Popped Secret: The Mysterious Origin of Corn — HHMI BioInteractive Video

Summary

This video explores the domestication of maize (corn) from its wild ancestor, teosinte. It highlights the work of geneticist George Beadle, who discovered that maize evolved from teosinte through changes in just a few genes. Collaborations between geneticists and archaeologists uncovered evidence showing maize was first cultivated around 9,000 years ago in Mexico's Balsas River region. The video demonstrates how small genetic changes, particularly in regulatory genes, led to dramatic differences between maize and teosinte, transforming it into one of the world’s most important crops.

Takeaways

• 🌽 Maize (corn) has been domesticated from a wild grass called teosinte, a process that began around 9,000 years ago in the Balsas River region of Mexico.
• 🌱 Teosinte, the ancestor of maize, looks drastically different from modern corn. It is a small, branched plant with hard kernels encased in a fruitcase.
• 🧬 Geneticist George Beadle proposed that teosinte was the ancestor of maize after finding that their chromosomes were nearly identical and they could produce fertile hybrids.
• 🔬 Beadle conducted a large-scale breeding experiment with 50,000 plants to show that only a few genes (around four or five) controlled the major differences between teosinte and maize.
• 🌾 Teosinte and maize have different genes related to branching and kernel encasement. The changes in just a few regulatory genes were enough to transform teosinte into maize.
• 🧑‍🌾 Early humans likely began cultivating teosinte despite its hard kernels, possibly using it like popcorn. This initial usage could have spurred the domestication process.
• 🧪 Genetic and archaeological evidence shows that all modern maize traces back to a single type of teosinte in the southwestern part of Mexico, near the Balsas River.
• 🕰️ Archaeological findings, including microfossils on ancient grinding stones, indicate that humans were processing maize as far back as 8,700 years ago.
• 🔍 Teosinte's transformation into maize involved the domestication of regulatory genes that affect the plant's traits, such as branching and kernel protection.
• 🌍 The collaboration of geneticists and archaeologists helped uncover the story of maize's domestication, highlighting the interdisciplinary nature of this scientific discovery.

Q & A

What is the main focus of the script?

The script focuses on the domestication of maize (corn) from its ancestor, teosinte, and how geneticists and archaeologists worked together to uncover its origins.

Why was maize's origin a mystery for a long time?

Unlike other crops, maize does not have a wild counterpart that closely resembles it, making it difficult to trace its origins. Early maize fossils already looked like modern maize, which further complicated its lineage.

Who was George Beadle, and what did he discover about maize?

George Beadle was a geneticist who discovered that teosinte, a wild grass from Central America, was closely related to maize. He proved that teosinte could be the ancestor of maize, challenging the existing belief that maize's ancestor was extinct.

What are the key differences between teosinte and modern maize?

Teosinte has a highly branched structure, produces small ears with a few hard kernels, and looks very different from maize, which has a single stalk, large ears, and hundreds of exposed kernels.

How did Beadle confirm that teosinte could be the ancestor of maize?

Beadle conducted a large breeding experiment, crossbreeding maize with teosinte. He found that changes in only four to five genes could account for the major differences between the two plants.

What role do regulatory genes play in the transformation from teosinte to maize?

Regulatory genes are responsible for controlling other genes' activities. Changes in a small number of these powerful genes caused dramatic differences between teosinte and maize, such as branching and kernel exposure.

Where and when was maize domesticated?

Maize was domesticated around 9,000 years ago in the Balsas River region of southwestern Mexico. Genetic and archaeological evidence both point to this area as the origin of maize cultivation.

What evidence did archaeologists find to support the geneticist’s theory about maize's origin?

Archaeologists found ancient grinding tools with maize microfossils in the Xihuatoxtla shelter, which were radiocarbon dated to about 8,700 years ago, confirming maize cultivation in the Balsas River region.

Why would early farmers have cultivated teosinte despite its hard kernels?

George Beadle hypothesized that early farmers might have used teosinte like popcorn. He tested this by popping teosinte kernels, which supported the idea that it was edible in this form, providing an incentive for cultivation.

How did scientists determine the timeline of maize domestication?

By comparing DNA sequences of maize and teosinte and calculating mutation rates, scientists were able to estimate that maize was domesticated around 9,000 years ago.

Outlines

00:00

🌽 The Ubiquity and Mystery of Maize

The first paragraph introduces an everyday scene where maize (corn) is a major part of life. It describes how humans have transformed wild plants like maize through domestication, turning them into valuable crops. Despite maize being omnipresent in modern diets, its origin was once a mystery. Unlike other crops whose wild ancestors are apparent, maize had no obvious precursor in nature, leading to an intriguing scientific puzzle. This sets the stage for the story of how geneticists and archaeologists worked together to uncover the origins of maize.

05:01

🔬 The Discovery of Teosinte

This paragraph recounts how maize’s origins were first linked to a Central American grass called teosinte, through the work of geneticist George Beadle. Beadle discovered that teosinte’s chromosomes were nearly identical to maize and that the two could produce fertile hybrids, suggesting they were closely related. Despite initial skepticism from botanists, due to the stark differences between the plants, Beadle's work opened the door to the possibility that teosinte was maize's ancestor.

10:04

🌱 A Groundbreaking Breeding Experiment

The third paragraph focuses on George Beadle's quest to determine how many genes were responsible for the transformation of teosinte into maize. He conducted a massive breeding experiment, growing 50,000 plants, and discovered that the differences between teosinte and maize were controlled by just four or five genes. This finding supported his theory that early humans could have relatively easily domesticated teosinte, leading to modern maize.

15:05

🧬 Tracing the DNA of Maize

This section explores the work of Dr. John Doebley and his team, who used DNA analysis to pinpoint the variety of teosinte most closely related to modern maize. They found that maize originated from a type of teosinte in the southwestern part of Mexico, near the Balsas River, about 9,000 years ago. This genetic evidence provided insight into when and where maize was first domesticated.

🗿 Archaeological Evidence Supports the Genetic Findings

In this paragraph, Dr. Dolores Piperno, an archaeologist, describes how her team worked to find physical evidence of maize domestication in the Balsas Valley, based on the geneticists’ predictions. They discovered grinding tools with maize microfossils at the Xihuatoxtla shelter, confirming that maize was being processed there nearly 9,000 years ago, aligning with the genetic evidence.

🍿 Teosinte as Popcorn? A Surprising Theory

Here, Dr. Doebley presents George Beadle’s hypothesis that early humans may have used teosinte similarly to how we use popcorn. The team tested this theory by popping teosinte kernels, demonstrating that they could be eaten this way, offering an explanation for why early humans might have started cultivating teosinte despite its initially hard, inedible seeds.

🎻 The Power of Regulatory Genes

This paragraph dives into the genetic mechanisms behind the transformation of teosinte into maize. Dr. Doebley explains that a small number of 'regulatory genes' played a crucial role in controlling other genes, dramatically changing the plant’s characteristics. This explains how just a few genes could cause such significant differences between teosinte and maize, likening these regulatory genes to a conductor directing an orchestra.

Keywords

💡Domestication

Domestication refers to the process by which humans transform wild plants or animals into forms that better suit human needs. In the video, maize (corn) is used as an example, having been selectively bred over thousands of years to become larger, sweeter, and more useful compared to its wild ancestor, teosinte.

💡Maize

Maize, commonly known as corn in the U.S., is the focus of the video as a crop that humans have significantly transformed through domestication. The video's narrative revolves around the origins of maize, its genetic differences from its ancestor, teosinte, and its importance as a food source across the world today.

💡Teosinte

Teosinte is a wild grass from Central America and the ancestor of maize. The video explains how teosinte and maize are genetically similar, yet visually and structurally different, with teosinte being smaller, more branched, and producing fewer edible kernels. The transformation from teosinte to maize was a major breakthrough in agricultural history.

💡Genetics

Genetics is a central theme of the video, focusing on how the genetic relationship between maize and teosinte was uncovered. The video discusses how changes in just a few genes, including regulatory genes, were responsible for turning the wild teosinte into the domesticated maize we know today. This discovery was pivotal in understanding maize's origins.

💡George Beadle

George Beadle was a geneticist whose work led to the discovery that teosinte was the ancestor of maize. The video highlights his groundbreaking experiments, including a large-scale breeding experiment that showed only a few genes control the major differences between maize and teosinte. His work earned him a Nobel Prize and helped solve the mystery of maize's origins.

💡Regulatory Genes

Regulatory genes are a special class of genes that control the activity of other genes. In the video, these genes play a crucial role in the transformation of teosinte into maize. A small number of mutations in regulatory genes, such as the 'fruitcase gene' and the 'branching gene,' caused dramatic physical changes in the plant's structure and appearance.

💡Archaeology

Archaeology is discussed in the video as a means of discovering when and where maize was first domesticated. Dr. Dolores Piperno's archaeological research in caves near the Balsas River in Mexico provided physical evidence of ancient maize, with microfossils from stone tools dating back nearly 9,000 years, supporting the genetic findings.

💡Microfossils

Microfossils are tiny plant remnants found on ancient tools that provide evidence of early crop use. The video highlights how microfossils discovered on grinding stones in Mexico helped archaeologists determine that humans were processing maize as early as 9,000 years ago. These microfossils were a key part of verifying maize’s domestication timeline.

💡Balsas River Valley

The Balsas River Valley in southwestern Mexico is identified in the video as the region where teosinte was first transformed into maize. Genetic analysis of teosinte varieties found in this region closely matched modern maize DNA, suggesting that domestication of maize occurred here approximately 9,000 years ago.

💡Popcorn Hypothesis

The 'Popcorn Hypothesis' was proposed by George Beadle to explain why ancient humans may have begun cultivating teosinte. Beadle suggested that early farmers could have used teosinte kernels as a form of popcorn. In the video, this hypothesis is tested, showing that teosinte kernels can indeed pop, offering a potential explanation for why humans initially valued the plant.

Highlights

Maize (corn) is a central crop in the U.S., used in numerous products, and its origin was a mystery for a long time.

Christopher Columbus’s crew were the first Europeans to encounter maize, but people in the Americas had been growing it for thousands of years.

Unlike other crops, maize's ancestor, teosinte, doesn’t resemble it closely, which led to initial confusion about its origin.

Geneticist George Beadle discovered that teosinte’s chromosomes were almost identical to those of maize, leading him to hypothesize that teosinte was maize's ancestor.

Beadle’s breeding experiments showed that just four or five genes were responsible for the transformation of teosinte into maize.

Teosinte plants look very different from maize, being bushy and branched, whereas maize has a single main stalk with a few ears.

Early humans likely domesticated teosinte around 9,000 years ago in the Balsas River Valley in southwestern Mexico.

DNA analysis from teosinte varieties helped pinpoint the location and timing of maize domestication.

Archaeologists found ancient plant-processing tools in the Xihuatoxtla shelter, providing evidence of early maize usage.

Radiocarbon dating of charcoal found alongside maize microfossils revealed that maize domestication began about 8,700 years ago.

Genetic changes between maize and teosinte were driven by regulatory genes, which control the activity of other genes.

The transformation of teosinte into maize was possible due to mutations in just a few powerful regulatory genes.

One gene controls the hard fruitcase of teosinte, and altering this gene produced kernels that resemble corn.

Beadle hypothesized that early humans might have used teosinte like popcorn, which was supported by modern experiments.

The research of geneticists and archaeologists revealed that maize was domesticated through human innovation, transforming an unassuming grass into one of the world’s most important crops.

Transcripts

[crickets]

[footsteps]

[chime]

[crowd noise]

[CASHIER:] That’s three dollars for six.

[LOSIN:] OK

[CASHIER:] Alright, thank you.

[LOSIN:] Great, thank you so much.

Have a great day.

[bag crumples]

[LOSIN:] 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.

Here in the U.S., most of us call it corn.

And we eat a lot of it.

There’s corn bread, corn chips, corn cereal.

If you look a little deeper, you’ll find corn starch

and corn syrup in hundreds of products.

And a lot of the meat we eat comes from animals

fed a corn-based diet.

So, maize is all around us, but, for a long time,

the origin of maize was a mystery.

The ancestors of wheat pretty much look like wheat.

The precursors of apples basically look 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 archaeologists working together

to discover where maize really came from.

[LOSIN:] Christopher Columbus’s crew were the first Europeans

to see maize.

But by the time Columbus arrived,

people all over the Americas had been growing maize

for thousands of years.

Archeological evidence from around the world

reveals that, starting around 10,000 years ago,

humans were beginning to live in larger settlements

and 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.

And 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.

And 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 the ancestor of maize

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.

[DOEBLEY:] So Neil, the reason I wanted to bring you out here

was to show you just how different corn and teosinte

are.

[LOSIN:] Yeah.

[DOEBLEY:] This is a teosinte plant,

and it doesn’t look anything like a typical corn plant.

[LOSIN:] No.

[DOEBLEY:] You can start by just looking at the base,

it just branches a lot.

So, it is a very bushy creature, and quite different from a corn

plant, such as you see here …

[LOSIN:] Yeah.

[DOEBLEY:] … where there’s just 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 an ear of 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.

And after his retirement, he returned to that question.

To silence the skeptics, Beadle had

to show how humans could have transformed this into this.

So, after his retirement, he launched

one of the biggest breeding experiments in history

to settle that question once and for all.

For Beadle, the key question was “How many genes control

the differences between maize and teosinte?” If that number

were small, then it wouldn’t have been too hard for early

humans to transform teosinte into maize.

He began by cross-breeding 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 one in four of the F2 plants

should look just like maize, and one in four

ought to look like teosinte.

If two genes are at work, this number drops to one in sixteen.

For three genes, it’s one in sixty-four, and so on.

If more than three 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 one in 500 plants looked identical to teosinte,

and a similar number looked just like maize.

That number suggested that changes in just four or five

genes were responsible for all the major differences

between the two plants.

So, George Beadle was right!

The real ancestor of maize was teosinte,

and it was right in front of us all along.

But many varieties of teosinte grow

throughout Mexico and Central America,

and humans have lived there for thousands of years.

So, 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 looked for the teosinte variety with DNA

sequences most similar to maize.

[DOEBLEY:] We’ve actually 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.

[DOEBLEY:] Our estimate is that the original domestication

of corn would’ve 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 Dr. Dolores Piperno at the Smithsonian Tropical

Research Institute in Panama to see that evidence.

[PIPERNO:] Hi, Neil.

How do you do?

Welcome to Panama.

[LOSIN:] So 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?”

[PIPERNO:] Teosinte is distributed all over

Mexico—highlands, lowlands, it gets into Nicaragua.

So the question for archeologists was,

where do we go?

And Dr. Doebley’s work told us exactly where to go.

[LOSIN:] Nine thousand years ago,

people living in this area were taking shelter and preparing

food in caves and rock shelters.

[PIPERNO:] When we went to the central Balsas Valley,

one of the things we did was to ask the local people,

“Do you know of any caves or big rock shelters?” And that’s how

we found the Xihuatoxtla shelter.

[LOSIN:] So people took shelter there, they slept there,

they probably ate there.

[PIPERNO:] 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 decompose.

But Dr. Piperno wasn’t looking for such obvious evidence.

[PIPERNO:] These were the earliest plant-processing

tools, … we call them plant grinding stones—that’s what

they were used for—and these were 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 tools’ surface, leaving behind “microfossils.”

[PIPERNO:] We found hundreds of these microfossils right

on the grind surface of the stone, and like the seeds,

they’re very highly diagnostic.

[LOSIN:] So even with these microscopic traces,

you can tell the difference between corn and teosinte?

[PIPERNO:] 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.

[LOSIN:] And so what was the oldest date of these maize

remains?

[PIPERNO:] The … it’s very 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.

[LOSIN:] One of the main differences between teosinte

and maize is that the teosinte seeds are encased in this

really hard fruitcase that makes it really difficult to eat.

So clearly that’s something that had to change.

[DOEBLEY:] That’s right.

And the remarkable thing is that having

a fruitcase versus not having a fruitcase

is basically controlled by a single gene.

[LOSIN:] A single gene?

[DOEBLEY:] 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 fruitcase gene

into teosinte plants.

When they did that, the teosinte kernels,

which are normally enclosed in a hard fruitcase,

become partially exposed, almost like little corn kernels.

When they did the opposite—putting the teosinte

fruitcase gene into maize plants—the fruitcase became

larger and started to cover up the maize kernels,

similar to teosinte.

[DOEBLEY:] One gene makes a pretty dramatic change.

[LOSIN:] So, another really obvious difference between

teosinte and corn is that teosinte produces dozens

of these little tiny ears on a plant that branches a lot,

and corn just produces a couple of ears on a plant that hardly

branches at all.

So what’s going on there?

[DOEBLEY:] There is one gene that we’ve identified that

plays a central role in that process.

And you 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 has shown that the fruitcase gene,

the branching gene, and just a few others—a small number

of genes, just 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?

[DOEBLEY:] They both belong to a, a special class of genes

called regulatory genes.

And these are genes that directly regulate

the activities of other genes.

[LOSIN:] And so, 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?

[DOEBLEY:] That’s right, they can turn other genes

on and off.

You could think of these genes as something

like the conductor of an orchestra.

And if you would take the conductor from one orchestra

and give that orchestra, say, a new conductor …

[LOSIN:] Just like we did moving some genes from teosinte

to maize or vice versa.

[DOEBLEY:] Right.

And you could get a very different quality of music,

even though all of the musicians and all of 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.

So, I understand now how teosinte was transformed

into maize, but the thing that’s still bothering me is that,

teosinte really doesn’t seem like a very good crop.

So, why would anyone have started

growing it in the first place?

[DOEBLEY:] Well, George Beadle actually had an idea about that

question.

And his idea was that they might have used it like popcorn.

[LOSIN:] Huh.

[DOEBLEY:] And Beadle did an experiment to test his

hypothesis that they used it like popcorn,

and we can do that same experiment here today.

[LOSIN:] Alright, let’s do it.

Remember, the nutritious kernels of teosinte

are trapped inside hard fruitcases.

But if they popped, like maize kernels, that could be one way

the earliest farmers could have eaten teosinte.

In Dr. Doebley’s lab, we were about to find out whether

the ancestor of maize could pop.

[DOEBLEY:] Oh, there goes one.

Ooh, that was a good one!

[LOSIN:] Ok, so we actually, we’ve got some popped teosinte

here, and uh, I’m gonna, I’m gonna actually give this a try.

[DOEBLEY:] Looks good to me.

[LOSIN:] And that’s basically just like popcorn.

That’s pretty cool.

[DOEBLEY:] Tastes like popcorn.

[LOSIN:] The archeological and genetic evidence tell us

a remarkable story.

About 9,000 years ago, people living in the Balsas River

region of Mexico began cultivating an unassuming grass

called teosinte and ended up transforming it

into the amazing crop we now call maize.

[music plays]