Opal phytoliths are one of several kinds of natural artifacts, including starches and pollen, that have been discovered within ancient plant residues and have been used to inform us about ancient diets.
The analysis of opal phytoliths has become, over the past thirty years or so, a workhorse of archaeological science. Its use in identifying plant species in archaeological contexts has been demonstrated time and time again in numerous sites, from numerous cultures and time periods. Phytolith analysis is an example of how inventive researchers in archaeology use the hard science processes of botany and geology to illuminate the soft-science interpretations of our human past.
What are Opal Phytoliths?
Silica, the mineral which makes up opal phytoliths, is the second most abundant element on the planet--the first being oxygen. Like most minerals, silica is found in both solid form (as in quartz or feldspar) and liquid form (called 'soluble silica'), and these forms are continually trying to become each other. Quartz is always being eroded down by water; and soluble silica is always being precipitated out of water.
Soluble silica is present in groundwater. Plants send their roots down into the earth and absorb groundwater; the soluble silica (along with a whole wealth of other minerals of course) comes with the water. The plants run the water through their cells, and the silica is deposited, depending on the species of plant, between the cells, within the cell walls, or even sometimes completely infilling the cells themselves. Essentially, the silica deposits can create three-dimensional copies of the cells: tiny, mineralized, carbon-copies--or rather silicon-copies--of the cell bodies.
These deposits don't harm the plants--in fact, the added density in the plant leaves may help keep them erect and out in the sunlight and better able to absorb the light and create chlorophyll. It may also help the plant fend off fungal infections; and there is evidence that the added density of the silica bodies makes the leaves less tasty to herbivores and chewing insects.
Opal Phytoliths and ArchaeologyOf most importance--well, of most importance to archaeologists, of course, not the plant--is that when the plant dies, the three-dimensional copies of the plant cells are left intact, and can remain intact and recognizable for long periods of time in most conditions--certainly as long as any archaeologist cares to think about. Rock dissolution is a very very slow process.
Silica bodies can be created in the cells of any part of any plant--the roots, the leaves, the stem, the fruit--and in any kinds of cells--hair follicles, skin and sub-skin cells, the works. Different species and families of plants deposit the silica in different places, so that some species have most of their silica bodies in the skin cells of leaves, while others deposit them in the hair cells of their roots. Still others don't produce silica bodies at all. But the upshot is, that given an assemblage of silica bodies, a phytolith analyst can often identify the species of plant they came from: as we'll see, this is a very useful trick indeed.
Benefits of Opal Phytolith Research
One of the benefits of opal phytolith research is that, unlike pollen which is produced by the living plant for the purposes of being carried far away by the wind, phytoliths are generally deposited in the soil where the plant decays. There are circumstances when phytoliths are released into the atmosphere--for example, during a forest fire--but for the most part, phytoliths are found very close to where the plant was living, or in pit features or ceramic vessels where the plant was stored. The most well-known use of phytolith research is identifying ancient agricultural fields.
Thus, retrieving phytoliths from the archaeological record is most often a soil-sampling process. Samples are taken at regular intervals across the length of the archaeological site, as well as within the different levels of vertical stratigraphy at a site. Say, for example, you have an archaeological site that has two occupation levels stacked on top of one another, and separated by a sterile layer; you'd be interested in the phytolith assemblage from all three, for comparison.
Of special interest to archaeologists studying the invention of agriculture is whether a particular plant is a domesticated species or not. Some plants, including maize, achiva, arrowroot, rice, bananas, plantain, wheat, barley, and squash rinds, do produce distinct phytoliths in their domesticated and wild varieties. One of the first studies in modern phytolith research was the identification of wild and domesticated varieties of maize corn over time, a problem with which researchers are still wrestling.
Thanks to Susan Mulholland for suggesting this topic, and loads of help with this series. A bibliography has been assembled for this project.