northern Mexico to El Salvador and Honduras in the south. Maize is widely thought to have been domesticated in the Rio Balsas region of western Mexico (6), although the chronology of its origins and dispersal are less well understood. In the Tehuacán Valley, where maize cobs seemingly were associated with Coxcatlán phase (7000-5500 BP) levels, direct AMS dating on a dozen cobs produced dates no older than 4700 ± 110 BP (7-9). Two cobs from Guilá Naquitz have now been dated to 5400 BP and are the oldest maize macrofossils currently known (10), while maize pollen has been found in 6000 BP levels at San Andrés near the Gulf Coast of Tabasco (11), Pope et al. 2001), and maize phytoliths by about 5500 BP at the Aguadulce rockshelter in Panama (12-13). Even in well documented archaeological excavations, however, the context and dating of archaeobotanical specimens may be problematic (14), and the identifica- tion of small numbers and certain forms of maize phytoliths has also been argued by some to be unreliable (15-16). In any event, even after its initial domestication, there was most likely considerable temporal and spatial variability in the incorpora- tion of maize into existing subsistence patterns, and it is extremely difficult to measure its importance relative to other food resources since there is no way to balance the uneven representation and preservation of botanical and faunal remains in the archaeological record. The consumption of maize can, however, be quantified through stable isotope analysis of human skeletal remains (17-20). Principles of Stable Isotope Analysis Carbon and nitrogen isotope ratios in human bone (and other tissues, when they are preserved) may be used to reconstruct prehistoric diet because of differential fractionation, between certain plant groups, of atmospheric carbon dioxide during ACS Maya isotopes paper pdf format.wpd January 30, 2002 2 photosynthesis and of nitrogen during fixation or absorption. There are two stable isotopes each of carbon (12C, 13C) and nitrogen (14N, 15N), with 12C and 14N by far the most common in nature. Small differences in the ratios of these isotopes (12C/13C, 15N /14N) can be measured by isotope ratio mass spectrometry using samples smaller than 1 milligram. High precision isotope measurements are reported using the delta notation ( 13C, 15N) relative to the internationally recognized PDB and AIR standards and are expressed in parts per thousand or per mil (‰). Empirical and experimental data have indicated that different bone tissues reflect different components of the diet (21-22). In general, bone collagen is produced primarily from the protein portion of the diet, while bone apatite and tooth enamel are produced from a mixture of dietary protein, carbohydrates and fats. Stable isotope analysis of both bone collagen and apatite thus permits quantitative estimates of several dietary components. Both bone collagen and apatite are constantly being resorbed and replenished, so that their isotopic composition reflects dietary averages over at least the last several years of an individual's life. The apatite in tooth enamel, however, is not turned over, and represents diet at the time of tooth formation, often from a pre-weaning age. Typically, grasses originally native to hot, arid environments follow the C4 (Hatch-Slack) photosynthetic pathway, and will have 13C values averaging about -12‰; trees, shrubs, and grasses from temperate regions, which follow the C3 (Calvin-Benson) photosynthetic pathway, will have 13C values averaging about -26‰. Metabolic fractionation in their consumers results in bone collagen 13C values of about -7‰ and -21‰ and bone apatite 13C values of about 0‰ and -14‰, respectively, for pure C4 and pure C3 diets. Stable carbon isotope analysis is particularly useful in New World dietary studies since maize is often the only C4 plant contributing significantly to human diets; its contribution to bone collagen and to bone apatite may be estimated by interpolation. Some caution is warranted, however, if succulent plants were present, since they utilize the alternative CAM (crassulacean acid metabolism) photosynthetic pathway which results in carbon isotope ratios similar to those of C4 plants. Nevertheless, CAM plants are unlikely to have been major sources of dietary protein, whether consumed directly or indirectly through herbivorous faunal intermediaries. While most plants C3 and C4 plants have similar carbon isotope ratios in most ecological settings, care must be taken in forested or other environments where atmospheric recycling can occur resulting in depleted carbon isotope ratios in plants and their consumers (23). Human diets which include forest animals, therefore, can significantly mask the consumption of maize or other C4 plants with enriched carbon isotope ratios if the values for these food resources have not been established. Carbon isotope ratios in C3 plants also are affected by water stress and ACS Maya isotopes paper pdf format.wpd January 30, 2002 3 thus correlate to altitude and other factors. This is an important factor to consider in establishing the isotopic baseline for C3 plant foods; C4 plants, with their photosynthetic adaptation to hot and arid environments, do not seem to vary to the same extent, so that a carbon isotope ratio of about -10‰ for maize may be used throughout Mesoamerica unless demonstrated otherwise. The carbon isotope ratios of marine and freshwater organisms are more variable, depending on local ecological circumstances, and often overlap with those of terrestrial plants and their consumers. These foods typically have much higher nitrogen isotope values, however, and their high protein content will contribute much more carbon to bone collagen than will plants foods which are only 10-20% protein. For all of these reasons, quantification of the contribution of maize or other plants to human diets is better accomplished through measurement of stable isotope ratios in bone apatite or tooth enamel, rather than bone collagen. The nitrogen isotope ratios for plants depend primarily on how they obtain their nitrogen - by symbiotic bacterial fixation of atmospheric nitrogen, or from soil nitrates - and these values are similarly passed along through the food chain accompanied by an approximately 2-3‰ positive shift for each trophic level. Human consumers of terrestrial plants and animals typically have 15N values in bone collagen of about 6-10‰ whereas dedicated consumers of freshwater or marine fish, and sea mammals, may have 15N values of 15-20‰ (24). Nitrogen isotope ratios also vary according to rainfall and other factors (25), and both carbon and nitrogen isotope ratios vary considerably among marine organisms (26). It is critical, therefore, to establish a site-relevant isotopic baseline for interpreting human skeletal data. Analyses of faunal remains provide a good estimate both of the animals themselves and the plants they consume. This is particularly important since in many areas while