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One of the most interesting challenges archaeologists face is reconstructing the diets of ancient peoples. Although bones and other animal remains can be preserved over thousands of years, most plant material decays quickly under normal conditions. Archaeologists occasionally find seeds and other small plant remains that have been charred by fire or otherwise preserved, but these are relatively rare, especially at sites that were not occupied for long periods of time. Thus, the importance of plants in the diets of early peoples has often been underestimated.

In their search for other lines of direct evidence for prehistoric plant use, archaeologists increasingly turn to opal phytoliths. Phytoliths are actually plant microfossils composed of silica. When a plant draws nutrients up through its roots, it also takes in amounts of monosilicic acid. Although the plant can use other nutrients, silica cannot be absorbed and instead is deposited in between cells. When the plant dies (or is cooked), these mineral deposits survive intact, potentially for several million years, while the surrounding organic portions of the plant quickly decay.

Although researchers have been aware of phytoliths for some time, it is only in the last 20 years that they have begun to study phytoliths intensively and use them to address archaeological problems. Researchers have determined that phytoliths are formed in a variety of three-dimensional shapes, such as conical, cylindrical, rectangular, or pyramidal. More importantly, different types of plants form particular shapes, or combinations of shapes. Therefore, by identifying individual phytolith shapes, or groups of shapes, one can identify the type of plant from which they came.

 
 



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Possible squash or gourd phytolith recovered from food residue on a ceramic sherd excavated from 21AN106 in 1996 (magnified 900 times).


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Corn phytolith recovered from food residue on a ceramic sherd excavated from 21AN106 in 1996 (magnified 900 times).


 


Thus, by examining phytoliths recovered from archaeological sites, one often can determine what plant foods the site's inhabitants were cooking and eating. Food residue preserved on the inside of ceramic vessels used for cooking is an excellent source for phytoliths. By analyzing phytoliths present in the food residue, archaeologists have identified direct evidence for cooking of wild rice, corn, and other wild or cultivated grasses at a number of sites in Minnesota.

In addition, using accelerator mass spectrometry (AMS), a relatively new form of radiocarbon dating that requires only very small amounts of organic matter, archaeologists have been able to date the food residues, and therefore the phytoliths, as well as the archaeological sites from which they were excavated. As a result, archaeologists are beginning to develop a clearer picture not only of what plants prehistoric peoples used, but also when these plants first came to be used. In turn, this information on diet and how it changed over time provides a better understanding of how prehistoric peoples lived.

 
 


 

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Updated 29 Jun 1999