One of the important questions in understanding the spread of agriculture into the Southwest from Mexico is when Southwestern peoples became dependent on it for their subsistence. It is generally accepted that this dependence was in place by the Pueblo I period, which is defined as starting around AD 750 in most areas, but there has traditionally been a sharp divergence of opinion on when this came about.
One school has it that it was only in the Pueblo I period, and in some areas perhaps even later, that agriculture finally shifted to becoming the main food source after having been a minor, auxiliary supplement to a hunting and gathering system for hundreds or even thousands of years. This view is generally, though not universally, associated with the idea that maize and squash were introduced quite early to the Southwest, as early as the Late Archaic period, and were cultivated on a small scale by local hunting and gathering peoples for a long time before the final shift to an agricultural economy, often associated with the “Pithouse to Pueblo Transition” thought to have taken place between the Basketmaker III and Pueblo I periods, when people are thought to have moved out of Basketmaker pithouses and into aboveground Pueblos, retaining the pithouse as a ceremonial structure ancestral to the later kiva. This view, in other words, emphasizes continuity of the Basketmaker period with Archaic traditions and sees a long, gradual process of adoption of cultigens, with total dependence on agriculture only coming very late. The key transition period in this view is between Basketmaker III and Pueblo I.
The other camp sees things very differently. They propose that agriculture was already the mainstay of Southwestern economic systems in the Basketmaker III period and possibly as early as Basketmaker II, and see the most important transition as being not between Basketmaker III and Pueblo I but between the Late Archaic and Basketmaker II (there is no Basketmaker I period). Furthermore, they see the introduction of agriculture as having been abrupt and caused by a migration of established farming populations into the Southwest from somewhere to the south. Jane Hill, one proponent of this view, sees this migration as associated with the spread of the Uto-Aztecan languages and proposes that the people in question originated in central Mexico, but not everyone who favors a migrational account agrees with her specific theory. The important thing is not so much exactly where the Basketmakers came from or what language they spoke but that they came from somewhere else, and brought agriculture with them. In this view, then, there is a profound discontinuity between Archaic hunter-gatherers and Basketmaker/Pueblo farmers, and most of the elements of the later Pueblo lifestyle were present already in some form in the Basketmaker period. The Pithouse to Pueblo Transition is therefore not as important in this account. It clearly marks some changes in society, but in this view the adoption or intensification of agricultural production was not one of them, since the Basketmakers were already as dependent on agriculture as they would ever get.
One way to test these hypotheses is to look at whether Basketmakers were in fact more similar in their diets to earlier Archaic populations (supporting the gradualist theory) or later Pueblo populations (supporting the migrationist theory). As I mentioned earlier, one early attempt to look at Basketmaker II subsistence in the Cedar Mesa area of southern Utah used multiple lines of evidence to argue that these Basketmakers, at least, were already very dependent on maize agriculture. These lines of evidence were:
- Settlement patterns, which were found to be quite similar to later Pueblo II and Pueblo III patterns in emphasizing locations with good agricultural potential.
- Material from the trash midden at Turkey Pen Cave, which showed abundant maize and squash remains.
- Analysis of coprolites from the Turkey Pen Cave midden, a very direct source of dietary information, which also showed abundant evidence of maize.
- Stable carbon isotope analysis of human skeletal material, which showed heavy use of maize in both Basketmaker and Pueblo remains, especially compared to a single Archaic burial.
This last will require some explanation. Basically, carbon has three naturally occurring isotopes: Carbon-12, the most common; Carbon-13, another, less common stable isotope; and Carbon-14, a radioactive isotope that decays over time at a known rate and becomes Carbon-12. The ratio of Carbon-12 to Carbon-14 is used in radiocarbon dating, but for the type of analysis we are talking about here the more important ratio is that between Carbon-13 and Carbon-12. This ratio is affected by the metabolic processes used by plants in carbon fixation during photosynthesis. There are three processes plants use to fix carbon, known as C3, C4, and CAM. The details are complicated, but the upshot of this distinction is that most plants, especially those in temperate climates, use the C3 pathway, but some plants, particularly grasses from hot, arid environments, use the C4 pathway instead. (The CAM pathway is also used by some plants in arid environments, but it is not very important for stable carbon isotope analysis.) Due to the differing nature of the chemical processes involved in the C3 and C4 pathways, they result in different ratios of Carbon-13 to Carbon-12, with C4 plants having noticeably higher ratios.
The importance of all this for studies of prehistoric diet is that maize is a C4 plant, while most plants native to the Southwest, like most plants in general, are C3. There are a few Southwestern C4 plants, such as some amaranths and chenopods, but it can generally be assumed that isotopic evidence for heavy reliance on C4 plants indicates a maize-based diet. The research on human remains from Cedar Mesa showed very high reliance on C4 plants among the Basketmaker and Pueblo individuals. The Archaic burial also showed some reliance on C4 plants, probably from chenopods and dropseed, both C4 plants that are known from Archaic period coprolites in the area, but much less than the later burials.
Objections could be raised to any one of these lines of evidence, but taken together they are quite strong. It’s important to note, however, that this type of evidence cannot directly confirm or refute the gradualist or migrationist hypotheses for the introduction of agriculture to the Southwest. After all, knowing that people were already dependent on agriculture in the Basketmaker II period does seem to contradict the idea that they only became dependent on agriculture much later, but it still doesn’t establish when and how agriculture was first introduced to the area. It could have been by a migration from the south at the beginning of the Basketmaker II period, but it could also have been by diffusion during the Late Archaic, practiced on a small scale for a few hundred years and gradually integrated more and more into the subsistence system until it began to dominate it by Basketmaker II. That is, the gradualists could still be right about the process even if they had the timing wrong. What the migrationists need is direct evidence for a migration, but that is fiendishly difficult to find in the archaeological record in general. Recent discoveries of Archaic maize and squash in more and more parts of the Southwest, including Chaco Canyon, pose a problem for the migrationists and seem to most easily support a gradualist account, but again, maybe the migration just took place much earlier.
It’s also possible that both sides are right. Maize and squash may have diffused northward, possibly from multiple sources in Mesoamerica, during the Late Archaic, and been adopted casually into local hunting and gathering economies over a long period of time, with a migration of agriculturalists coming later and causing major changes in those local adaptations that ended up resulting in widespread agricultural dependence throughout the region. The wide variety of types of maize found in Archaic contexts, and the differences between them and the more standardized types used during the Basketmaker and Pueblo periods, argues in favor of something like this.
Much recent research on issues like this using stable-isotope techniques has come out of the Archaeological Center Research Facility at the University of Utah Department of Anthropology. One important paper looked at Western Basketmaker II burials, mostly from the Kayenta area and excavated by the pioneering expeditions of Alfred Kidder and Samuel Guernsey in the early twentieth century, analyzing both carbon isotope ratios (to test for dependence on maize) and the ratio of Nitrogen-15 to Nitrogen-14 (a measure of the amount of meat in the diet). They also radiocarbon dated all the remains. The results were perhaps not unexpected, but still interesting. The researchers found plenty of evidence of maize dependence in all the individuals sampled, comparable to other samples from later Pueblo sites in the same region as well as Chaco great house burials (more on that later), suggesting, like Matson and Chisholm before them, that people in the Southwest were already as dependent on maize during Basketmaker II as their descendants would be later. They also found evidence for relatively low levels of meat consumption among most individuals. Although they are clearly part of the migrationist camp, they are quite aware that their data here, while interesting, are not dispositive when it comes to the gradualist/migrationist question:
Our data strongly suggest that in the study area the presence of Basketmaker material culture signifies heavy reliance on maize. However, these data do not indicate that the appearance of maize was synonymous with the appearance of Basketmaker groups or resolve questions regarding possible farming activities among Late Archaic foragers and their relationship to the Basketmaker complex.
There are still a lot of open questions about the introduction of agriculture to the Southwest, but I find research like that presented in these two papers pretty convincing in showing that however it arrived, it was there and very important at least as early as the Basketmaker II period.
Coltrain, J., Janetski, J., & Carlyle, S. (2007). The Stable- and Radio-Isotope Chemistry of Western Basketmaker Burials: Implications for Early Puebloan Diets and Origins American Antiquity, 72 (2) DOI: 10.2307/40035815
Matson, R., & Chisholm, B. (1991). Basketmaker II Subsistence: Carbon Isotopes and Other Dietary Indicators from Cedar Mesa, Utah American Antiquity, 56 (3) DOI: 10.2307/280894