Back to the Stone Age

It was once thought that the earliest people in the Americas arrived 13,500-11,500 years ago. This culture was called "Clovis" and was characterized by remarkable "points" (arrowheads) with pressuring plating--which produces strong but sharp edges--and fluting--shaping at the base that forms a hollow where a shaft could be bound to produce a spear. Although some scholars argued that there were earlier settlements, the stone tools used as justification for this theory were so much simpler than Clovis tools that many archaeologists disagreed over whether or not they were actually stone tools as opposed to natural rocks. Even when more complex artifacts and features were found, the dating was often controversial. A site in Monte Verde, Chile, however, finally provided convincing evidence of a "pre-Clovis" settlement dating to 14,800 years ago (using radiocarbon dating of bones and charcoal). The site was particularly well preserved because it had become a bog soon after occupation which provided anoxic conditions in which even wood structures were preserved (since degrading bacteria could not live there). Current consensus--as much as there is one--among archaeologists now, is therefore that humans crossed the Bering Land Bridge into North America at least 16,000 years ago and the Neotropics (New World tropics) by at least 15,000 years ago. Some even argue for earlier dates (as far back as 35,000 years ago), but that is still extremely controversial.

This means that humans have been in the Neotropics for a long time. How did they live? It has long been thought that early prehistoric peoples relied mostly on large game animals for sustenance. While they undoubtedly hunted for meat, more evidence is emerging regarding the use of plants in prehistoric times.  Although archaeology has long been limited to the examination of pottery, bones, and stone remains, more recent paleobotany techniques have allowed us a glimpse into past ecology and agriculture.

Our professor for the first half of the class is a pioneer of some of this research. She uses phytoliths (formed by plants taking up water with soluble silica which crystallizes within the plant) and starch granules (the remains of plant cell organelles called amyloplasts which store starch)--all of which can be, but are not always, diagnostic of certain species or general types of plants. By looking for these signatures on stone tools she can reconstruct an approximate timeline for the development of agriculture. For example, she has found starch grains of corn and other crops on the grinding surfaces of stone tools in much higher concentrations than on other surfaces or in the surrounding soil. Starch granule analysis can even be done on teeth, giving us a direct indication of ancient peoples' diets. In some cases the domestic and wild versions of a plant have different phytoliths and/or starch grains, which allows us to check when signs of domestication become present in the archaeological record at different sites.

This type of evidence adds to pollen and charcoal data from lake sediments which show the increase of grass species (which invade cleared areas), decrease of woody tree species (mature, undisturbed forest), and increased charcoal levels which indicate the use of slash and burn agriculture in Panama at least 7,600-7,200 years ago. Altogether, the current evidence supports origins of crop domestication and agriculture in both Mexico and South America independently about 10,000-9,000 years ago. This is concurrent with the development of agriculture in both China and Southwest Asia, probably because global climate was transitioning from the last ice age (Pleistocene) into the current interglacial (Holocene) which changed the state of ecosystems such that farming became more energetically rewarding than hunting and gathering (though the reasons for the simultaneous emergence of agriculture throughout the world are also controversial, ranging from purely climatological to purely social reasons).

The exciting thing about starch grain analysis is that it allows for identification of different crop species being ground or eaten at different archaeological sites through time. (Phytoliths can also provide this information, but both phytoliths and starch grains are only distinctive for certain species, and are deposited in different amounts by different plants and processes, so this adds another tool to the toolkit, as it were.) The use of starch grains in archaeological work is still quite new, however, so there are many questions about how to interpret starch grain results. For instance, which crops and distinguishable and which look the same? Which are damaged by grinding and how? We did some exploratory work on these questions, grinding up corn, yam, beans, squash, and manioc (also called yuca or tapioca) to compare fresh and ground starch granules. We collected suitable stones from the Chagres River when we visited the Embera, brought them back to the lab, and started grinding!

Thanks to Chhaya for the photo!

As you can see, it's a rather messy process. Grinding releases the starch grains from the plant cells and some are left behind on the stones. We washed them off and looked at the resultant liquid under a light microscope and were able to observe some alterations due to grinding such as an increased occurrence of fractures on the starch grains.

Here are some photos taken through the microscope (thanks to Lee for these).

Bean, highly fractured.

Yam

Squash

The grinding stones also because smooth and sometimes striated from the grinding process, signs of human alteration. It's fascinating to get a glimpse of how such study and reconstruction of the lives of ancient peoples is done (though one can't help feeling a little silly banging rocks together for a grade...)!