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The nearest relatives of corn and the purely New World grasses in the genera Manisuris and Tripsacum and the many Tripsacum-corn intermediates called teosinte. These have been included in a separate genus, Euchlaena, but should be considered as a variable hybrid involving maize and Tripsacum. Wiles (1967) provided a thorough survey of present knowledge of teosinte and its relationships.

The species of Tripsacum may be arranged in three groups:

1. Slender, Manisuris-like species, such as T. floridanum and T. lanceolatum, with restricted distribution and usually thirty six chromosomes.

2. More vigorous species that may have hybrid or polyploid vigor sometimes appear to have been involved in hybridization with maize, teosinte, or maize-contaminated species of Tripsacum, and may have thirty-six or seventy-two chromosomes—for example, T. dactyloides, T. laxum, and T. austral.

3. Vigorous tropical species like T. pilosum, which apparently bear considerable amounts of maize germplasm and usually have seventy two occasions.

The center of diversity of Tripsacum and of teosinte lies in Mexico. There, too, are located most of the archaeological sites in which Tripsacum or teosinte have been found.

The oldest known corn has been found in Mexico. Most ancient are some pollen grains identified among fossil pollen isolated form a drill core taken at a depth of more than seventy meters below present-day Mexico City (Barghoorn, Wolfe, and Clisby, 1954). The identifications were questioned (Kurtz, Liverman, and Tucker, 1960), but later studies of Barghoorn and his associates confirm that at least some of the fossil pollen grains were those of corn (Mangelsdorf, Macneish, and Galinat, 1964: 539). The pollen is believed to have been deposited during the last interglacial period, estimated to have occurred about eighty thousand years ago.

The oldest corn plants remains were found by MacNeish about 125 miles east-southeast of Mexico City, in the Tehuacán Valley. The earliest are from levels dated about 4800-3500 B.C. and are believed to be wild corn. The cobs are small, usually eight-rowed, the kernels partially covered by the glumes as in weak forms of pod corn, but the structure is essentially that of modern corn. The Tehuacán corn is likely to have been wild, self-propagating, and not dependent upon man for its care, and thus may be considered to be wild corn.

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