Before Cortés: Mesoamerica's Own Logic · Day 1 of 7

The crop that had to be cooked with lime

In a rock shelter called Xihuatoxtla, in the Balsas River valley of Guerrero in southern Mexico, archaeologists found grinding stones with starch grains stuck in the pits of the surface. The grains were maize. They date to roughly 8,700 years ago. Somebody was already sitting there crushing the stuff into flour.

What they were grinding did not look like corn. Its wild ancestor is a grass called teosinte, and it still grows on those hillsides. A teosinte ear is about the length of your little finger. It carries two rows of five to twelve kernels, each one sealed inside a shell so hard you can crack a tooth on it. When the seeds ripen the ear shatters on purpose and throws them onto the ground, which is excellent behaviour for a wild grass and hopeless for a farmer. The plant grows bushy, with many stems and many tiny ears.

Maize is the same species, rearranged. One stalk instead of many. One or two big ears instead of dozens of small ones. Hundreds of kernels held in place on a cob that does not shatter, with the hard shell reduced to a papery husk you can rub off with your thumb. For a long time botanists assumed that a change this large needed a long list of genetic changes. It does not. A handful of genes do most of the work. One, called teosinte branched1, switches the plant from bushy to single-stalked. Another, teosinte glume architecture1, softens the shell around the kernel. The plant was waiting on a small number of lucky mutations and people willing to keep replanting them.

It was still slow. The oldest actual cobs we have come from Guilá Naquitz, a cave in Oaxaca, and date to about 6,250 years ago. They are a few centimetres long, with maybe six rows of kernels. Thousands of years after the first grinding stones, maize was still a snack, not a diet. Real dependence on it — maize as most of what people ate — came later, in the third and second millennia BC, and it came with beans and squash grown in the same plot, the combination later called the milpa.

Once it arrived, maize was extraordinary. It yields more food per hectare than almost anything else you can grow with a digging stick. It dries hard and stores for a year or more in a granary. It does not need a threshing floor, an ox, or a plow. It grows from sea level to well over 2,000 metres, in wet soil and dry, on slopes too steep for a field of anything else. If you want to feed a hundred thousand people in a mountain basin without draft animals, maize is the answer, and in Mesoamerica it was more or less the only answer.

There is a catch, and the catch nearly disqualifies the whole plan.

Maize contains niacin, which we also call vitamin B3. But in maize the niacin is chemically bound up in a form the human gut cannot release. You can eat maize all day and absorb almost none of it. Maize is also short on tryptophan, one of the building blocks of protein, which the body can otherwise convert into niacin. So a diet that is mostly plain maize is a diet sliding toward niacin deficiency, and niacin deficiency has a name: pellagra. It starts with cracked, scaling skin on sun-exposed arms and necks. Then diarrhoea. Then confusion and memory loss. Then, often, death. Doctors in Spain described it in 1735. Northern Italy suffered it through the nineteenth century. In the American South it became an epidemic after maize-heavy diets took hold in poor rural districts; Joseph Goldberger proved from 1914 onward that it was caused by diet and not by an infection, and the commonly cited toll for the United States between 1906 and 1940 runs to around three million cases and about a hundred thousand deaths.

Mesoamerica ate maize for three thousand years and did not get pellagra. The reason is a cooking step.

You take dried maize kernels and boil them in water with something strongly alkaline in it — wood ash, or better, cal, slaked lime made by burning limestone or shells and slaking the result with water. You let the kernels steep, sometimes overnight. Then you rinse them, rub off the loosened skins, and grind the soft wet grain into dough. The dough is masa. Flattened and cooked on a griddle it is a tortilla; wrapped and steamed it is a tamal. The Nahuatl word for the process gives us the modern term: nextamalli, from nextli, ashes, and tamalli, dough. Nixtamalization.

The alkali does four things at once. It breaks the chemical bond holding the niacin, so the vitamin becomes available to the body. It improves the balance of amino acids in the grain, which makes the protein in maize usable rather than lopsided. It loads the grain with calcium from the lime — nixtamalized masa carries many times the calcium of untreated maize, and in much of rural Mexico tortillas have long been the main source of calcium in the diet, with no milk anywhere in the picture. And it softens the kernel so it can be ground at all, and makes a dough that holds together instead of crumbling.

That last point mattered every single day. Lime cooking turned maize from a hard seed into a plastic dough. It also cost labour. Grinding wet maize on a metate, a stone slab worked with a stone roller, took hours each morning, and it fell to women. Skeletons from Mesoamerican sites show the wear of kneeling and pushing. A city of a hundred thousand people was also a city of tens of thousands of grinding stones going before dawn.

We do not know who worked it out, or when, with any precision. Lime-processing gear turns up in the archaeological record well before the first great cities, which is the answer you would expect: dense settlement came after the cooking method, not before. No pellagra appears in the prehispanic skeletal or documentary record.

Here is the part worth carrying away. After 1492 Europe took the plant and left the recipe. Maize crossed the Atlantic in a sack, spread across Italy, the Balkans, Egypt and the American South, and fed millions of poor people who had never heard of lime water. Pellagra followed it everywhere it went.