The cultural and chronological context of early Holocene maize and squash domestication in the Central Balsas River Valley, Mexico

Abstract

Molecular evidence indicates that the wild ancestor of maize is presently native to the seasonally dry tropical forest of the Central Balsas watershed in southwestern Mexico. We report here on archaeological investigations in a region of the Central Balsas located near the Iguala Valley in Guerrero state that show for the first time a long sequence of human occupation and plant exploitation reaching back to the early Holocene. One of the sites excavated, the Xihuatoxtla Shelter, contains well-stratified deposits and a stone tool assemblage of bifacially flaked points, simple flake tools, and numerous handstones and milling stone bases radiocarbon dated to at least 8700 calendrical years B.P. As reported in a companion paper (Piperno DR, et al., in this issue of PNAS), starch grain and phytolith residues from the ground and chipped stone tools, plus phytoliths from directly associated sediments, provide evidence for maize (Zea mays L.) and domesticated squash (Cucurbita spp.) in contexts contemporaneous with and stratigraphically below the 8700 calendrical years B.P. date. The radiocarbon determinations, stratigraphic integrity of Xihuatoxtla's deposits, and characteristics of the stone tool assemblages associated with the maize and squash remains all indicate that these plants were early Holocene domesticates. Early agriculture in this region of Mexico appears to have involved small groups of cultivators who were shifting their settlements seasonally and engaging in a variety of subsistence pursuits.

Fig. 1.

The study area in northern Guerrero.

Fig. 2.

A view of the enormous boulder that formed the Xihuatoxtla Shelter.

Fig. 3.

Stratigraphy of units 1 and 2 in the Xihuatoxtla Shelter.

Fig. 4.

Handstones and milling stone bases from the preceramic layers in the Xihuatoxtla Shelter. (A) Small handstone (318e) from layer E that yielded 80 maize starch grains, maize cob phytoliths, and 29 squash phytoliths. (B) Complete milling stone base (316d) from layer D that yielded 68 maize starch grains as well as 4 yam (Dioscorea sp.), 3 legume, and 1 Marantaceae starch grains. (C) Handstone fragment (318d) from layer E that yielded 22 maize starch grains, maize cob phytoliths, and 28 squash phytoliths. (D) Handstone (322c) from layer E that yielded 11 maize starch grains, maize cob phytoliths, and 7 squash phytoliths. (E) Small handstone (365a) from layer C that yielded 24 maize starch grains and maize cob phytoliths. (F) Handstone (319d) from layer E that yielded 8 maize starch grains, maize cob phytoliths, and 37 squash phytoliths. (G) Slab milling stone fragment (316c) from layer D that yielded 2 maize starch grains, maize cob phytoliths, and 29 squash phytoliths.

Fig. 5.

Bifacial points and preforms from north central Balsas Valley sites. (A) “Pedernales” point base from Xihuatoxtla, layer E. (B) Point from Xihuatoxtla, layer E (distal end). (C) Point base from El Abra. (D) Reworked stemmed point from Temaxcalapa. (E) Preform fragment from Temaxcalapa. (F) Point base from Temaxcalapa. (G) Preform base from Temaxcalapa.