Dating El Gigante

美国国家科学基金会的资金将为位于洪都拉斯中部的吉甘特洞穴生成一个精确的放射性碳年表。那里的分层沉积物产生了保存极为完好的植物遗骸样本，以及动物骨骼、陶器和其他文化材料。玉米的大型化石尤其丰富，这是一种主要作物，今天直接或间接地为全世界人类摄入的卡路里贡献了约21%。尽管具有重要意义，但人们对导致玉米出现的古老选择过程知之甚少。DNA测试表明，玉米是从一年生大刍草进化而来的，大刍草是一种原产于墨西哥、危地马拉和洪都拉斯亚热带地区的草。玉米进化的年代仍然存在争议，主要是因为只有少数几个地点有丰富的宏观化石——主要是墨西哥特瓦坎和瓦哈卡山谷的干燥洞穴。特华坎的玉米可以追溯到公元前5000年，特华坎洞穴年表为近30年来玉米的进化提供了总体框架。1989年，随着一种新形式的放射性碳定年法的应用，这种重建突然瓦解了。标准的放射性碳方法需要大量的有机材料样本。特华坎的玉米样本太珍贵了，不能在20世纪60年代被破坏，所以它们被间接地测定了年代——也就是说，通过使用来自同一地层的假定关联的碳。加速器质谱仪（AMS）测年只需要微量的有机物样本。因此，玉米和其他植物的大型化石可以直接确定年代，从而消除了这种关联的假设。对老特华坎样品的重新研究表明，玉米在那里出现的时间比之前认为的要晚得多，地层关联也不可信。埃尔·吉甘特的玉米样本与特华坎的样本相当，其中包括人类利用的其他植物的碎片。它主要在2000 - 7000年前使用。因此，这个洞穴增加了对调查中美洲植物驯化起源有用的遗址名单。在接下来的一年里，大约30个来自埃尔吉甘特的玉米和其他植物大化石样本，以及其他文化材料，如一个皮包，将用AMS方法测定年代。这些日期将测试地层学的可靠性，从而形成未来研究El Gigante收集的框架。如果地层学是可信的，那么在5000年或更长的时间里，人类使用玉米和其他植物性食物的图景将会更加清晰。如果像在特华坎一样，地层学是混乱的，我们将拥有一个无与伦比的古迷宫种群，时间间隔很短，大约在公元前2000-2500年。这项研究的一个广泛影响将是为对洞穴沉积物的年代含义进行充分的独立测试设定一个标准。另一个将是把注意力集中在中美洲的西南边缘，这个地区以前缺乏早期植物材料的分层沉积物，这对于了解早期玉米从其有限的家乡传播开来以及新大陆粮食生产传播的更大问题至关重要。

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.