The Origins of Mexican Cultivated Plants

There is no simple definition of the term “domestication.” There is a difference between the domestication of such an animal as the dog or pig, which can and often does revert to the “wild” state, and such a creature as the Egyptian chicken, the reproduction of which entirely depends upon the presence of human populations for it has lost the ability to incubate its own eggs. We are clearly dealing here with a broad spectrum, in which the degree of domestication may vary widely; one might therefore adopt the definition proposed many years ago by the Russian geneticist Nikokai Vavilov, and say that domestication is co-evolution, directed by the interference of humans. Essentially, this implies that human beings have in some way tampered with the reproduction of a certain species—a process that may be systematic or totally unwitting.

As in animals, in its most extreme form, plant domestication ends up with species that cannot reproduce by themselves, and which are therefore without wild populations. In the case of cereals and other plants that reproduce by means of seeds, this implies that artificial selection has resulted in species that lack the ability to disperse their seeds. It is no accident that all the important food plants of the world belong in this category of entirely captive populations, since the reduction of the ability to self-reproduce has resulted in greatly increased food values in the plants concerned.

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Crucial evidence has been fond by the Piperno team in the rockshelter of Xihuatoxtla, in the Balsas basin. The combination of stratigraphy, radiocarbon dating, and microscopic analysis of phytoliths and starch grains make it virtually certain that maize was first transformed from wild teosinte to a domesticate early in the seventh millennium BCE, and spread from Guerrero to most of Mesoamerica and then south to the rest of the American tropics. Archaeologists Mary Pohl and Kevin Pope have found large maize phytoliths (probable domesticates) dated to 4800 BCE at San Andrés on the Gulf Coast of Tabasco. There are no known wild species of Zea native to coastal Tabasco, which means these plants were introduced to the region, almost certainly by humans. At the same level, the archaeologists found evidence of large-scale forest clearance of the type associated with maize cultivation in this area. If this Tabasco material is true maize cultivation, then it is the earliest record of such activity that we have.

In the highlights, the earliest firmly dated maize cob was found in Guliá Naquitz cave in Oaxaca and dated to 4300 BCE. Cobs from caves in Tehuacan, Puebla, found by Richard S. MacNeish and long thought through radiocarbon dating of associated materials to date to around 5000 BCE, have been subjected to direct dating of the cobs themselves and found to originate from 3500 BCE and after (figure 10). This Tehuacan sequence is crucial for the domestication of maize in particular and the Archaic period in general, for it is mainly through the work of MacNeish and his colleagues that we obtained our first systematic picture of this period (see “The Tehucan Valley and early Domestication,” p. 32).

There is still much to learn about the evolution of domesticated maize, but the most important general fact remains: many thousands of years BCE, the human inhabitants of Mesoamerica brought a wild form of maize under their control through a slow process of selection. This was indeed the most crucial process along the road leading to the great Pre-Columbian civilizations.