nature

Closing in on the domestication of the Americas' principal native grain
by Eric Jackson

North Americans call it "corn," but in England and in most English Bible translations that word applies to all food cereals, so we'll use the Spanish-cognizant and specific word, "maize." Hot buttered corn on the cob? That's a popular way of eating maize.

Over much of a professional lifetime's work, Dr. Dolores Piperno has been tracing maize back to its origins. At one point, the oldest residues of the grain were found in Panama, and that largely demolished the notion that maize was of Andean origin. But the search has taken her farther back and to the north and west, to the point that we very likely now know the approximate location of maize's first domestication. The time line is a bit less precise.

Piperno, an anthropologist, developed a science of analyzing phytoliths --- microscopic stone-like bits found in plant starches and pollens, with different sizes and shapes corresponding to different plants. Using techniques that Piperno pioneered it's possible to dig a grinding stone or cutting implement from an ancient sediment in which all organic material's DNA would have long ago degraded past the point of recovery and identification, wash the implement and examine the dust that comes off, and find phytoliths that will identify which plants were cut or ground with that utensil.

Plant identifications made under microscopes have thus been telling us a story of vast cultural and trading networks in the Americas, of the spread of maize, peppers and squash far from the places where they were domesticated from their wild ancestors. For example, long before any human migrations of which we have any evidence in oral traditions, linguistic analysis or DNA, the phytoliths tell us of certain peppers making it from high in the Andes to the coral islands of the Bahamas. The peoples of the ancient Americas, so it seems, were not so primitive about trade and cultural exchanges. By the time of the European Conquest certain crops were found throughout most of the Americas and were so well established that nobody remembered their origins.

Now a team of five scientists, three of them associated with the Smithsonian Tropical Research Institute (STRI), have tracked the origin of maize to the Central Balsas River Valle in the Mexican state of Guerrero. Piperno, an emeritus professor from Princeton who has long been associated with STRI and is currently also affiliated with the National Museum of Natural History in Washington; Anthony J. Ranere, an archaeologist with STRI and Temple University; STRI researcher Irene Holst; Temple anthropologist Ruth Dickau and University of Exeter (UK) archaeologist José Iriarte have published an article about their findings in the scientific journal PNAS.

While other colleagues were looking in the Mexican highlands, Piperno, having noticed that the wild ancestor of maize, a local strain of teosinte (Zea mays ssp. parviglumis) survives and still grows wild in the lowlands of the Central Balsas River Valley, began looking in low places and in lake sediments found charcoal traces and squash and corn phytoliths that indicated that at least 7,000 years ago there was slash-and-burn agriculture going on in the area.

(The identification of the Balsas River Valley strain of teosinte as maize's ancestor was made by molecular biologists Hugh Iltis and John Doebley about a decade ago. This is one of five species of teosinte grass, whose combined ranges cover parts of Mexico, Guatemala and Nicaragua.)

Going back 7,600 years, Piperno had previously found maize in Panama, but her lake sediment findings in Mexico were "at least." At this point the archaeologists, led by Dr. Ranere, went looking for ancient dwellings. They found several, all but one of which had been contaminated, eroded or otherwise compromised. The jackpot was the Xihuatoxtla Shelter, a spot under an enormous overhanging boulder near the contemporary town of Tlaxmalac.

Digging down through the layers, the scientists found ceramic shards and stone tools toward the top, then stone tools in layers from times before the development of ceramic. Here and there bits of charcoal that could be radioactive Carbon-dated were found, allowing the researchers to put approximate dates on those layers. Both in the ceramic and pre-ceramic layers, the food preparation utensils were covered with maize phytoliths. The oldest bit of carbon dated from the shelter was from approximately 8,700 years ago. However, there was another layer of human detritus below that, in which there was a grinding stone with maize phytoliths but no carbon to establish a date.

Thus we can say that most probably, maize was first domesticated in the Central Balsas River Valley, sometime before 6700 BCE. Archaeological finds elsewhere in the valley show human habitation going back to at least 7000 BCE, and suggest that it actually goes back thousands of years before that. The patterns that the researchers found was of crops planted at the edges of lakes, on banks cleared by fire and in soil exposed during dry season when lake levels went down.

It appears that in space, scientists are within less than 100 kilometers of knowing the location at which maize was first domesticated. In time, however, they may be off by millennia.

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