PERSPECTIVE The diffusion of maize to the southwestern United States and its impact William L. Merrilla, Robert J. Hardb,1, Jonathan B. Mabryc, Gayle J. Fritzd, Karen R. Adamse, John R. Roneyf, and A. C. MacWilliamsg aDepartment of Anthropology, National Museum of Natural History, Smithsonian Institution, P.O. Box 37102, Washington, DC 20013-7012; bDepartment of Anthropology, One UTSA Circle, University of Texas at San Antonio, San Antonio, TX 78249; cHistoric Preservation Office, City of Tucson, P.O. Box 27210, Tucson, AZ 85726; dDepartment of Anthropology, Campus Box 1114, One Brookings Drive, Washington University, St. Louis, MO 63130; eCrow Canyon Archaeological Center, 23390 Road K, Cortez, CO 81321; fColinas Cultural Resource Consulting, 6100 North 4th Street, Private Mailbox #300, Albuquerque, NM 87107; and gDepartment of Archaeology, 2500 University Drive Northwest, University of Calgary, Calgary, Alberta, Canada T2N 1N4 Edited by Linda S. Cordell, University of Colorado, Boulder, CO, and approved October 30, 2009 (received for review June 22, 2009) Our understanding of the initial period of agriculture in the southwestern United States has been transformed by recent discoveries that establish the presence of maize there by 2100 cal. B.C. (calibrated calendrical years before the Christian era) and document the processes by which it was integrated into local foraging economies. Here we review archaeological, paleoecological, linguistic, and genetic data to evaluate the hypothesis that Proto-Uto-Aztecan (PUA) farmers migrating from a homeland in Mesoamerica intro- duced maize agriculture to the region. We conclude that this hypothesis is untenable and that the available data indicate instead a Great Basin homeland for the PUA, the breakup of this speech community into northern and southern divisions Ϸ6900 cal. B.C. and the dispersal of maize agriculture from Mesoamerica to the US Southwest via group-to-group diffusion across a Southern Uto- Aztecan linguistic continuum. early agriculture ͉ migration ͉ US Southwest ͉ Mesoamerica ͉ Uto-Aztecan he dispersal of maize agricul- led to the diversification of PUA into dis- maize (Zea mays spp. mays) was intro- ture from Mesoamerica to the tinct dialects and eventually the distinct duced to the southwestern United States southwestern United States is a languages that comprise the Uto-Aztecan no later than 2100 cal. B.C. (calibrated perennial topic of renewed in- (UA) language family, distributed at the calendrical years before the Christian era) Tterest as part of a broad, interdiscipli- time of European contact from the Great (Table 1 and Table S2) (25, 26). The ar- nary effort to understand the develop- Basin of western North America to Cen- rival of maize in this region was preceded ment of maize agriculture in the New tral America (see Table S1) (15–17). by a long period during which its wild an- World (1–3). At present, there are two The credibility of Bellwood’s proposal cestor, Balsas teosinte (Zea mays ssp. par- competing models of the mechanisms was bolstered by the strong endorsement viglumis), native to southwestern Mexico, of the Uto-Aztecanist linguist Jane Hill, through which this dispersal took place. was transformed through human interven- who in a series of publications presented The first proposes that maize agriculture linguistic, cultural, and human genetic tion into a productive grain crop (3, 27– diffused northward as a complex com- data that she interpreted as supporting his 29). The oldest known maize macrofossils, prised of the cultigen itself and the perspective (9, 18–21). Largely as a result recovered from Guila´Naquitz Cave in the knowledge associated with its cultivation of her work, the Bellwood–Hill hypothesis southern Mexican state of Oaxaca, date to and use, transmitted from one foraging is now regarded as offering a plausible 4280 cal. B.C. (30, 31). For the purposes band to another without major popula- scenario for the origins of maize agricul- of this essay, we will assume that the ini- tion movements (4–6). The second ar- ture in the US Southwest (22–24), but it tial dispersal of maize from Mesoamerica gues that maize agriculture was intro- has not yet received the critical evaluation to the southwestern United States began duced to the southwestern United States that the evidence currently available al- Ϸ4300 cal. B.C. through the long-distance migration of lows. Here we review this evidence, con- The earliest evidence for maize in the Mesoamerican farmers (7–9). cluding that it strongly supports the alter- southwestern United States comes from Over the years, most scholars have con- native view that maize agriculture spread three open sites and two rockshelters in sidered group-to-group diffusion more from Mesoamerica to the Southwest via Arizona and western New Mexico (Table likely, but since 2000 the long-distance group-to-group diffusion. We propose that 1, Fig. 1) (32–37). Ranging in elevation migration model has gained adherents Uto-Aztecans played a crucial role in this from Ϸ700 to 2200 m, these sites are no- due primarily to perspectives developed by diffusion but more than a millennium af- table for the diversity of their settings. the archaeologist Peter Bellwood regard- ter the PUA speech community began Clearwater and Las Capas are located ing the expansion of agriculture and lan- dividing into subgroups and migrating out along the middle Santa Cruz River in the guage families in diverse regions of the of the homeland that its members shared world (8, 10–14). Bellwood suggested that at that point in their history. We offer new data to argue that, at the time of the maize agriculture expanded from its area Author contributions: W.L.M., R.J.H., J.B.M., G.J.F., K.R.A., of origin in Mesoamerica to the US initial breakup of this speech community, J.R.R., and A.C.M. designed research; W.L.M., R.J.H., J.B.M., its members were foragers located in the G.J.F., K.R.A., J.R.R., and A.C.M. performed research; Southwest as a consequence of the migra- Great Basin rather than farmers in Me- W.L.M., R.J.H., J.B.M., G.J.F., K.R.A., J.R.R., and A.C.M. ana- tion of farmers who were members of the soamerica and that this breakup began lyzed data; and W.L.M. and R.J.H. wrote the paper. Proto-Uto-Aztecan (PUA) speech com- Ϸ6900 B.C., significantly earlier than pre- The authors declare no conflict of interest. munity. He proposed that this migration viously proposed. This article is a PNAS Direct Submission. Ϸ began 3500 B.C. and involved the 1To whom correspondence should be addressed. Email: spread of PUA speakers across the region The Introduction of Maize Agriculture to the [email protected]. extending between Mesoamerica and the Southwestern United States. Multiple AMS This article contains supporting information online at www. US Southwest. This population expansion radiocarbon dates firmly establish that pnas.org/cgi/content/full/0906075106/DCSupplemental. www.pnas.org͞cgi͞doi͞10.1073͞pnas.0906075106 PNAS ͉ December 15, 2009 ͉ vol. 106 ͉ no. 50 ͉ 21019–21026 Downloaded by guest on September 26, 2021 Table 1. The earliest maize sites in the US Southwest Calibrated date (2 ␴ and Site Location Latitude Elevation Type median) (refs. 25, 26) Old Corn Colorado Plateau, western New Mexico 34°30’ 1,910m Open 2460–2060 B.C. (2260 B.C.) McEuen Cave Gila Mountains, eastern Arizona 33°14’ 1,340m Rockshelter 2270–1940 B.C. (2080 B.C.) Clearwater Tucson Basin, southern Arizona 32°13’ 720m Open 2200–1960 B.C. (2080 B.C.) Las Capas Tucson Basin, southern Arizona 32°20’ 680m Open 2200–1940 B.C. (2050 B.C.) Three Fir Shelter Colorado Plateau, northeastern Arizona 36°35’ 2,240m Rockshelter 2470–1540 B.C. (1990 B.C.) The calendrical dates are calibrated to the maximum 2 ␴ range with the median (25, 26). Uncalibrated radiocarbon dates and references are in Table S2. basin and range country of the upper So- The near contemporaneity of the appear- aged Ϸ1.3 km per year, using the noran Desert. McEuen Cave is found in a ance of maize in Tamaulipas and the US Ϸ2,800-km highway distance between deep canyon in the eastern Gila River Southwest, as well as the genetic differ- Oaxaca City and Tucson as a proxy for valley, in the mountainous transition zone ences between the early maize recovered the actual distance covered. At this pace, between the southern basin and range in the two areas, suggest that maize did it would have proceeded an average of country and the Colorado Plateau. The not diffuse to the Southwest through one degree of latitude (120 km) every 92 Old Corn site is in an upland valley on northeastern Mexico (40). Mexico’s Pa- years, allowing adaptations to local condi- the southeastern edge of the Colorado cific coast, the Sierra Madre Occidental, tions to take place over numerous genera- Plateau, and the Three Fir Shelter is situ- and the eastern and western flanks of this tions of maize. ated on the central Colorado Plateau, cordillera have been proposed as likely near the north rim of Black Mesa. The routes (41, 42), but the absence of addi- The Proto-Uto-Aztecans as Mesoamerican fact that maize appears almost simulta- tional data precludes identifying both the Maize Farmers. Bellwood proposed that neously in these upland and lowland sites point(s) of origin of the early southwest- migrating Mesoamerican farming popula- suggests that the ancient landrace(s) were ern maize and the route(s) of its tions could have diffused maize agricul- tolerant of a wide range of environmental dispersal.