IMAGE: Smithsonian scientists Dolores Piperno (right) and Irene Holst (left) growing teosinte under modern (chamber on the left) and past (chamber on the right) climate conditions. Photo: STRI photographer Sean Mattson.
Equipped with only a pair of garden shed-sized, perfectly ordinary-looking glass greenhouses at the Smithsonian Tropical Research Institute in Panama, archaeobotanist Dolores Piperno and her team have spent the past four years engaging in agricultural time-travel, in a quest to understand the origins of corn.
Corn is ubiquitous, yes — but it’s also mysterious. It’s the most grown crop in the world, turning up in a quarter of everything for sale at your local supermarket, and consumed in such industrial quantities that, in a carbon analysis, “North Americans look like corn chips with legs.”
But while corn was busy taking over the world, paleo-archaeologists were equally industriously trying to work out where it had come from. Until relatively recently, writes evolutionary biologist Sean B. Carroll in The New York Times, “many botanists did not see any connection between maize and other living plants. Some concluded that the crop plant arose through the domestication by early agriculturalists of a wild maize that was now extinct, or at least undiscovered.”
IMAGE: Teosinte and modern corn compared. Credit: Nicolle Rager Fuller, National Science Foundation.
The problem is that corn looks nothing like the plant that turns out to be its closest wild relative, a grass called teosinte that can still be founding growing in southwestern Mexico. Although botanists in the 1930s were able to breed fertile hybrids from teosinte and corn (hinting at their genetic ties), the differences in the appearance and form of the two plants were so significant that it was easier to believe that teosinte was more closely related to rice than today’s giant yellow cob-producing maize.
As geneticist John Doebley, whose landmark 1990 paper supplied DNA evidence that a particular subspecies of teosinte, Z. mays subspecies parviglumis, was indeed corn’s long lost ancestor, put it: “The stunning morphological differences between the ears of maize and teosinte seemed to exclude the possibility that teosinte could be the progenitor of maize.”
And yet it was—with the lack of family resemblance being attributed to more than a thousand years of artificial selection by human farmers.
Which leaves the question, what on earth made these fledgling farmers choose to invest their time and energy into cultivating such an awkward, skinny-earned, tough-kernelled, hard-to-harvest grass in the first place?
This is the question that Dolores Piperno managed to answer by using her time-traveling Panamanian greenhouses to speculatively reconstruct the teosinte that would have been around when Mesoamerican foragers-becoming-farmers were first beginning to domesticate it (as opposed to the wild teosinte we know—and, for the most part, consider a weed—today).
IMAGE: Teosinte growing in the greenhouse set to late-glacial conditions, from “Teosinte before domestication: Experimental study of growth and phenotypic variability in Late Pleistocene and early Holocene environments,” by Dolores R. Piperno, Irene Holst, Klaus Winter, and Owen McMillan, published January 31, 2014, in Quaternary International.
In a blog post on the Smithsonian site, Piperno explains that she and research collaborator Irene Holst planted a dozen identical seeds of teosinte in both glass cubes on the same day, but, in one of them, they tweaked the climate to recreate “the conditions that this wild grass probably faced 11,000-10,000 years ago,” during the late-glacial period, when “it’s 2C degrees cooler and the atmospheric carbon dioxide levels are around 265 parts per million.”
In other words, while one set of teosinte seeds germinated and grew under today’s temperature and atmospheric carbon dioxide levels, their neighbours, just a couple of metres away, lived their lives in a late-Pleistocene bubble.
Piperno repeated the experiment three times, and then grew out their descendents in an atmospheric recreation of the early Holocene. For each growth cycle, the time-traveling teosinte was accompanied by a contemporary analogue in the second greenhouse. The results were quite a surprise.
IMAGE: (A) shows a “maize-like phenotype plant from the Late Glacial Chamber. Like maize, it has a single tassel that terminates the main stem, female ears at the main stem (arrows) that terminate a few, very short lateral branches, and no secondary branching. The inset at the upper right is a close-up of one of the female ears.” (B) shows “teosinte in the Modern Control Chamber. Like in modern natural populations, it has many long, primary lateral branches (example, upper white arrow) terminated by tassels (black arrow).” From “Teosinte before domestication: Experimental study of growth and phenotypic variability in Late Pleistocene and early Holocene environments,” by Dolores R. Piperno, Irene Holst, Klaus Winter, and Owen McMillan, published January 31, 2014, in Quaternary International.
It turns out that eleven thousand years ago, in the late-glacial period, teosinte plants actually looked a lot more like contemporary corn than teosinte does today. In response to the lower temperatures and different atmospheric composition of the past, Piperno explains, a teosinte plant will “exhibit characteristics more like corn; a single main stem topped by a single tassle, a few, very short branches tipped by female ears and synchronous seed maturation.”
What’s more, the early Holocene plants, grown in atmospheric conditions that mimicked those at the point at which teosinte gatherers would made the transition to “persistent cultivators,” maintained their maize-like qualities in a second generation. They were easier and more efficient to harvest than contemporary teosinte, and the single stem architecture meant their nutrients were more concentrated, too.
Of course, the seed size and yield of these artificially ancient plants was much lower than that of their great-great-great grandchild, corn, today. Nonetheless, Piperno’s experiment adds an intriguing layer of complexity to the human/corn relationship. Rather than being one of the more extreme examples of the power of human desire to reshape nature, the form of today’s corn is, in part, the product of the plant’s own ability to change in response to its environment.
And the mystery of why early farmers chose to make maize a staple seems a little less mysterious when it turns out that its ancestors naturally exhibited some of the compelling characteristics that have led to its contemporary conquest of 84 million acres of the United States.
IMAGE: Table showing temperature in the greenhouses, from “Teosinte before domestication: Experimental study of growth and phenotypic variability in Late Pleistocene and early Holocene environments,” by Dolores R. Piperno, Irene Holst, Klaus Winter, and Owen McMillan, published January 31, 2014, in Quaternary International.
IMAGE: Dolores Piperno measuring teosinte. Photo by STRI photographer Sean Mattson.
Meanwhile, Piperno’s paper ends on a thought-provoking note, suggesting that her time-traveling greenhouses could be used to explore the possible range of plant morphological responses to our climate-changed future. Indeed, it’s possible to imagine generating a speculative 3D corn “morphospace” that maps the formal adaptations of corn to a series of potential atmospheres, as a way of understanding the plant’s own design vocabulary.
More practically, leaked projections from an IPCC report due out in March suggest that, under current models of climate change, maize yield will decrease by two percent each decade this century—but these assumptions are derived from twentieth-century data on the impact of heat waves on corn harvests. Piperno’s work implies that the shape of corn itself might change, too, in response to shifts in temperature and atmospheric carbon—and that could have a dramatic impact on its harvestability.