Archaeological evidence of teosinte domestication from Guilá Naquitz, Oaxaca

The maize ear (the cob plus its attached grains) is distinguished from the teosinte female inflorescence by morphological characteristics that confer selective disadvantage for surviving in the wild. Maize has a rigid, polystichous (3- to 12-ranked) rachis with tenaciously attached grains that require human intervention for dispersal and propagation. The teosinte female inflorescence, on the other hand, is distichous (two-ranked) and naturally disarticulates, disseminating a single grain in each cupulate fruitcase. Each rachis segment or rachid in the maize ear bears a pair of fertile grain-bearing spikelets at the base of a cupule (the indurate invagination of each rachis segment) that are oriented perpendicular to the rachis axis. The teosinte inflorescence, in contrast, has a single grain-bearing spikelet per rachid that is oriented parallel to the rachis and is partially enclosed by the lateral margins of the invaginated rachis segment. Teosinte grains are thus protected inside the fruitcase formed by the invaginated rachid and lower glume when they are individually dispersed (7–9). In many grain crops, the first step in domestication resulted from human selection for a rachis that did not naturally disarticulate. Once this rigid-rachis phenotype had been fixed genetically, human selection and agroecosystem creation and maintenance led to an increase in grain size (10–12). The predicted initial evolutionary changes documented for other grain crops such as barley and wheat are evident in the early inflorescence fragments of Zea mays encountered at Guilá Naquitz.

All three specimens from Guilá Naquitz have a nondisarticulating rachis. Breaks in the rachis near the inflorescence apex occur through the internode instead of at the node (Fig. 1), which indicates that the plants bearing these inflorescences were domesticated and depended on human dispersal and propagation. All three inflorescences are also distichous (two-ranked); one fragment (Fig. 1, specimen c, D10) has four rows of grain (two-ranked with two rows per rank), and the other two have two rows of grain (two-ranked with one row per rank; Fig. 1, specimens a and b, C9). Like teosinte, the two-rowed specimens are two-ranked (distichous) and possess a single grain-bearing spikelet per rachis segment. The single four-rowed specimen is like maize, because it has paired spikelets on each rachis segment. The stratigraphic and temporal association of these cobs with distinct morphologies suggests that the Guilá Naquitz specimens represent domesticated plants that were subject to human selection for paired spikelets, that is, ears with four or more rows of grain.

The lower glume of each spikelet in the two-rowed inflorescence fragments (C9) is perpendicular to the rachis, short, dull, soft, and flexible in contrast to the rachis, which is mottled, shiny, rigid, and very indurate. The single grain-bearing spikelets alternate on the axis and are oriented at an approximately 45° angle to each other instead of being opposed at 180° as in the four-rowed specimen (Fig. 1, specimens b and c). The rachis segments are very short in comparison to the female inflorescence of both the presumed ancestor, teosinte, and modern descendants, extant Mexican maize races. Rachis segments are approximately one-half as long as those of the shortest of the extant Mexican maize races, Mexican annual teosintes, and teosinte homozygous for tga1 (7, 13, 14). Cupules of the two-rowed specimens are as wide as they are long and very shallow (Fig. 2; refs. 4 and 15). The two-rowed cobs from Guilá Naquitz are similar to those of teosinte, because each rachis segment has a single spikelet; the rachis is distichous and has a mottled, smooth, and shiny surface; and because both are indurate and possess alternately stacked rachis segments. These cobs differ from teosinte by having spikelets oriented perpendicularly to the rachis, very short and shallow cupules, and a nondisarticulating rachis.

The four-rowed inflorescence is similar to four archaeological cobs from the Tehuacán valley, three from San Marcos, and one from Coxcatlán cave. The rachis of this Guilá Naquitz cob is distichous, but each rachis segment has two grain-bearing spikelets per node; one is sessile, and the other is distinctly pedicellate, a characteristic of maize. The lower glumes are indurate but less so than those of the two other specimens at Guilá Naquitz (Fig. 1, Lower, specimen c). Rachis internodes alternate, the spikelets of one rank alternating with spikelets of the opposing rank. Rachis internodes are twice as long but only as wide as the internodes of the two-rowed specimens (Figs. 1 and 2). The cupules are shallow, seeming to be flat and slightly reflexed at the apex. This single cob from Guilá Naquitz, like the few aforementioned specimens from Tehuacán, possesses three of four derived morphological attributes characteristic of the female inflorescence of maize: a rigid rachis, paired grain-bearing spikelets, and spikelets oriented perpendicularly to the axis. This specimen is distichous like teosinte; thus, it lacks the fourth derived characteristic, polystichy. The other two cobs from Guilá Naquitz share only two of these derived characteristics: a rigid rachis; and a perpendicular orientation of spikelets to the rachis.

By 5,400 ¹⁴C years B.P., human cultivators had made genetic changes in teosinte that resulted in a nondisarticulating rachis, a reduced rachid length, spikelets reoriented perpendicularly to the rachis, and opened cupulate fruitcases, which exposed the grain. The precise genetic focus of human selection on teosinte is not obvious from the phenotypes of the archaeological specimens. However, inferences based on existing genetic evidence suggest that at least two genetic loci were subject to human manipulation at this early date (8, 16–18). The teosinte branched locus (tb1) has demonstrated effects on the condensation of lateral branches and the rachis segments of inflorescences located on lateral branches in maize and teosinte. The rachis segments of the Guilá Naquitz specimens are remarkably short, as short as the earliest maize from the San Marcos cave, and considerably shorter than any extant teosinte or Mexican land race. The teosinte glume architecture locus (tga1) controls the development of the cupulate fruitcase and the degree of glume induration in teosinte and maize. The fact that the glumes of all three cob fragments from Guilá Naquitz seem flexible and less indurate than the rachis or the glumes of teosinte suggests that this locus was the focus of human selection after or simultaneously with shortened rachis internodes and a rigid rachis phenotype. The cooccurrence of two-rowed and four-rowed distichous inflorescences at Guilá Naquitz suggests that these and other genetic factors had not yet reached fixation and that modifiers had not yet accumulated to stabilize the maize-like phenotype with paired spikelets.