Wednesday, April 8, 2009

Maize domestication

It’s taken me a couple of weeks to find and read the new articles about the domestication of maize in Mexico that came out in the Proceedings of the National Academy of Sciences. Here are the full references:

Dolores R. Piperno, Irene Holst, Ruth Dickau, and José Irarte (2009). The Cultural and Chronological Context of Early Holocene Maize and Squash Domestication in the Central Balsas River Valley, Mexico. Proceedings of the National Academy of Sciences vol. 106, No. 13, pp. 1514-1518.

Piperno, Dolores R., Anthony J. Ranere, Irene Holst, José Irarte, and Ruth Dickau (2009). Starch Grain and Phytolith Evidence for Early Ninth Millennium B.P. Maize from the Central Balsas River Valley, Mexico. Proceedings of the National Academy of Sciences vol. 106, No. 13, pp. 1519-1524.

There was also a commentary about this pair of articles published in the same issue:

Hastorf, Christine A. (2009). Rio Balsas Most Likely Region for Maize Domestication. . Proceedings of the National Academy of Sciences vol. 106, No. 13, pp. 4957-4958.

You can read the abstracts on the journal website here.

These are important articles. The Ranere team excavated a number of rock shelters in the region of Iguala, Guerrero, looking for deposits containing evidence of the beginning of plant domestication in the Archaic period. They chose this area because the genetic evidence—that is, analyses of the DNA of maize varieties and teosinte varieties—pointed to this region as the heartland of corn domestication. Interestingly, they are focusing on the moister piedmont between 700-900 m elevation, rather than the arid highlands.

The first article, in which Ranere is the first author, describes the archaeological sequence that they found, mainly at Xihuatoxtla rock shelter. They seem to make a fairly sound argument, based on the stratigraphy, carbon dating, and artifact sequence, that they found a series of Archaic deposits. The second article describes the microbotanical evidence—phytoliths and starch grains—for the presence of maize cultivation and processing.

Here is a Google Earth snapshot of the Xihuatoxtla area where the Rockshelter is located.

These are nice articles, and they report very significant results. We’ve all been waiting to hear these results for some years now. We’ve known that the current earliest dates for maize domestication were much too late, and that the answer probably lay in the Balsas River Valley. These articles provide strong support for domesticated maize exploitation in the early ninth millennium (2-sigma calibrated date: 8990-8610 B.P.).

Nevertheless, I was a little disappointed. Perhaps I’m an intellectual Neanderthal, but I would have liked to seen some macrobotanical evidence in addition to the microbotanicals. I was disappointed that they didn’t find a single cob or kernel. Give me a cupule! In North America, we find carbonized maize remains in hot and humid parts of the southeastern U.S., which is not that different environmentally from the tropics. I’ve excavated maize macroremains in the Yazoo Basin in Mississippi, and I’ve also worked in various parts of southern Mesoamerica, and frankly, the Mississippi Delta feels more tropical. I’ve worked at Copán, Honduras, which is at 600 m elevation, about the same as these sites in Guerrero, and it’s fairly cool and pleasant by comparison. I still find myself a teeny bit skeptical about phytolith and starch grain evidence, if only because they are young fields and so few people do them that I don’t get the sense that there’s a community of experts who have arrived at a consensus on various important issues. The authors of these articles are undoubtedly world experts on precisely these issues—the identification of maize starch and phytoliths—one couldn’t ask for a better group. And they deserve tremendous credit for having developed the approaches painstakingly over many years. But I would be happier if their results could be independently validated. Or, of course, they could keep digging till they find a hearth full of carbonized corn cobs.

I also would have been happier if the most significant of the deposits, at Xihuatoxtla rock shelter, had been deeper with better stratigraphic separation. The deposits there are barely 1 meter deep and the first 50 cm are post-Archaic. In fact, there are sherds down 60 cm in Layer C too. So, it’s really just Layers D and E, only 35 cm thick, that contain the whole Archaic part of the sequence, stretching back to almost 9000 B.P. Are we to believe that those two strata all date to 8990-8610 B.P., the age of the sole radiocarbon date from those strata? Shouldn’t the deposits have built up over time, like the ones above them apparently did? Actually, that carbon sample came from the middle of Stratum D, so some of this material may be even older than that date. But I think there’s no doubt that their conclusions would have been much stronger if they had found a deeper stratigraphic profile associated with more radiocarbon dates. Ideally, one would like deeper strata without any maize exploitation overlain by the earliest maize processing. I expect the Ranere team was disappointed in some respects too. In addition, I think some discussion, however brief (these are articles after all, not monographs), on the geomorphology and taphonomy of the deposits might have alleviated concerns about their shallowness. The map of the excavation in the Supplementary materials suggests that the ground drops away in front of the rock shelter, a common situation which leads to downslope movement of materials over time. So, evidence that the deposits are intact would have solidified their argument.

Overall, the articles report on an impressive achievement. Most of my doubts are based on what they found, which is not under their control. It’s no accident that no one has excavated Archaic sites in the valley of the Río Balsas till now. These guys have spent years doing survey to find sites, performing paleoenvironmental reconstruction, excavating sites and performing artifact analyses. And it’s probably not an easy area to work. I love Mexico. I’d rather be there right now. But Guerrero is not the safest place in the world.

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