The information presented here represents preliminary research as the result of ten-weeks of investigation in-residence at the National Museum of Natural History. This is not an official publication of the information.

As preliminary information, results and/or findings should not be cited as part of conclusive work. Please contact the authors first if you wish to utilize the information presented here.

Domestication: an introduction

Domestication, as a technological innovation, occurred in a limited set of places and times throughout the world. The process can be considered an economic strategy to lessen the risk of food procurement. It is defined as the human creation of a new plant or animal (Smith 1998). Therefore, there are identifiable markers that differentiate a domesticated species from its wild relatives. In the case of foodstuffs, the domestication process creates a more efficient product, with more consumable material available from fewer plants. For sunflowers, this means suppression of lateral flowers in favor of one large central head as well as increased seed size (Heiser, 1985).

The domestication process of the sunflower is revealed in the archaeological record by the increase in seed size over time. While some seeds are found in dry environments favorable for preservation, most are not. The majority of seeds recovered were preserved through carbonization, a process that allowed for their survival to the present day, but one that is also problematic. Heating and ensuing carbonization changes the morphology of the seed. It affects dimensions of the seed differently, so that some degree of reconstructive measure is necessary to determine the original size of the seed. These reconstructive compensations can only be attained though experimentation.

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Methods of Experimentation

The experiments completed during the RTP summer term focused on three variables believed to influence the morphological changes due to carbonization: temperature at which the seed is carbonized, time of exposure, and moisture content of the seed. All seeds were carbonized in a Fisher Scientific Isotemp® muffle furnace. This instrument allowed temperatures to be controlled in one degree increments. Time of exposure was recorded by electronic timer. Moisture content was measured as percent oil content, as given by USDA files. When all variables are taken into account, will seeds change shape in a predictable and systematic way? To test this, both seeds (sunflower fruit) and achenes (seeds plus their outer pericarp) were subjected to experiments based on the variables mentioned above.
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Additionally, seeds were heated in two different environments: oxygen rich and reducing (oxygen deficient). The purpose of the two environments was to explore the act of carbonization. When carbonized in antiquity, it is assumed that seeds fell to the bottom of a fire, underneath the burning wood coals that would effectively use all available oxygen. This allows a seed undergo thermal decomposition without ignition. However, is there a difference between the effects of the two environments? To answer this question, a ceramic reducing furnace was made to house samples inside the muffle furnace. The reducing furnace interior sandwiches the samples between two layers of powdered carbon, which acts as a reducing agent. Any oxygen in the reducing furnace is used by the carbon, effectively preventing ignition of the samples.

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Results

The graph above shows original and carbonized lengths of wild seeds carbonized in a reducing (oxygen-deficient) environment. One hypothesis, that seed shrinkage increases concurrently with original seed size, fails. Rather, the results seem anything but systematic. Samples ranked by oil content, another hypothesized shrinkage determinant, also yields little patterning at this time.

This is also exampled in the graph below, which shows shrinkage percentage of the wild seeds in a reducing environment. The data seem to follow no systematic pattern in particular, but rather seem dependent on the individual characteristics of each seed. However, the amount of shrinkage does seem dependent on the time for which a seed is heated. This conclusion will make contextual information of any recovered seed very important. For example, the seed could have been carbonized in a household fire that may have burned continuously for day, or in a processing station fire that lasted only a few hours.

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Past Studies

Since 1978, archaeologists have applied a set of static correction factors to all carbonized seeds found in archaeological contexts. Estimations of original seed size are obtained by adding 30% to the carbonized length and 45% to the width (Yarnell, 1978). Additionally, Yarnell cites personal notes of Heiser (1953) that claim achene length and width can be estimated at 11% and 27% greater than the carbonized measurement. As the results of experimentation show, applying a ubiquitous correction factor does not accurately estimate original sizes of carbonized seeds.

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Continuing Research

Although no new corrections for size can be reported at this time, it is important that the past standard has been proven insufficient. Continuing research will explore the mechanisms and process of carbonization. Melson and Potts (2002) report 300°C as the spontaneous ignition point of dry plant matter. Further testing will provide a similar critical point for carbonization of dry material in an oxygen deprived environment. The effect of heat exposure time will be further questioned and tested. Is there an upper limit to carbonization, at which point a seed will not shrink past? What are the differences between hours and days in a carbonizing environment? As these questions are answered, new tools for seed reconstruction will be disseminated to the archaeological community. With new reconstructive measurements, more accurate analysis of the domestication process of Helianthus annuus can be completed

Significance The study of plant domestication is an important issue in understanding the past, but is also critical in the health and productivity of modern species. By properly identifying ancestral sources of domesticated plants, botanists and agronomists today can maintain variation in populations, thereby adding to future health of the species. This is crucial for economically important crops such as the sunflower. Seeds are becoming increasingly known for health benefits, being sources of vitamin E and other phytochemicals.

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References and Acknowledgments

Heiser, Charles B. 1953 The Archaeological Record of the Cultivated Sunflower with Remarks Concerning the Origin of Indian Agriculture in Eastern North America. Manuscript. Files of the Author.

1985 Some botanical considerations of the early domesticated plants
north of Mexico. In Prehistoric Food Production of North America, edited by R. Ford, pp. 149-203. Museum of Anthropology, Anthropological Paper No. 75, University of Michigan, Ann Arbor.

Melson, William G. and Richard Potts 2002 Origin of Reddened and Melted Zones in Pleistocene Sediments of the Olorgesailie Basin, Southern Kenya Rift. Journal of Archaeological Science 29:307-316.

Smith, Bruce D. 1998 The Emergence of Agriculture. W.H. Freeman and Co., New York.

Yarnell, Richard A. 1978 Domestication of Sunflower and Sumpweed in Eastern North America. In The Nature and Status of Ethnobotany, edited by R. Ford, pp. 289-299. Museum of Anthropology, Anthropological Paper No. 67, University of Michigan, Ann Arbor.

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Acknowledgements

Thanks to Bruce Smith for his advising par excellence. Many thanks to Bill Melson, who shared clay, firing instructions, and scientific methods. Melinda Zeder provided assisted with data analysis and visual representation. Thanks also to the USDA-GRIN for providing samples, and to the Smithsonian Women’s Committee for the generous grant through the RTP.

This research was supported by the Smithsonian Women’s Committee

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