Natural History Highlight
Phytoliths: An Archaeobotanist's Best Friend
How can scientists tell us what ancient peoples ate? How do we know when humans began to cultivate and grow the grains they ate rather than gathering wild foods? What tell-tale signs might be left by the plants themselves that might give us clues to answer these questions?
Recent research and publications by Museum of Natural History and Smithsonian Tropical Research Center scientist Dolores Piperno highlight the growing importance of phytolith analysis in the study of ancient plant communities and the use of plants by prehistoric cultures. Piperno has just published a new review and synthesis of phytolith studies world wide called “Phytoliths: A Comprehensive Guide for Archaeologists and Paleoecologists” (AltaMira Press, Lanham, MD). Phytoliths are microscopic pieces of silica that form in the cells of many kinds of plants. They can be used to identify plant remains in archaeological and other ancient contexts, including deposits from underneath lakes that record shifts in vegetation and climate through time. Because phytoliths are like pieces of hard glass, they remain well-preserved through very long periods of time once plants die and decay. Phytoliths were first used in archaeology by Piperno and others for documenting prehistoric agriculture in tropical zones, where other types of plant fossils had long-decayed from the humid climate. Now phytoliths are being applied around the world in all types of environments. Piperno’s work with phytoliths from an archaeological site located in the southern Peruvian Andes published recently in the journal Nature (with SI colleague Linda Perry, who studied microscopic starch grains from the site) helped to demonstrate for the first time that phytolith data can be very useful in tracking plant domestication in cold mountainous zones. In addition, a team of European researchers recently reported the oldest known phytoliths from the dung of dinosaurs that lived in India 70 million years ago.
Piperno is currently working in tropical Mexico studying pre-Columbian human occupations and agriculture in a little-studied and now-remote region where molecular research on modern varieties of wild maize indicates maize was probably domesticated. Maize or Indian corn was the most important crop grown in the Americas before Europeans arrived, but not much is known about its earliest history in its Mexican homeland. Piperno and her team of researchers from Temple University and the Smithsonian Tropical Research Institute have found sites dating to 9000 years ago that they hope will yield records of maize use and early domestication. Phytoliths and other plant microfossils such as starch grains may prove to be critical links in this story. Piperno’s team is also studying the history of the tropical forest that grew in this region before humans cut it for agriculture.
Piperno’s election to the National Academy of Sciences
Recently elected to the National Academy of Sciences, Dolores Piperno (STRI/NMNH) was recognized for her pioneering work in the development of new approaches to documenting plant use and domestication in the archaeological record of tropical forest regions. Piperno has been a world leader in identifying two new categories of archaeological evidence for ancient plants in tropical forests: opal phytoliths - microscopic silica fragments produced by plants that are both taxonomically distinctive and impervious to decay in tropical soils; and starch grains, which are sometimes remarkably preserved in small crevices in grinding slabs and other ancient tools, providing evidence of ancient use of otherwise rarely documented root crops. Piperno has employed these two new classes of evidence to document the initial cultivation of a number of crop plants in the New World tropical regions. Her approaches and methods have been adopted by a new generation of researchers studying agricultural origins world-wide. Before Piperno’s work, there was little archaeological evidence ever recovered of ancient plant remains in tropical forest soils due to their poor preservation in such environments.
Phytoliths from maize leaves. Arrows point to inclusions of carbon from the cells in which the phytoliths formed that we use to radiocarbon date phytoliths directly and determine precise ages for them. (Click for higher resolution)
Diagnostic phytoliths from maize cobs, showing how phytoliths from different plant structures can be identified. (Click for higher resolution)
Phytoliths from the domesticated squash species Cucurbita moschata. (Click for higher resolution)
Phytoliths from the domesticated squash species Cucurbita maxima, showing how phytolith diversity in squashes is considerable and sometimes species-specific. (Click for higher resolution)
Species-specific phytoliths from Lagenaria siceraria (bottle gourd). (Click for higher resolution)
Diagnostic phytoliths from important neotropical tree species. (Click for higher resolution)
Diagnostic phytoliths from the seed covering of sedges, showing how important herbaceous plants leave phytolith records. (Click for higher resolution.)
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