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Popped Secret: The Mysterious Origin of Corn

18 minutes

(female) Three dollars for six. Thank you. Thank you so much. This is an everyday scene, but it's actually pretty amazing. We've taken dozens of wild plants and transformed them into useful crops through the process of domestication. Humans have carefully bred these plants for generations to make them bigger, sweeter, more colorful. And it's hard to find a plant that we've transformed more completely than this one--maize. In the U.S., most call it corn. We eat a lot of it. There's corn bread, corn chips, corn cereal. If you look deeper, you'll find corn starch and corn syrup in hundreds of products. Much of the meat we eat comes from animals fed a corn-based diet. So maize is all around us. For years, the origin of maize was a mystery. The ancestors of wheat pretty much looked like wheat. The precursors of apples basically looked like apples. But there's nothing in nature today that looks like this. This is the story of an unexpected collaboration, the story of geneticists and archeologists working together to discover where maize really came from. Christopher Columbus's crew were the first Europeans to see maize. By the time Columbus arrived, people all over the Americas had been growing maize for thousands of years. Archaeological evidence from around the world reveals that starting around 10,000 years ago, humans were beginning to live in larger settlements and to manipulate wild plant and animal species to better suit their needs. In the case of plants, this process of domestication led to plants that we call crops, like wheat, apples, and potatoes. In most cases, the wild relatives of these crops can still be found in nature. But you can't find anything that looks like maize growing in the wild today. Even the earliest fossil ears of maize, which are more than 6,000 years old, already look, essentially, like today's crop. So, where did maize come from? Many scientists thought that the ancestor of maize must be extinct. But a brilliant young geneticist discovered something that made him think that maize's ancestor was right in front of us. His name was George Beadle. Beadle was studying a grass from Central America called teosinte. He found that teosinte's chromosomes looked nearly identical to those of maize. He also showed that teosinte and maize could produce fertile, hybrid offspring, meaning that they must be closely related. Beadle concluded that teosinte was likely the ancestor of maize. But many botanists doubted the young scientist's claims. Maize expert Dr. John Doebley at the University of Wisconsin told me why. Neil, I want to show you just how different corn and teosinte are. This is a teosinte plant. It doesn't look anything like a typical corn plant. Look down at the base. It just branches a lot. It is a bushy creature. Quite different from corn plants, such as you see here, where there's a single main stalk, no branches, except for these two short branches, each of which has an ear on it.

(Losin) The dramatic difference in branching between teosinte and maize is just the beginning. When you look at corn, you can see hundreds of kernels exposed on the cob. But teosinte is different. Each ear only has a handful of kernels, each enclosed in a fruit case that's so hard, you might crack a tooth if you tried to eat it. It was no wonder that botanists doubted that teosinte could be the ancestor of maize. Beadle moved on to other questions in genetics, which ultimately earned him the Nobel Prize. But the origin of maize continued to intrigue him. After his retirement, he returned to that question. To silence the skeptics, Beadle had to show how humans could have transformed this into this. After his retirement, he launched one of the biggest breeding experiments in history to settle that question once and for all. The key question was how many genes control the differences between maize and teosinte. If that number were small, it wouldn't have been too hard for early humans to transform teosinte into maize. He began by crossbreeding maize with teosinte. In most plants and animals, individuals inherit two copies of each gene-- one from each parent. So, the offspring from this first generation cross between teosinte and maize, the F1 generation, would have one copy of each gene from teosinte and one from maize. These F1 plants would then be crossed with one another to produce the F2 generation. This is where things get interesting. If only one gene differs between teosinte and maize, then 1 in 4 of the F2 plants should look just like maize, and 1 in 4 ought to look like teosinte. If 2 genes are at work, this number drops to 1 in 16. For 3 genes, it's 1 in 64, and so on. If more than 3 genes were involved, Beadle was going to need a lot of plants. He decided to grow 50,000 F2 plants for his experiment. And what did he find? About 1 in 500 plants looked identical to teosinte. And a similar number looked just like maize. The number suggested that changes in just 4 or 5 genes were responsible for all the major differences between the two plants. George Beadle was right. The real ancestor of maize was teosinte, and it was right in front of us all along. Many varieties of teosinte grow throughout Mexico and Central America. Humans have lived there for thousands of years. Where and when did they first transform teosinte into maize? Doebley's team set out to find the answer. They collected DNA samples from different teosinte varieties throughout Mexico to compare their DNA sequences to those of modern maize. The more closely related two groups of organisms are, the more similar their DNA sequences will be. Doebley's team looks for the teosinte variety with DNA sequences most similar to maize. We've figured out that all of modern corn traces back to one type of teosinte in the southwestern part of Mexico near a river called the Balsas River.

(Losin) The relatively small number of DNA sequence differences between maize and the Balsas River teosinte yielded another critical piece of information.

(Doebley) We can take teosinte and corn and ask how many mutations do they differ by and then knowing the rate at which mutations occur, make a prediction about how long ago their paths separated.

(Losin) The more differences in the DNA of two groups of organisms, the longer it's been since their ancestors were all one species. Our estimate is that the original domestication of corn would have taken place sometime around 9,000 years ago.

(Losin) Based on genetics, Doebley's team had come up with a hypothesis about where and when maize was domesticated. But the ultimate test would require independent evidence from outside the field of genetics. I visited Doctor Delores Piperno, at the Smithsonian Tropical Research Institute in Panama, to see that evidence. You're an archaeologist. What did you think when this geneticist from Wisconsin, analyzing DNA, said, "Here's where we need to look for the earliest evidence of maize domestication."? Teosinte is distributed all over Mexico-- highlands, lowlands, it gets down into Nicaragua. The question for archaeologists was where do we go? Dr. Doebley's work told us exactly where to go.

(Losin) Nine thousand years ago, inhabitants here were taking shelter and preparing food in caves and rock shelters. We went to the central Balsas Valley and asked local people, "Do you know of any caves or rock shelters?" That's how we found the Xihuatoxtla Shelter. People took shelter, slept, and probably ate there. They ate there, they cooked their food there.

(Losin) But finding evidence of ancient maize wouldn't be easy. In the tropical environment of ancient Mexico, the cobs and kernels would typically be scavenged or decomposed. But Dr. Piperno wasn't looking for such obvious evidence. These were the earliest plant-processing tools. We call them plant grinding stones. That was their use. These are no more than river cobbles.

(Losin) Dr. Piperno showed me how ancient people used these stone tools to grind up maize and other crops. In the process, tiny plant pieces might be deposited on the tool's surface... leaving behind microfossils.

(Piperno) We found hundreds of these microfossils on the grind surface of the stone. And like the seeds, they're very highly diagnostic. Even with these microscopic traces, you can tell the difference between corn and teosinte? Yes, we can tell the difference.

(Losin) Finding maize microfossils on the grinding tools meant that the humans living in the Xihuatoxtla Shelter were processing maize for food. But how long ago? Archaeologists can calculate the age of ancient remains using radiocarbon dating. But microfossils are too small to date using this method. So, Dr. Piperno used charcoal found in the same sediment layer as the grinding stones to determine the age of the microfossils. What was the oldest date of these maize remains? It's interesting how well the genetic and archaeological data fit together. The oldest charcoal date we received back was about 8,700 years ago.

(Losin) That date coincided almost perfectly with the date Dr. Doebley predicted from the genetic evidence. So, nearly 9,000 years ago, humans had already produced an early version of maize. But how was teosinte transformed into maize? Back in Dr. Doebley's lab in Wisconsin, I learned about the genetic changes involved. One of the main differences between teosinte and maize is that the teosinte seeds are encased in this hard fruit case that makes it difficult to eat. So clearly, that's something that had to change. That's right, and what's remarkable is that having a fruit case versus not having a fruit case is basically controlled by a single gene.

(Losin) To test this gene's function, Dr. Doebley's team did a clever experiment. They carefully crossbred maize and teosinte to introduce the maize version of the fruit case gene into teosinte plants. When they did that, the teosinte kernels, which are normally enclosed in a hard fruit case, became partially exposed, almost like little corn kernels. When they did the opposite, putting the teosinte fruit case gene into maize plants, the fruit case became larger and started to cover up the maize kernels, similar to teosinte. One gene makes a pretty dramatic change. Another obvious difference between teosinte and corn is that teosinte produces dozens of these tiny ears on a plant that branches a lot. Corn produces a couple of ears on a plant that hardly branches at all. What's going on? One gene we've identified plays a central role in that process. We call it the branching gene.

(Losin) Dr. Doebley explained how putting the teosinte version of the branching gene into maize made the maize plants more branched, like teosinte. And putting the maize version of the gene into teosinte made the teosinte plants less branched. Dr. Doebley showed that the fruit case gene, the branching gene, and others, a small number of genes as George Beadle predicted, were responsible for setting in motion all the major differences between maize and teosinte. But how could so few genes cause such huge changes? Why were these genes so powerful? They both belong to a special class of genes called regulatory genes. These are genes that directly regulate the activities of other genes. When we move the teosinte version of one of these genes into a corn plant or vice versa, we're actually changing more than just that one gene? Yes, they can turn other genes on and off. You could think of these genes like an orchestra conductor. If you take the conductor from one orchestra and give that orchestra a new conductor...

(Losin) Like moving genes from teosinte to maize or vice versa.

(Doebley) You could get a different quality of music, even though all of the musicians and all the instruments remain the same.

(Losin) These regulatory genes probably influence the activity of hundreds of other genes, which explains how mutations in just a few regulatory genes could dramatically transform teosinte. But there was still one thing I couldn't figure out. I understand now how teosinte was transformed into maize. What is still bothering me is that teosinte doesn't seem like a very good crop. Why would anybody grow it in the first place? George Beadle had an idea about that question. His idea was they might have used it like popcorn. Beadle did an experiment to test his popcorn hypothesis. We can do that same experiment today. Alright, let's do it.

(Losin) Remember, the nutritious kernels of teosinte are trapped inside hard fruit cases. But if they popped, like maize kernels, the earliest farmers could have eaten teosinte. In Dr. Doebley's lab, we were about to discover whether the ancestor of maize could pop.

(Doebley) That was a good one. We've got some popped teosinte here. Give this a try. Looks good to me. That's like popcorn. Tastes like popcorn.

[chimes ring]

(Losin) The archaeological and genetic evidence tell us a remarkable story. About 9,000 years ago, people living in the Balsas River Region of Mexico began growing an unassuming grass called teosinte, and ended up transforming it into the amazing crop we now call maize.

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Ten thousand years ago, corn didn’t exist anywhere in the world, and until recently scientists argued vehemently about its origins. Today the crop is consumed voraciously by humans, by livestock, and as a major part of processed foods. So where did it come from? Evolutionary biologist Neil Losin tells the story of the genetic changes involved in the transformation of a wild grass called teosinte into corn. Evidence from genetics supports archeological findings pinpointing corn’s origins to a very particular time and place in Mexico.

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