Popcorn origins glimpsed in the ruins of antiquity and the greenhouse time machine
Before there was corn (Zea mays subsp. mays), and corn ruled the world, there was the wild grass, teosinte. Corn, known as maize outside the Americas, easily hybridizes with its wild sibling, but these two incarnations of Z. mays do not look alike. Subtle genetic alterations in the regulation of development change Z. mays profoundly, turning an edible wild weed into the economic powerhouse con. Teosinte’s body plan structure and mechanics are also plastic in different climatic conditions, Smithsonian researchers have recently found.
The ur-maize, Zea mays parviglumis, is one of four wild teosinte subspecies that grow in Mexico’s Rio Balsas valley and in parts of Guatemala and Nicaragua. Its domesticated sibling has conquered large swathes of the world. Maize monoculture covers 80 million acres of the US alone, and big patches of Brazil, Indonesia, and India. But its alterations assure that its sole means of dispersal and competitive advantage is Homo sapiens.
Teosinte grows tall like its cultivated counterpart, but long side branches with many ear-like spikes emerge from its main stalk at junctures where the vertical maize grows only single, giant ears.It yields seeds over the course of months, bringing forth narrow ears of 5 – 12 kernels, sealed safely inside dark, mottled, rock-like casings, which peel away from the cob at maturity to disperse their genetic legacy to the wide world. The hard case protects the seed from the digestive tracts of birds and mammals – and you. Teosinte is far too hard to eat with joy, or easily grind into meal.
Through cultivation and selection, the ancient people of Mesoamerica derived strains of Z. mays that produce hundreds of naked, soft kernels, maturing in synchrony and clinging to the cob for easy harvest.
They began this domestication about 9,000 years ago, based on genetic evidence and carbon-14 dating of starch grains and the silicon molds of cellular interiors found at Xihuatoxtla shelter in the Rio Balsas drainage of modern day Guerrero, Mexico.
But domestication takes time. How did the ancient Americans eat their early efforts, if teosinte seeds are so hard and uncooperatively recalcitrant? They made popcorn, says Dolores Piperno, curator of New World archaeology at the Smithsonian’s National Museum of Natural History.
Piperno and colleagues have unearthed a long history of maize from the ancient floors of prehistoric settlements on the dry northern coast of Peru, near Trujillo, far afield from the native range of teosinte. Charred kernels and cob fragments are at least 6500 year old, and of a size, hardness, and popping ability typical of popcorn varietals. A hard hull aids the popping action by sealing in and superpressurizing water as the kernel heats, melting the starch of the seed. When the hull abruptly bursts, the steam and starch poof. Piperno and the research team also found evidence of corn flour.
Piperno has been growing teosinte in a greenhouse that simulates the early Holocene climate of 10 to 12 thousand years ago, at the dawn of maize domestication, and she has observed a surprising change that may have encouraged the early experiments in cultivation. In temperatures 2 – 3 degrees Celsius cooler, with lower atmospheric carbon dioxide (270 ppmv; CO2 subsequently held at 330 ppmv for 10 thousand years, and is at 405 ppmv in our current industrial era), teosinte grows a tall main stem topped with a male tassel, and short side branches end in a single, female spike. Its seeds mature in synchrony. It looks like maize.
- Piperno, D.R., et al., Teosinte before domestication: Experimental study of growth and phenotypic variability in Late Pleistocene and early Holocene environments. Quaternary International (2014). http://dx.doi.org/10.1016/j.quaint.2013.12.049
- Grobman, A., et al. Preceramic maize from Paredones and Huaca Prieta, Peru, PNAS 109(5) 1755 http://dx.doi.org/10.1073/pnas.1120270109