Dating El Gigante

点击翻译按钮获取中文摘要

National Science Foundation funds will generate an accurate radiocarbon chronology for El Gigante Cave, located in central Honduras. Stratified deposits there have produced extremely well-preserved samples of plant remains, along with animal bone, pottery, and other cultural material. Particularly abundant are macrofossils of maize, a staple crop which today contributes, directly or indirectly, about 21% of the calories ingested by humans worldwide. Despite its significance, the ancient selection processes that led to the emergence of maize is poorly understood. DNA testing shows that maize evolved from annual teosinte, a grass native to sub-tropical regions of Mexico, Guatemala, and Honduras. Still controversial is the chronology of maize evolution, mainly because only a handful of sites have abundant macrofossils - mainly dry caves in the Tehuacan and Oaxaca valleys of Mexico. Maize from Tehuacan was long ago dated to 5000 BC, and the Tehuacan cave chronology provided the general framework for the evolution of maize for almost three decades. This reconstruction abruptly fell apart in 1989 with the application of a new form of radiocarbon dating. Standard radiocarbon methods required large samples of organic material. Tehuacan maize samples were too precious to destroy in the 1960s, so they were indirectly dated - i.e., by using carbon in assumed association from the same strata. Accelerator mass spectrometer (AMS) dating requires only minute samples of organic matter. Macrofossils of maize and other plants can thus be directly dated, eliminating the association assumption. Restudy of the old Tehuacan samples showed that maize was much more recent there than previously thought, and that stratigraphic associations could not be trusted. El Gigante's maize sample is comparable to that from Tehuacan, and includes fragments of other plants utilized by humans. It was mainly used between 2000 - 7000 years ago. The cave thus augments the meager roster of sites useful for investigating the origins of Mesoamerican plant domestication. During the next year some 30 samples of maize and other plant macrofossils from El Gigante, along with other cultural material such as a leather bag, will be dated by the AMS method. These dates will test the reliability of the stratigraphy, and hence form the framework for future studies of the El Gigante collections. If the stratigraphy can be trusted, a much better picture of human use of maize and other plant foods for a period of 5000 years or more will emerge. If, as at Tehuacan, stratigraphy is confused, we will have an unparalleled population of ancient maze from short interval of time - roughly 2000-2500 BC. One broad impact of the study will be to set a standard for adequate independent testing of the chronological implications of cave deposits. Another will be to focus attention on the southwestern margins of Mesoamerica, a region formerly lacking stratified deposits of early plant material that is essential to understanding the spread of early maize from its restricted homeland, and the larger issue of the spread of New World food production.