INAUGURAL ARTICLE by a Recently Elected Academy Member:Late Pleistocene and Holocene environmental history of the Iguala Valley, Central Balsas Watershed of Mexico

Paleoecological records from Guatemala to the Amazon Basin demonstrate that lowland Neotropical environments were not stable in the past (e.g., refs. 1–3). Shifts in climate and vegetation associated with the last phases of tropical deglaciation were particularly strong as the Pleistocene was ending at ≈12,000–10,000 B.P. (≈14,000–11,200 cal B.P.),†† when temperature and precipitation rose considerably and lowland tropical forest replaced the herbaceous and cool-adapted flora that dominated the ice age vegetation of many regions. Such dramatic ecological changes have been linked to the origins of agriculture in a number of areas of the world, including the Americas (2, 4). Mexico was one of the world's great centers of plant domestication, and among the many crops it contributed, none is of longer or more intense interest to investigators than the premier cereal crop of the Americas, maize. Molecular research indicates that the wild ancestor of maize is an annual species of teosinte, Zea mays ssp. parviglumis (Iltis and Doebley) (the race Balsas) presently found at elevations between 500 and 1,800 m in the central parts of the Rio Balsas drainage in tropical southwestern Mexico (5, 6) (Fig. 1). Other major crops may have been domesticated there; for example, the “silverseeded squash,” Cucurbita argyrosperma Huber (7). The natural vegetation of much of the region is, or was before intensive human disturbance, species-diverse tropical deciduous forest. Extant forests are considered centers of diversification for important genera of trees such as Bursera and Leucaena, contain many endemic species, and are of high conservation interest (8). Fig. 1. Map of Mexico showing the location of the Iguala region and sites mentioned in the text. Despite its importance, no data relating to early agricultural evolution and associated environmental history are available from the Balsas drainage. Evidence from Guila Naquitz Cave, located in the semiarid highlands of Oaxaca, indicates that plant domestication in Mexico (C. pepo L. squash) occurred by 9,000 B.P. (10,000 cal B.P.) (9). Currently, however, the earliest evidence for Mexican maize consists of cobs recovered from Guila Naquitz dated to 5,400 B.P. (6,200 cal B.P.) (10), and pollen and phytoliths from San Andres, Tabasco, with an age of 6,200 B.P. (7,300 cal B.P.) (11, 12). Both sites are outside of the present distributional range of wild maize, and neither provided evidence for a premaize use of teosinte. This article presents the results of paleoecological studies carried out on three lakes and a swamp located in the central Balsas watershed that provide information on the natural- and human-induced changes in vegetation and climate since the late Pleistocene. The sites, called Ixtacyola, Ixtapa, Tuxpan, and Chaucles, are situated in and near the Iguala Valley in northern Guerrero state (Fig. 1). The work is part of a large initiative combining paleoecological and archaeological studies in the Central Balsas drainage. During six field seasons undertaken from 1999 to 2005, we carried out reconnaissance, testing, and coring of the sites reported on here.