Corn (maize) was domesticated in the earlier part of the Holocene in Mexico from a wild plant called teosinte. Subsequent to the discovery of this area of origin by MacNeish, a great deal of research has gone on to track the spread of maize across the New World, its diversification, its effects on Native American lifeways, and so on.
How do you tell if corn was grown in a particular area? There are several possibilities, including looking for pollen in swamps and lakes or at archaeological sites, finding macro-fossils (don't be fooled by the name .. macrofossils are tiny, like individual corn grains) or by examining phyotliths on sites, in cores, or stuck to cooking pots. Phytoliths are silica bodies that form in many plants and that typically have shapes that can be used to identify the plant to some level of taxonomic certainty (depending on the plant species and the state of research on phyotoliths for that species).
This post reports on a new paper in PNAS on the problem of distinguishing between wild teosinte and domesticated corn in areas where the two may be expected to have overlapped.
Holst, Moreno and Piperno examined modern teosinte (wild Maize), domesticated maize, and various closely related grasses to determine the viability of separating them on the basis of pollen and starch granules. They also discuss phytoliths.
Pollen is, of course, plant "sperm" and is thus often a very good means of distinguishing between species. However, one may not expect this to be the case when comparing a cultivated crop with a wild crop. This is in part because distinguishing characteristics of pollen that can be used by researchers parallel distinguishing traits that the plants themselves use. In wild plants, there is probably a selective benefit to reproductive isolation between species (to avoid inviable hybridization or fertilization), gut with cultivated crops, this selection is one way ... it may benefit the wild version of the plant, but not the cultivated version to the same degree. As I would have expected from theory, it turns out that this is indeed the case. Holst et al. show that pollen is an unreliable indicator of teosinte vs. maize and therefore is an unreliable tool in areas where the two may overlap. They conclude that "[t]here is presently no valid method for separating maize and teosinte pollen on a morphological basis."
However, there is some hope, according to these researchers, of distinguishing between the two types of corn using starch grain analysis.
Starch grain analysis, a recently developed tool of archaeobotany, appears to be of significant utility in distinguishing the seeds of teosinte from maize. We propose that the differences in starch grain morphology and size between wild and domesticated maize defined in this study may be associated with domestication genes in Zea that have been documented in the starch biosynthesis pathway.
With respect to phytoliths, this paper has some problems. There is a long term, well known debate between one of the authors of this paper (Piperno) and others, including Bob Thomson and Robert Lusteck of the University of Minnesota. Thompson developed a method of distinguishing between populations of maize, and between maize and teosinte, that in my view works better than the method previously developed by Piperno and Pearsall (and others). The authors of this paper utterly ignore Thompson's work, which I view as an inexcusable scholarly oversight, at best.
Anyway, this paper asserts that "... phytoliths effectively discriminate the female reproductive structures of Tripsacum, teosinte, and maize. " True, but only if you use Thomson's method!
Ultimately, the present paper suggests the combined use of starch grains and phytoliths to make the distinction between wild, uncultivated teosinte and cultivated maize in those regions (in Mexico) where the two are likely to overlap.
The authors conclude:
We have found contrasting degrees of utility for the discrimination of microscopic remains of maize, teosinte, and Tripsacum in ancient sedimentary records. Starch grains and phytoliths appear to be more useful than pollen in discriminating wild from domesticated maize. ...
The investigation of much larger samples of teosinte than studied previously enabled the establishment of new and more accurate pollen size parameters for teosinte. It is apparent that size standards commonly used for identifying maize pollen in Mexico and other regions of Mesoamerica are not reliable....
The present-day geographical distribution of teosinte is fairly well understood, and the various species and subspecies of wild Zea often have disjunct and restricted distributions...
These significant uncertainties relating to pollen records can be addressed by carrying out multiproxy microfossil studies. In paleoecological contexts, it will be important to have phytolith data, which should often enable investigators to assess whether teosinte or maize, or both, were present when Zea pollen is identified or not recovered .... Phytoliths can identify the remains of maize from both leaves and cobs, detect teosinte fruitcase remains, and rule out, or provide evidence consistent with, leaf phytolith decay from a number of teosinte races, including Balsas ...
It appears that phytoliths and starch grains acting as complementary sources of information will be of significant utility in archaeological records, where both of these microfossils are likely to occur.
Holst2007. Holst, I., Moreno, J. E. & Piperno, D. R. (2007). Identification of teosinte, maize, and Tripsacum in Mesoamerica by using pollen, starch grains, and phytoliths. Proc Natl Acad Sci U S A 104, 17608-17613.