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A recent study by Dr. James Watson and his colleagues examined changes in projectile points, mortuary practices, and bioarchaeology to provide new insights into migration patterns and the introduction of maize into the Sonoran Desert during the Early Agricultural period (EAP).

The study is in the journal American Antiquity.

"Some archaeologists suggest that plant encouragement may have occurred in the Sonoran Desert prior to the arrival of maize in the area. But the only solid evidence of cultivation in the region begins with maize," explains Dr. Watson.

Two hypotheses contribute to the debate about when maize first arrived in the region. The first proposes that maize, along with other crops such as beans and squash, was first introduced into the Tucson Basin around 4,500 years ago, together with small-scale migrations of farming populations from Mesoamerica.

Proponents of the second hypothesis argue that maize and other crops diffused into the area, traveling along existing trade networks and through cultural contact. These crops were then gradually incorporated into the subsistence strategies of local hunter-gatherers.

Archaeological data have been used to support both hypotheses. However, in the study by Dr. Watson, three different archaeological data sources are used to identify subtle differences in population composition related to small-scale migration within the Sonoran Desert, associated with the spread of maize.

"Maize enters the Sonoran Desert at a time when water was apparently plentiful (for a desert—i.e., not a drought period) and would have likely aided in its spread and the investment of people in farming," says Dr. Watson.

It was during this time that four different projectile point types could be identified in the archaeological record. Of these, two are of particular interest: the Empire and the San Pedro type points. The San Pedro points are characterized by wide blades with side or corner notches forming extending stems.

Empire points, by contrast, are typically long, thick, finely serrated blades. One of the archaeological sites, Las Capas, is dominated by Empire points, despite nearby contemporary sites only producing San Pedro points.

The Las Capas inhabitants only began production of San Pedro type points after an occupational hiatus. It is suggested that the Empire points may have been of non-local origin, being used by migrants until the descendants of these migrants adopted the local tradition or were replaced by local populations.

Mortuary practices also showed differences, indicating both local and non-local traditions during the EAP. Sharing similar mortuary features can reflect an adherence to a shared group identity or origin.

When comparing Tucson Basin-linked burials to non-local La Playa-linked burials (another site that predominantly used Empire points), researchers noted that, in general, local groups had less variation in their burials. Most local burials preferred placing their dead in flexed positions, typically with little or no pigmentation (i.e., ocher and hematite).

Meanwhile, non-local groups had a much greater diversity of burial positions, including extended and prone (face down), and typically used more pigmentation in their burials.

Grave goods were generally few; however, if they did occur, these were typically shell jewelry, food processing objects, and projectile points, the latter of which are exclusively associated with male burials.

Finally, Dr. Watson and his team examined the bioarchaeology of the EAP populations. Various studies indicated that males had higher genetic diversity and variation. This increase in gene flow may have been the result of male exogamous marriage patterns, which influenced male migration for mate selection.

When taken together, the projectile points, mortuary practices, and bioarchaeology, the evidence suggests that small-scale male-dominated migrations likely expanded northwards during the EAP. These migrants brought with them knowledge of maize cultivation as well as distinct projectile points and mortuary practices.

While Dr. Watson is not certain, it seems this migration pattern associated with maize is unique to the Sonoran Desert. "I'm not familiar enough with other areas to identify if similar migrations occurred, but overall there does not appear to have been a similar movement of people associated with maize in the rest of northern North America at this time."

Further evidence suggests that some of these early San Pedro phase farmers eventually migrated to the Colorado Plateau, contributing to the origins of the Western Basketmaker II era.

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