It’s odd that there’s such a hole in the historical literature when it comes to maize in the north. But it was such a novel thing, and it seems that words failed the European adventurers. Two explorers, commissioned by King Francois I of France, Giovanni Verrazano and Jacques Cartier, may have referred to maize in rather oblique terms that have been missed in the past. These two were both exploring, and writing about their discoveries, in the 1520s and 1530s. Verrazano writes about an excellent and delectable ‘legume’ that he tastes when meeting Native Americans living near Chesapeake Bay. Later French texts describe maize as a legume. Cartier, exploring what would become Quebec, describes ceremonial feasts involving ‘gros mil’ – a term for sorghum, which has surely been appropriated here for maize.
It seems clear that there were ample opportunities for an early introduction of North American varieties of maize into northern Europe – from the end of the fifteenth century into the first half of the sixteenth. More recent genetic analyses strongly suggest that there were indeed multiple introductions of Northern Flints into Europe. Cabot and son, Verrazano and Cartier, are just a few of the pioneers who could have brought Northern Flints back with them. As well as coming back with official voyages of discovery, maize probably hitch-hiked with unofficial Atlantic fishing expeditions. And in contrast to the tropical Caribbean maize, the North American varieties were already adapted to temperate climates – they would have thrived immediately in central and northern Europe.
The genetic story of maize plays out in a similar way in eastern Asia. Maize in tropical latitudes, from Indonesia to China, is closest to Mexican maize. But this time, history supplies the details – the Portuguese introduced maize into south-east Asia as early as 1496, and another wave of maize arrived with the Spanish colonisation of the Philippines in the sixteenth century. The genetic map of maize in Africa is complicated, with early introductions of South American maize to the west coast by Portuguese colonisers in the sixteenth century. This history echoes in the African names for maize – mielie or mealies – which derive from the Portuguese word for maize, milho. Later, from the nineteenth century onwards, varieties from the southern half of North America, known as ‘Southern Dents’, were introduced to eastern and southern Africa. Up in the north-west corner of Africa, there’s evidence of Caribbean ancestry – just as in southern Spain. That Caribbean genetic signal is also scattered across western Asia, from Nepal to Afghanistan. Linguistic and historical clues support the role of Turkish, Arab and other Muslim traders in the spread of maize from the Middle East, by sea and by land – from the Red Sea and the Persian Gulf out into the Arabian Sea and eastwards to the Bay of Bengal; along the Silk Road and through the Himalayas.
But it’s the DNA of maize from the middling latitudes of its new homes around the world that’s most fascinating. In the north of Spain and the south of France, European maize is equally related to North American and Caribbean types. It looks like hybridisation created the perfect in-betweener – as early as the seventeenth century. Strains of maize which had diverged away from each other, adapting to different environments, in the Americas, were brought back together in the foothills of the Pyrenees.
The dissemination of maize across the world was astonishingly fast. Genetic analysis and molecular dating suggests that maize was domesticated around 9,000 years ago in the Americas. It stayed in this region for 8,500 years, going global in just the last 500 years. But in fact its spread was even faster than this implies – the documentary evidence shows that maize had spread right across Eurasia, from Spain to China, in just six decades after Columbus first brought it over from the Caribbean. In some ways, this spread and adoption seems quite extraordinary – these were regions of the world where agriculture had been practised for millennia, and there were already well-established fields of wheat and rice to provide populations with staple foods. The historical records show that farmers didn’t immediately swap their traditional crops for this new grain. Instead, maize was often grown on marginal land, by impoverished farmers trying to eke out a living in relatively barren areas. It was considered a food of the poor – and yet, once it had a foothold in the Old World, the global future of maize was assured. Its sheer variety, and ability to grow in such a wide range of environments, meant that – as soon as it crossed the Atlantic – it was poised to spread throughout the world.
American origin
Back in the Americas, genetic studies have been crucial, not only in estimating the timing of maize domestication, but in tracking down the identity of the wild progenitor, establishing how many times maize was domesticated, and where this happened. Maize is a subspecies – Zea mays mays – and there are three other subspecies within the same species – all of which are wild, and known more colloquially as teosinte: a name that comes from the Aztec language of Guatemala. The Aztecs venerated maize, in the forms of the goddess Chicomecoatl and the god Cinteotl.
The three teosintes – Zea mays huehuetenangensis, mexicana and parviglumis – grow wild in Guatemala and Mexico. Although the teosintes look quite distinct from their domesticated cousin, maize hybridises freely with all of them. If we imagine evolution as a branching tree, it seems likely that one of these cousins will be closer to maize than the others, and may even represent the surviving wild descendants of the original population that was also domesticated.
Analysis of enzymes in maize and the teosintes had suggested that one of the wild subspecies was indeed more similar to maize than the others. And in 2002, this was confirmed by a large genetic study. Having tested 264 samples in total – of maize and the three teosintes – the geneticists found that Mexican annual teosinte Zea mays parviglumis was closest to the domesticate.
As the study contained so much data on American maize populations – 193 of the 264 samples were from maize – it was also possible to construct a phylogeny, a family tree, for this domesticate. All the maize lineages – from the temperate-adapted Northern Flints to the tropical types in Colombia, Venezuela and the Caribbean – tracked back and coalesced, converging on a single stem. So maize was domesticated just once. Or at least, if it was domesticated several times, only one, branching lineage has survived to the present. The stem of the phylogenetic tree was rooted in Mexico. But pinning down the place where the domestication first started was tricky. The most primitive form of domesticated maize on that phylogenetic tree grows in the highlands of Mexico. But the closest wild relative is a lowland plant: it’s the Zea mays parviglumis of the Balsas River Basin of central Mexico, or Balsas teosinte.
By the time this genetic information emerged, the earliest evidence of maize in the archaeological record – in the form of whole cobs – came from the Mexican highlands, dating to 6,200 years ago. So it seemed that, either Balsas teosinte had been carried up into the mountains to be planted, or it was first domesticated down in the valleys, spreading to higher altitudes later.
Over thousands of years, climate and environments have changed quite a bit, and species will have shifted accordingly. But, given the new genetic data and the identification of the closest wild relative to maize, archaeologists believed it was still worth having a good look down in the Balsas Valley. And so they began to scour the area for traces of ancient cultivation and domestication. What they needed was something which would clearly distinguish the wild from the domestic.
When it starts growing, teosinte can be difficult to distinguish from its domesticated cousin, making it a vexatious weed in maize fields. But when it matures, it looks quite different. Each teosinte plant is bush-like, with branching stalks – whereas maize grows with a single, tall stalk. Teosinte ears are small and simple, with a staggered row of about a dozen kernels attached to a central rachis. Maize cobs are huge in comparison, crammed with hundreds of kernels. Teosinte kernels are small, and each contained in a hard case; maize kernels are large and naked. And just like wild wheat, wild teosinte ears shatter at maturity, whereas maize kernels stay firmly attached to a non-shattering rachis. Geneticists have b
een able to pinpoint just a handful of genes which have undergone mutations to produce the differences in branching, kernel size, fruitcases and seed shattering between teosinte and maize.
This is all very well, but down in the tropical lowlands, preservation of plant remains is pathetic at best – the archaeologists had no hope of finding whole plants, whole cobs, or even intact kernels. Instead, they turned their attention to much smaller components of plants – phytoliths and starch granules. Phytoliths are silica-rich and very resistant to degradation, meaning that they stick around, even in tropical places, for an incredibly long time. Both the phytoliths and starch granules of teosinte are – very usefully – characteristically distinct compared with those of maize.
The first evidence of these microscopic traces of early maize were discovered in lake sediments in the Balsas River Valley. The archaeologists followed up by excavating four prehistoric rock shelters in the region – and one of them, the Xihuatoxtla Shelter, yielded precious, early evidence of maize. Stone tools from the cave – in a layer dating to 8,700 years ago – contained diagnostic maize starch granules tucked into cracks and crevices. Maize phytoliths were also found on the stone tools, as well as being scattered throughout samples of sediment from inside the rock shelter.
The phytoliths provided further clues as to how the ancient Mexicans were using maize. It’s been suggested in the past that maize may have been cultivated, first and foremost, for its stalks. The hard fruitcases of ripe teosinte kernels would have made them unpalatable, whereas the sugary pith of the stalk could have been eaten or even used to make a fermented drink – a sort of teosinte rum. Phytoliths are different in the stalk and cobs of maize, and the archaeologists working on the samples from Xihuatoxtla found plenty of cob phytoliths but none from stalks. It seems that it was the grain that the early cultivators were most interested in – at least at this site. And the kernels appeared to have already undergone a genetic change associated with domestication, shedding their hard fruitcases – as no phytoliths from such cases were found. Other sites in Panama, dating to around 6,000 to 7,000 years ago (4000 to 5000 BCE), have presented a similar picture – of the use of cobs, not stalks. It’s still possible that hunter-gatherers may have used the sugary stalks of teosinte more than its grains, and switched to a focus on grains later, when the plant had already begun to develop domesticated features. But perhaps the difficulty of processing teosinte kernels has been overplayed. They can be made edible by soaking and grinding, and some Mexican farmers still use teosinte seeds to feed their livestock.
This discovery of early maize, in the seasonal tropical forest of the Mexican lowlands, is important. It significantly predates – by two and a half millennia – the previous evidence which was used to argue for an origin of domestication of this crop in the highlands. It also makes a lot more sense – Balsas teosinte, the closest relative of maize, grows naturally in the lowlands, not up in the mountains.
Yet, after all this sleuthing, there’s still a big, juicy question that remains. After 1493, this home-grown American crop rapidly spread all over the world, into myriad environments, getting a toehold even in some of the world’s most inhospitable landscapes. The global success of maize depended on its large portfolio of variation – but how had it developed all that astonishing variety, coming from a single origin in the lowlands of south-western Mexico?
Extraordinary and conspicuous diversity
In his book The Variation of Animals and Plants Under Domestication, published nine years after his Origin of Species, in 1868, Darwin wrote about the American origin, antiquity and wonderful variety of maize:
Zea mays … is undoubtedly of American origin, and was grown by the aborigines throughout the continent from New England to Chili. Its cultivation must have been extremely ancient … I found on the coast of Peru heads of maize, together with eighteen species of recent sea-shell, embedded in a beach which had been upraised at least 85 feet above the level of the sea. In accordance with this ancient cultivation, numerous American varieties have arisen …
Darwin didn’t know about that close relationship between annual Mexican teosinte, particularly that in the Balsas Valley, and maize. ‘The aboriginal form [of maize],’ he wrote, ‘has not as yet been discovered in the wild state.’ But then he gives an account of a young Native American man who told the French botanist Auguste de Saint-Hilaire about a curiously maize-like plant – but with husked seeds – that ‘grew wild in the humid forests of his native land’.
Darwin was impressed and intrigued by the ‘extraordinary and conspicuous manner’ in which maize varied. He believed that the dissimilarities between varieties had arisen as the crop spread into northern latitudes, developing an ‘inherited acclimatisation’ to different environments. He writes about the experiments of the botanist Johann Metzger, who tried growing various American varieties of maize in Germany – with remarkable results.
Metzger grew some plants from seeds obtained from a tropical region in America. And this is how Darwin described the outcome:
During the first year the plants were twelve feet high, and a few seeds were perfected; the lower seeds in the ear kept true to their proper form, but the upper seeds became slightly changed. In the second generation the plants were from nine to ten feet in height, and ripened their seed better; the depression on the outer side of the seed had almost disappeared, and the original beautiful white colour had become duskier. Some of the seeds had even become yellow, and in their now rounded form they approached common European maize. In the third generation nearly all resemblance to the original and very distinct American parent-form was lost. In the sixth generation this maize perfectly resembled a European variety.
This is such an astonishingly quick transformation. It seems much too quick to be down to a genetic change in the plants. It sounds more like physiological adaptation, or – if you can bear even more technical jargon – phenotypic plasticity. This concept relates to the latent potential – which is still governed by genes – for organisms to adjust, during a lifetime, to particular environments. Adult organisms usually have a limited ability to adapt physiologically or anatomically in this way. But organisms nurtured from birth, or grown from seed, in a different environment to their parents can end up looking quite dissimilar, and functioning differently too.
Darwin’s writing is brilliant in so many ways. He builds arguments beautifully, and he illustrates big ideas with carefully described, often personally experienced, details – like those ancient maize cobs which he found in the raised beach in Peru, 85 feet above sea level. Sometimes he’s laying out his argument, and providing evidence to support a particular theory. But at other times, you can almost feel the whirring of his mental cogs. He’s endlessly inquisitive and excited by new pieces of information that reach him. With Metzger’s tropical American maize grown in Germany, Darwin’s much less surprised by changes to the stem, and the time it took for seeds to ripen, than he is by the transformation of the seeds themselves. He writes: ‘It is a much more surprising fact that the seeds should have undergone so rapid and great a change.’ But then he almost argues with himself, introducing the dialectic into his own monologue: ‘As … flowers, with their product the seed, are formed by the metamorphosis of the stem and leaves, any modification in these latter organs would be apt to extend, through correlation, to the organs of fructification.’
In other words, flowers – and their seeds – develop out of the tissues of stem and leaves. So if stem and leaves are being modified by climate, perhaps it’s not so surprising after all that seeds change so much as well. In this passage, Darwin gets very close to understanding something that we can now appreciate from a genetic perspective. Separate parts of an organism are not always controlled by separate genes – far from it. The relationship between DNA, on the one hand, and the form and function of a whole organism, on the other, is much more complicated than that. A change in a particular gene can have widespread effects throughout the body of an organism – whe
ther that’s a human, a dog, or a maize plant.
With his discussion about the astonishing changes observed in tropical maize after just a few generations of growing in that less favourable climate in Germany, Darwin is also getting very close to that, much more recently articulated, idea of phenotypic plasticity. What we now know is that this doesn’t require a change in the DNA itself – what might be called a ‘true’ evolutionary change. It just requires a modification to the way the organism reads, or expresses, its DNA. Even without genetic mutations, phenotypic plasticity can be a source of extraordinary novelty. And yet so much research into the transformation of wild species into domesticated ones focuses purely on genetic mutations, sometimes forgetting just how much the phenotype can vary, without a change to the underlying DNA code. Metzger’s tropical maize, transplanted into a temperate climate, is a fantastic example of just how malleable the phenotype can be. And one recent study uncovered an even more surprising degree of plasticity than Metzger had demonstrated with his American maize.
Dolores Piperno is an archaeobotanist at the Smithsonian Museum in Washington DC. She led the investigation that found maize phytoliths in the Xihuatoxtla Shelter in the Balsas Valley. But as well as looking for ancient traces of long-dead plants, her research also involves doing experiments with their living counterparts. She led a team from the Smithsonian Tropical Research Institute in Panama, which – between 2009 and 2012 – set about examining just how important a factor phenotypic plasticity might have been in the variation produced in maize, as it became domesticated. They took the wild ancestor of maize, Zea mays parviglumis, and grew it in glasshouses under two sets of climatic conditions. One climate replicated that of the end of the Ice Age, between 16,000 to 11,000 years ago. The other was a control chamber, replicating the modern climate. As the plants in each chamber grew, the results were astonishing.
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