Tehuacan Valley - Heart of Agriculture Invention in America

Early Evidence of American Domestication Process

Cactus in the Ethnobotanical Garden in Oaxaca
Cactus in the Ethnobotanical Garden in Oaxaca. Rod Waddington

The Tehuacán Valley, or more precisely the Tehuacán-Cuicatlán valley, is located in southeastern Puebla state and northwestern Oaxaca state in central Mexico. It is the southernmost arid area of Mexico, its aridity caused by the rain shadow of the Sierra Madre Oriental mountain range. Annual mean temperature averages 21 degrees C (70 F) and rainfall 400 millimeters (16 inches).

In the 1960s, the Tehuacán Valley was the focus of a large-scale survey called the Tehuacán Project, led by American archaeologist Richard S. MacNeish.

MacNeish and his team were looking for the Late Archaic origins of maize. The valley was selected because of its climate and its high level of biological diversity (more on that later).

MacNeish's large, multi-discipline project identified nearly 500 cave and open-air sites, including the 10,000-year-long occupied San Marcos, Purron, and Coxcatlán caves. Extensive excavations in the valley's caves, particularly Coxcatlán Cave, led to the discovery of the earliest appearance at the time of several important American plant domesticates: not just maize, but bottle gourd, squash, and beans. Excavations recovered over 100,000 plant remains, as well as other artifacts.

Coxcatlán Cave

Coxcatlán Cave is a rock shelter that was occupied by humans for nearly 10,000 years. Identified by MacNeish during his survey in the 1960s, the cave includes an area of about 240 square meters (2,600 square feet) beneath a rock overhang about 30 meters (100 feet) long by 8 m (26 ft) deep.

Large-scale excavations conducted by MacNeish and colleagues included about 150 sq m (1600 sq ft) of that horizontal range and vertically down to the bedrock of the cave, some 2-3 m (6.5-10 ft) or more to bedrock.

Excavations at the site identified at least 42 discrete occupation levels, within that 2-3 m of sediment.

Features identified at the site include hearths, cache pits, ash scatters, and organic deposits. The documented occupations varied considerably in terms of size, seasonal duration, and number and variety of artifacts and activity areas. Most importantly, the earliest dates on domesticated forms of squash, beans and maize were identified within Coxcatlán's cultural levels. And the process of domestication was in evidence as well—especially in terms of maize cobs, which are documented here as growing larger and with an increased number of rows over time.

Dating Coxcatlán

Comparative analysis grouped the 42 occupations into 28 habitation zones and seven cultural phases. Unfortunately, conventional radiocarbon dates on organic materials (like carbon and wood) within the cultural phases were not consistent within the phases or zones. That was likely the result of vertical displacement by human activities such pit-digging, or by rodent or insect disturbance called bioturbation. Bioturbation is a common issue in cave deposits and indeed many archaeological sites.

However, the recognized mixing led to an extensive controversy during the 1970s and 1980s, with several scholars raising doubts about the validity of the dates for the first maize, squash, and beans.

By the late 1980s, AMS radiocarbon methodologies which allow for smaller samples were available and the plant remains themselves—seeds, cobs, and rinds--could be dated. The following table lists the calibrated dates for the earliest direct-dated examples recovered from Coxcatlán cave.

  • Cucurbita argyrosperma (cushaw gourd) 115 cal BC
  • Phaseolus vulgaris (common bean) cal 380 BC
  • Zea mays (maize) 3540 cal BC
  • Lagenaria siceraria (bottle gourd) 5250 BC
  • Cucurbita pepo (pumpkins, zucchini) 5960 BC

A DNA study (Janzen and Hubbard 2016) of a cob from Tehuacan dated to 5310 cal BP found that the cob was genetically closer to modern maize than to its wild progenitor teosinte, suggesting that maize domestication was well underway before Coxcatlan was occupied.

Ethnobotany

One of the reasons MacNeish selected the Tehuacán valley is because of its level of biological diversity: a high diversity is a common characteristic of places where first domestications are documented.

In the 21st century, the Tehuacán-Cuicatlán valley has been the focus of extensive ethnobotanical studies—ethnobotanists are interested in how people use and manage plants. These studies reveal the valley has the highest biological diversity of all the arid zones in North America, as well as one of the richest areas in Mexico for ethnobiological knowledge. One study (Davila and colleagues 2002) recorded over 2,700 species of flowering plants within an area of approximately 10,000 square kilometers (3,800 square miles).

The valley also has a high human cultural diversity, with Nahua, Popoloca, Mazatec, Chinantec, Ixcatec, Cuicatec, and Mixtec groups together accounting for 30% of the total population. Local people have amassed an immense amount of traditional knowledge including the names, uses, and ecological information on nearly 1,600 plant species. They also practice a variety of agricultural and silviculture techniques including the care, management and preservation of nearly 120 native plant species.

In Situ and Ex Situ Plant Management

The ethnobotanists studies documented local practices in habitats where the plants naturally occur, called in situ management techniques:

  • Tolerance, where useful wild plants are left standing
  • Enhancement, activities that increase the plant population density and availability of useful plant species
  • Protection, actions which favor permanence of particular plants through care

Ex situ management practiced in Tehuacan involves seed sowing, planting of vegetative propagules and transplanting of entire plants from their natural habitats into managed areas such as agricultural systems or home-gardens.

Sources

This article is part of the About.com guide to sPlant Domestication, and part of the Dictionary of Archaeology