Origins of Maize: A Further Paradox Resolved

van Heerwaarden et al. (2011) provide evidence of approximately 20% introgression of mexicana to maize varieties growing at over 1500 m. Below this altitude, only 1% such introgression is observed. Given the phylogenetic proximity of mexicana to parviglumis, these altitudespecific differences in admixture are more than sufficient to explain the close affinity of highland maize to teosinte; i.e., highland maize populations show genetic similarity to parviglumis because of mexicana genes acquired after domestication. On the basis of their findings, van Heerwaarden et al. (2011) reconstruct a maize phylogeny excluding both mexicana and parviglumis. In place, the authors infer ancestral states with respect to extant maize populations and calculate drift accordingly. By this analysis, they identify varieties from Western Mexico as ancestral, and the highland varieties as a divergent group. As such, the authors place the center of maize domestication within the geographic range of the parviglumis ancestor. Molecular characterization of the highland landraces has not been the only difficulty to reconcile with a center of domestication in Balsas. Long before the availability of large-scale genetic datasets, the highland landraces were already thought to be primitive on the basis of morphological characterization (Wellhausen et al., 1952). Furthermore, much of the iconic archeological work of Richard MacNeish was carried out in the highland caves of Tehuacán, Mexico (Hastorf, 2009). Although MacNeish’s pragmatic choice of field sites was based on the expectation that material would be better preserved in such localities, it no doubt served to promote the idea that ancient groups would be found in the highlands. In recent years, however, archeologists, prompted by the molecular data, have turned their attention to the Balsas region. Subsequent phytolith and cultural finds, while less spectacular than Origins of maize: a further paradox resolved