Starch grain and phytolith evidence for early ninth millennium B.P. maize from the Central Balsas River Valley, Mexico

Abstract

Questions that still surround the origin and early dispersals of maize (Zea mays L.) result in large part from the absence of information on its early history from the Balsas River Valley of tropical southwestern Mexico, where its wild ancestor is native. We report starch grain and phytolith data from the Xihuatoxtla shelter, located in the Central Balsas Valley, that indicate that maize was present by 8,700 calendrical years ago (cal. B.P.). Phytolith data also indicate an early preceramic presence of a domesticated species of squash, possibly Cucurbita argyrosperma. The starch and phytolith data also allow an evaluation of current hypotheses about how early maize was used, and provide evidence as to the tempo and timing of human selection pressure on 2 major domestication genes in Zea and Cucurbita. Our data confirm an early Holocene chronology for maize domestication that has been previously indicated by archaeological and paleoecological phytolith, starch grain, and pollen data from south of Mexico, and reshift the focus back to an origin in the seasonal tropical forest rather than in the semiarid highlands.

Fig. 1.

Starch grains from maize recovered from early preceramic grinding stones 318d (A and B) and 318e (C and D). The grains have irregular shapes and surface contours, along with defined compression facets and transverse fissures (A) or y-shaped and other fissures (B–D).

Fig. 2.

(A–C) Cucurbita phytoliths from early preceramic sediments directly associated with grinding stones 319d (A and B) and 318 e (C) with sizes and morphological attributes (surface cavities and marks; faint scalloped impressions) characteristic of domesticated plants. (D) A phytolith from a hybrid between the wild species C. argyrosperma ssp. sororia and the domesticated species C. argyrosperma that is heterozygous at the Hr genetic locus and exhibits the same morphological attributes as the archaeological phytoliths, a result of human selection at this genetic locus.