The word “phytolith” comes from the Greek words for “plant” and “stone,” and that does a good job summarizing what a phytolith is: they are basically tiny rocks inside of plant cells. When plants grow, they absorb water and nutrients through their roots. During this process, they also absorb dissolved minerals like silica, which is essentially the same material as rocks. These minerals get deposited inside plant cells, and get left behind in the soil after the plant dies because they are inorganic–they don’t decompose like the rest of the carbon-based plant tissue. The silica molecules form a perfect “cast” of the cell, or the little spaces in between the cells. To me, phytoliths look a little bit like snowflakes under the microscope.
Like pollen, different plants produce phytoliths with different shapes and sizes, so we can match the phytoliths we extract from archaeological excavations with certain plants that grew at that site. Also like pollen, archaeological sediment needs to undergo a series of chemical rinses and processes in order to separate the phytoliths from the surrounding dirt. The final product is a dusting of phytoliths on a slide, which goes under the microscope.
Phytolith analysis is often used to learn about ancient diets, environments, agriculture, and plant domestication. Often, archaeologists will use phytoliths alongside other methods, like pollen or DNA analysis. At an archaeological site in northeast China, a team of researchers used phytoliths to reconstruct ancient environments and better understand agricultural practices. Based on the discovery of millet and other phytoliths, the archaeologists found that the environment in the area of the site was warm and wet about 6,500 to 5,600 years ago, which was ideal for agriculture, and led to a prosperous time in the area.
I used phytolith analysis in my Ph.D. dissertation on sediments from Massachusetts. I studied phytolith data from two 17th century English homestead sites, where I found evidence of corn agriculture. This was interesting and a little bit surprising, because English colonists did not like eating corn in the early 1600s–they much preferred wheat, which was more common in Europe. In my research, I used corn to talk about how English colonists adapted to life in the Americas through the consumption of Indigenous foods.
This blog post was partially adapted from a YouTube video I wrote with Smiti Nathan for her channel @smitinathan.
When you think of pollen, you might think about hay fever and the yellow coating on your car in the spring. But pollen analysis, also known as palynology, can also be a valuable source of evidence for archaeologists.
Many different types of plants produce pollen in their flowers as part of their reproductive cycle. Plants also vary in their pollen dispersal mechanisms–some rely on insects like bees or flies to spread their pollen, some rely on animals like bats, and others release pollen on the wind in a process called “pollen rain.” Through these mechanisms, pollen becomes incorporated into sediments that can then be recovered by archaeologists.
Individual pollen grains are tiny–you need a microscope to be able to see it, so archaeologists call pollen a microbotanical, as opposed to remains that can be seen with the naked eye, which are known as macrobotanicals. Pollen grains are so tiny, in fact, that many are smaller than the width of a human hair. Different plants produce pollen grains that look unique–for example, pollen grains produced by sunflowers are round and covered in tiny spikes. Using modern comparisons, archaeologists can identify pollen from archaeological sediments.
Pollen analysis requires a series of chemical rinses in order to separate the pollen grains from the surrounding soil. This includes hydrochloric acid to melt away calcium carbonate deposits, like little bits of shell, followed by hydrofluoric acid to dissolve silt and sand, among other steps that require many hours in the lab.
Pollen analysis has many different applications, ranging from reconstructing diet to figuring out what trees grew in ancient forests. One of my favorite uses of pollen analysis is in the famous case of Ötzi the Iceman, a 5,300-year-old mummy found by hikers in the glaciers of the Alps in 1991. Ötzi is famous for being incredibly well-preserved; so well-preserved, in fact, that archaeologists have been able to figure out exactly what and when his last meal was. By using pollen and DNA analysis from his stomach contents and belongings, researchers have been able to figure out that he ate wheat bread and wild fruits shortly before his death. Also, based on the presence of the pollen of certain summer-flowering trees, the scientists working on Ötzi were able to deduce that he died in early summer.
I have used pollen analysis for my Master’s thesis in New Mexico. My data showed how the environment around a 17th century Spanish colonial ranch changed over the course of 100 years. I found that this region near Santa Fe was much marshier than it is today, with high levels of pollen from cottonwood and willow trees. I also found pollen from wheat and corn, which shows that colonists were eating and growing both of these important crops.