Human Footprint

(crowd chanting "Eat!

Eat!")

Eat!

Eat!

Eat!

Eat!

(crunching) (slurping) (Shane) When I was a kid, most of the adults I knew woke up in the morning, got ready for work, and disappeared into the town's chicken factory.

(clucking) I never really thought about what happened behind those walls or just how many chickens were actually in there.

It all just seemed...normal.

(crowd shouting "Eat!

Eat!")

Americans love chicken like preachers love Jesus.

Just look at us.

(crowd shouting "Eat!

Eat!")

But we're not the only ones.

People everywhere love to eat this bird.

We fry it, we roast it, grill, bake, and smoke it.

Dry-rubbed or jerked, chicken has no boundaries.

And don't even get me started on eggs.

We're talking 2 trillion per year.

(cheeping) To satisfy our appetite, farmers produce 2,000 chickens... every second.

But chickens aren't the only ones thriving in the world we've built.

(crowd) 3...2...1... (Shane) Get ready to meet the animals and plants that humans have chosen to succeed-- the species I like to call The Replacements.

Welcome to the Age of Humans... where one species can change everything.

And what we do reveals who we truly are.

This is "Human Footprint."

(soft clucking) The chicken is one of the chosen ones.

As humans spread across the planet, many plants and animals struggled in our wake, but not all.

A few special creatures, like the chicken, came along for the ride.

We even offered them a hand.

Now, all over the world, the species we've chosen have replaced the ones we haven't.

Think about this: Humans have transformed 40% of the world's land surface.

We use some of it to raise livestock, which outweigh all wild mammals and birds combined by a factor of 10.

And on most of the rest, we grow just a few crop species that we've lifted from biological obscurity to global domination.

But who are The Replacements?

It turns out, they're not always the species you'd expect.

Scientists like to think that they have a unique appetite for knowledge, but I think it's just part of being human.

(Nate Rose, "Curious George") I need to know everything Who and the what and the where I need everything (Shane) We all want to understand the past, make sense of the present, and ponder the possibilities of the future, but sometimes our drive to know has unexpected consequences.

Just ask our first Replacement: the African clawed frog.

(Jim) Xenopus means "strange foot."

(Shane) Jim Hanken is the Curator of Herpetology at the Harvard Museum of Comparative Zoology.

He was one of my mentors in grad school, and amphibians are his thing.

(Jim) 3 of the hind toes have a black, keratinized claw.

(Shane) I'm assuming that's where "African clawed frog" comes from.

(Jim) African clawed frog.

You got it.

(Shane) For millions of years, African clawed frogs were minding their own business in South African ponds, but then, something changed.

In grad school, we talked about Xenopus like the model system.

(Shane) Model systems are the species we study because there's something about their biology that gives us deeper insight into how life works.

Model systems are literally the "lab rats" of the scientific world.

It always struck me as the weirdest choice.

Yeah, it's a historical accident.

In the early 1930s, biologists working in southern Africa realized that you could use female Xenopus as an assay for human pregnancy.

(sound of needle scratching record) (Shane) Hold up.

A frog that's a pregnancy test?

(woman) In Ancient Egypt, people were peeing on wheat, barley, and other kinds of grain.

If it sprouts, she's pregnant.

If it doesn't sprout, she's not pregnant.

And it was actually found to be accurate 75% to 80% of the time.

So, this is a pregnancy test that worked, developed 3,000 years ago or more.

(Shane) Kelsey Tyssowski is a biologist at Harvard who's written about the history of pregnancy tests.

So, it was called the Hogben test after Lancelot Hogben, who-- (Shane) Lancelot Hogben.

Yes.

Got to be British if you're named Lancelot.

(Shane) Now, everyone's got their thing, and apparently, Hogben's thing was injecting animals with urine.

What he discovered is that if you inject a pregnant woman's urine into a female Xenopus, the frog will ovulate and lay eggs.

What is it exactly in the urine-- There's a hormone in the urine of pregnant women called human chorionic gonadotropin, hCG, and it goes up really, really high... (Shane) Kelsey explained that when an embryo first implants in a woman's uterus, her hCG levels spike.

And that hormone turns out to be very similar to another hormone called luteinizing hormone, which causes ovulation.

OK. (Shane) hCG in human urine can cause other mammals to ovulate, too, and in the early 20th century, doctors could actually order a mouse- or rabbit-based pregnancy test.

(Film narrator) They inject into animals... (Shane) The problem was, they had to kill and dissect the animal to find out the result.

So, the Hogben test was a revolution.

Demand exploded as Xenopus became the go-to pregnancy test around the globe.

(Kelsey) The fact that Xenopus were really useful for pregnancy testing inspired people to, you know, figure out, "How do we keep a lot of Xenopus like, living in a lab space?"

(Shane) Xenopus labs started popping up everywhere.

And scientists began to realize that they could use these frogs and their eggs to answer all kinds of questions.

Are you pregnant?

(Shane) Researchers eventually came up with easier ways to measure hCG in urine-- no frogs required.

But by then, Xenopus was firmly established as a model system in biology.

These frogs weren't going anywhere.

How many frogs do you think you have here?

(man) We have over 10,000 frogs.

(Shane) The epicenter of modern Xenopus research is tucked away in a basement in a quiet town on the coast of Massachusetts: the National Xenopus Resource.

(man) We focus on transgenic, inbred, and mutant frogs.

(Shane) Marko Horb is the director here.

And despite all this talk about mutants, he's more of a Willy Wonka than a Dr. Moreau.

(Marko) Somebody wants a mutant, they tell me what gene, I have technicians that work on that, they'll make the mutant-- it's like a shop.

OK. You want something, you order it from me, we make it, and we--we give it to you.

(Shane) Different research questions require frogs that carry different combinations of genes.

When a scientist needs a particular collection of genes that hasn't been assembled before, Marko's the dude that'll put it together for them.

And yeah, this is actually what he likes to do in his spare time.

What are Xenopus being used for?

(Marko) Developmental biology, how organs develop and how different cells are formed.

People use the eggs and the oocytes from the frog... (Shane) Marko's talking about research that's already led to new discoveries, technologies, medicines, and will undoubtedly lead to many more.

But in our quest for one kind of knowledge, I can't help but wonder if we're leaving another behind.

In the early 1990s, frogs around the world started...disappearing.

In just a few decades, dozens of wild species had been reduced to memories and a few pickled specimens in museums.

(Shane) Let's see, we got Rheobatrachus... (Shane) That's Atelopus... Colostethus.

this is a poison frog.

Pseudoeurycea.

Craugastor.

Everything that we're looking at here has gone extinct.

All--all of these are believed to be extinct.

(Shane) The killer is a fungus called chytrid.

(Jim) These are fungi that proliferate in the skin of amphibians.

The frog stresses out physiologically and dies.

Biologists literally walking along a mountain stream would see large numbers of dead or dying frogs on the rocks next to the stream or floating in the water.

It was just unheard of.

(Shane) Chytrid has probably existed and infected certain amphibians for thousands of years.

What's new is the massive global trade in amphibians for pets, food, and, yeah, science.

Chytrid has spread all over the world, exposing frogs and salamanders to a disease many of them have never faced before.

It's the largest pandemic the world has ever seen.

And in just a few decades, it's driven dozens of amphibian species to extinction.

And yet, this thing is doing--is doing perfectly fine, more than fine.

Some species can resist it very readily, other species cannot.

Xenopus tolerates chytrid infection.

It's a tough frog.

(Shane) We don't know for sure what role Xenopus are playing in the chytrid pandemic, but they certainly aren't helping.

They can carry chytrid without getting sick.

And thanks to us, they're world travelers.

Escaped and released frogs have made Xenopus an invasive species on 4 continents.

Research on these frogs has improved our lives, but as we transform one frog species into a global phenomenon, we left others on the brink of collapse.

It just goes to show that when our species plays favorites, the consequences can be global, whether the species we've chosen is a weird-looking frog or an adorable companion.

All right, see what we got here.

Spending time on the Internet is like staring into an abyss that not only stares back, but it calls you names.

It's one of the most expansive, complex innovations in human history.

So why am I using it to watch cat videos?

From YouTube to memes, social media to NFTs, cats rule the Web.

(meowing) And it's not just the virtual world.

Look around.

Domestic cats are everywhere, and their populations continue to grow.

Meanwhile, most wild predators are in steep decline.

How did this one species take over our brains, our homes, and ecosystems around the globe?

(man) So, we don't have cream and sugar-- (Shane) That'll work.

(man) But we have this foofy French vanilla stuff.

(Shane) I will foof it up.

(man) It's like dessert.

Shane, you get the honored "Nelson as a kitten" mug.

(Shane) Oh, I feel the honor, thank you.

(Shane) Jonathan Losos and I go way back.

He was one of my Ph.D. advisors in grad school.

I call him my academic Papa.

Cheers.

Great to see you.

(Shane) In the biology world, Jonathan is a legend for his research on lizards, but outside the lab, cats are his passion.

(Jonathan) I've always been into cats, but it never occurred to me to do anything professionally with them because they're just cats!

(Shane) But he just couldn't help himself, and now he's written a whole book on the science of cats.

You actually own cats.

I--We have 4 cats here in the house.

I'm not sure "own" is the proper verb.

OK.

But they live here.

And they run the show, put it that way.

So how do we--how do we get from the truly wild cat to these guys that are running around your house?

(Jonathan) The idea is this: that when humans adopted an agricultural lifestyle, we started raising crops and storing them, and that of course attracted rodents, and this was in the native range of the African wild cat.

(Shane) These wild cats started hanging around human settlements.

The less afraid of us they were, the better they did.

Over time, cats evolved to be friendlier, or at least more tolerant of human company.

We started appreciating them, too, and soon, we were bringing them with us around the world.

(Shane) Let me see one of these cats.

All right.

I'll go get one right here.

He's in his little basket over here.

It's showtime, Nelson.

This is Nelson.

He's the one on your mug.

Let me see what all this-- all this noise is about.

(Shane) Now, if I look uncomfortable here, it's because I'm very allergic to cats.

A key phase in domestication, in general, is when humans take over the breeding process.

(Shane) Cats became semi-domestic simply by hanging around and catching mice.

It's only for the last few hundred years that we've controlled who mated with whom, producing dozens of distinct breeds, like the American Burmese in my lap.

Here you go, Nelson.

We'll put you back in your basket.

(Shane) I know I'm gonna pay for that later.

(Shane) I had to see these breeds for myself, but where?

Well, the best place is a cat show.

(distorted voice) Cat show...cat show... (female announcer) Hey, judges and clerks, if you'll head to your rings, that would be great.

We are gonna get this show started.

(Shane) I want--I want to go talk to-- to those folks with the hairless cat.

Do you have to put sweaters on him?

We have sweaters, clothes, their own heated room.

That's awesome!

(Shane) Of all the kinds of cats that you could have, why Siamese?

They're a natural breed, meaning only God made them, no man made 'em.

Cats are very good companions.

We're going to stop for a second and put him away.

It's a toy tiger, it's a Toyger.

(Shane) Sweet mercy.

Yeah, this is a Bengal.

His full name is Salvador's Wizard From Oz.

He's an American Shorthair.

(woman) Each breed has a written standard that all the judges are trained on.

(judge) He's long, he's tall.

(woman) Number 352, they're kind of a stocky breed.

They're supposed to feel like a sack of oats.

Like a sack of oats, specifically.

A sack of oats, yes.

Sack of oats.

Is he--is he supposed to feel like a sack of oats as well?

No.

Only the Americans are supposed to feel like a sack of oats.

(Shane) Even Jonathan loves him a good cat show.

We have shown the European Burmese at some shows.

Actually, Nelson, who you held, was doing really well.

He was chasing the rank of Grand Premiere... (people) Wow!

except he decided he doesn't like cat shows, so much so that he started hissing at judges.

(purring) (Shane) Nelson isn't the only one who isn't cut out for the cat-show life.

There are between 600 million and a billion domestic cats in the world, and the pampered cats I saw at the cat show are in the minority.

Most cats live outdoors and they're probably the most abundant predatory mammal the planet has ever seen.

So you mentioned that a couple of your cats are inside/outside cats.

You know, when they're out cruising, do you have any idea what they're doing out there?

Well, mostly no.

(Nelson meows loudly) Ha ha ha!

He says, "None of your business is what I'm doing out there."

(chuckling) (Shane) Cats would be appalled to know that researchers are working hard to uncover their secret lives outdoors.

Roland Kays knows more about animal tracking than almost anyone on the planet.

What he gets up to in his own time... that's his business.

(Megan Thee Stallion, "Girls in the Hood") Bein' good, I'm a bad *** I'm sick of mother***s tryin' to tell me how to live You better hope I never run across your man (Roland) Hey, welcome.

Hey, how's it going?

(Shane) Thank the good Lord for antihistamines, 'cause this was not in my contract.

(Roland) Is this not the perfect place to talk about cats?

There's, uh, definitely a lot of cats in here.

(purring) It's a cat coffee shop.

And if you want to, you can walk out of here with your very own cat.

OK.

I will pass, but I-- I appreciate the sentiment.

(Shane) You know you got too many cats when you start giving them away for free.

When cats go out, cats hunt, right?

I mean, this is why we domesticated them in the first place.

One cat catching a few things isn't a big deal.

It's when you magnify that by 600 million, that all of a sudden, maybe it is a big deal.

(Shane) The problem is, scientists know next to nothing about what cats do in the wild or what their impact might be.

Roland's answer?

Track cats the way we track wild predators: by fitting them with GPS-enabled collars.

Several of those cats were right here in Raleigh, including a cat named Kraken.

(knocking on door) Hello!

(woman) Hey, how are you?

(Roland) Good, how are you?

(woman) Come on in.

(Roland) Thanks, thanks.

(woman) So she came to us as a stray before we decided to bring her inside.

(Shane) Is it like a mystery to you like what her life is like out there?

Oh, yeah.

Absolutely.

I found out about Mohammad's study, and so we signed her up.

So, this is the tracker that we are using on the cats.

What we have here is the microchip that has both the accelerometer and the GPS.

(Shane) The accelerometer measures the cat's movement-- left-right, forward-reverse.

And the GPS tells us where the cat is.

(Shane) Cool.

So should we release the Kraken?

I'm so glad I got to say that.

(Fugees, "The Beast") Warn the town the beast is loose Word 'em up, y'all... Warn the town the beast... (Roland) I think this is Kraken right here It went down here to this house, and then it came back, and you can see one trek way over there.

That was a bit of a surprise.

(Shane) Roland's study isn't just the biggest study of cat movement in the world, it's the largest tracking study of any species.

Roland sent GPS collars to collaborators all over the world.

So this is just a map of the Earth, and I've got pink dots anywhere that we were tracking cats.

And so, we've got Allie, Aley, Amber Rose, Amelia, right--just A--A to Z, probably we've got all kinds of different-- different ones.

(Shane) It turns out, most cats don't stray too far from home.

(Roland) Our average across all 900 cats was 3.5 hectares.

(Shane) That's about 6 or 7 football fields.

(Roland) And the next thing we had to figure out is, how do we put that into perspective?

(Shane) Roland compared data from domestic cats to their closest wild relatives.

What we found was that the domestic cats have a 4 to 10 times more ecological impact on their prey than do wild species, but that's gonna be concentrated within basically 100 meters of someone's house.

(Shane) One of their biggest footprints can be seen on migrating bird populations.

(Roland) There are some estimates in the United States that cats kill sort of 1 to 3 billion birds a year.

Billion.

Yes.

But it's even more small mammals.

It's like 7-10 billion small mammals.

(Shane) And the impacts of cats in places like Australia, New Zealand, and oceanic islands have been devastating.

(Roland) There are examples of animals that have been completely hunted to extinction by cats.

(Shane) Some biologists consider them the worst invasive species alive, but Roland's got a plan to curb the impact of these cuddly killers.

It starts with the accelerometer in the collar.

(Roland) The accelerometer basically measures the orientation and the movement of the collar in 3 dimensions.

So, here, you can see-- look how rhythmic this is.

Choo-choo choo-choo-choo.

Oh, OK.

So that's a walking or running cat.

(Shane) If the accelerometer can recognize a walking or running cat, what else can it tell us?

And so what our goal is is to get artificial intelligence algorithms on the collar recognizing, this cat's about to hunt.

So we could have basically a bird alarm call go off on the cat's collars and all the birds would be alerted, "Look out!

Predator here."

That's brilliant.

So, that's--thank you.

That's really cool.

That's one idea.

(Shane) We've made a pact with cats.

Even as they deplete the natural world around us, they seem to enrich another part of our lives.

Sure, their modern existence can be pretty bizarre, and damn, do they make my eyes itch.

But compared to their wild relatives, one thing's for sure: Cats are evolutionary winners.

(purring) (bird cries) A lot of folks love cats because they seem so self-sufficient.

But in the age of humans, that's not a prerequisite for success.

For the next Replacement, we'll move heaven and earth, even in places it could never survive on its own.

(film narrator) Because of abundant sunshine, golfers have access to scores of courses year-round.

(Shane) Here in Palm Springs, to walk the walk, you gotta dress the part.

(zip) (Shane) Lined up there.

(man) Perfect, looks good.

Yep.

(Shane) Yep.

Bam!

Yep.

(man) You got it!

(Shane) I did not.

All right.

Nope, it's still there.

OK, let's see what we can do.

(club hits ball) Boom.

I was feeling real good about myself until just now.

(chuckling) (Shane) Turf grass.

It covers more of the U.S. than almost any other plant.

And there's nowhere grass is more pampered than the golf courses of Palm Springs, California.

(Shane) Good golf requires good grass, in your opinion.

I would think so, yes.

OK, and good grass requires you.

Well, I like to think so.

(Frank Bentley, "Element") Gotta aim for the top like (Shane) Jonas takes care of some of the world's most prestigious golf courses.

And it's not just a profession.

(Bentley) I'm in my element, it's evident That there's levels to the game (Shane) It's a love affair.

Been breaking my back to make it out Got me feeling like (Jonas) I look at parks, football stadiums, soccer fields, and golf courses.

I love it all.

(whirring) (Shane) According to Jonas, this whole golf thing started with a bunch of Scots drinking whiskey and whacking balls with sticks.

Some of those Scots immigrated to the U.S. and brought the game with them, along with its emerald green landscape.

(cheering) (Jonas) Oh ho ho ho!

Dang!

That thing's out there!

(Shane) Tiger, here I come.

(echoed cheering) You've literally created an oasis.

There's not a whole lot of people that can say that.

Yeah.

(Shane) Turns out, grass isn't just grass.

You got your Bermudas, your fescues, your rye, and each species plays a different role.

(Jonas) A good Bermuda grass is gonna be your base for probably most of the golf courses here in the valley, just because the temperatures in the summer get so warm.

And that's the only grass that really will be able to thrive and survive through that.

That's good, hardworking, sturdy...

It's--it's solid.

(Shane) But these turf grasses are just a tiny subset of the 12,000 species of grass that grow on every continent-- many of which are disappearing, thanks to us.

OK. All right, let's see it.

Just take her back, then go.

Ohh!

That was heartbreaking.

Oh, a little piece of my soul died just now.

(mowers chugging) (Shane) Humans go to ridiculous lengths to make turf happy.

(Jonas) We have a set of mowers that will mow down to eight hundredths of an inch.

We have large mowers.

We have a greens roller to firm up the surface and make it faster.

(Jonas) Little bit, little more.

Not that much.

Right there.

(Shane) OK. Back and straight through.

Bingo.

(Shane) Yeah!

(Jonas) Hello.

You're going pro!

(Shane) I mean, if you're just measuring success and impact, if grass was playing golf, it definitely got the hole-in-one.

(Shane) Underneath this thin veneer of green lies a massive, engineered life-support system.

And without it, this desert oasis would turn to dust.

(Jonas) What you're looking at right here is essentially what we call our war room.

400 acres, several thousand sprinkler heads on--on 2 golf courses.

Wow.

So it's pretty massive.

Like, how do you decide what gets watered when?

So, up here, 358,000 gallons of water run last night.

So, this is putting out a lot of-- like, 300,000 some-odd gallons of water-- Yes, yep.

every single day.

Yep, on this course.

Then you have this golf course which would be very similar to it.

Where is all this water coming from?

'Cause we're basically in the middle of a desert.

(Jonas) Right.

Absolutely.

Yeah.

Here on this property, we're fortunate to be a part of the Mid Valley Pipeline, which is where they connect water to the Colorado River, so it comes, you know, from a long way away.

(Shane) To keep these courses green, millions of gallons of water travel hundreds of miles to be sprayed out of thousands of sprinklers.

If that sounds like a lot, that's because it is.

But golf courses are just the beginning.

There are more than 40 million acres of turf in the U.S. Each year they consume 80 million pounds of fertilizer and trillions of gallons of water, while legions of people and machines keep them manicured and green.

It's the most resource-intensive plant in the country.

Yet the vast majority of it grows, not on a farm, but in neat squares around our homes-- little patches of paradise perfect for a drink with a friend.

This is Marc Norman.

He's an urban planner.

For Marc, building a great city or neighborhood is kind of like making a great cocktail.

You need to know all the ingredients and predict how they'll interact once mixed together.

(splash) Thank you.

Cheers.

Cheers.

Oh, that is a tasty beverage.

As a biologist, it's kind of weird to me, right?

It's like wherever you go in whatever environment, humans settle down and then there's grass.

(chuckling) (Marc) Yeah, every place that we inhabit is artificial in some way.

Although this is a pretty nice situation.

It is.

I'm not gonna lie.

This is some good living.

(chuckling) (Marc) I'm going to blame the Brits-- OK, I'm--I'm OK with that.

Yeah.

This notion of the sort of manor, of the estate.

(posh English voice) Oh, hello!

(Marc) You mix that with the kind of individualism of the U.S., and you have now people with quarter-acre lots, and that's their castle, surrounded by this space.

So, it's--it's status is--is what you're saying.

It signals that I own this territory.

Uh-huh.

And also that I maintain it.

Mm-hmm.

(Marc) I think baked into the way we expanded as a country, where we had too much land, people needed to populate that.

(Shane) Like Manifest Destiny.

(Marc) Manifest Destiny, exactly.

If you farmed it, or ranched it, or timbered it, for 5 years, then that land was yours, OK?

I mean, if you were white and male.

Yeah, of course.

(Marc) But that sets a precedent, right, like, that you can't just sit on land.

You have to prove that you're cultivating it.

(Shane) But grass isn't just baked into American psychology.

It forms the economic foundation of American home ownership.

(Marc) Let's say you're driving around a neighborhood in suburban Boston or Tulsa or Northern California.

They look relatively similar, and that's because these standards had to be met in order for those homebuyers to get a 30-year mortgage.

They had covenants saying you had to have grass and you had to cut it once a week.

Wow.

OK. (chuckles) That is very specific.

(Marc) Right, and we have homeowner's associations that enforce that.

Like legally enforce that?

Legally enforce that.

OK. Are the same rules applied to the back, like you have to, like, mow your backyard the same way?

You know, the regulations might not go to the back of the house.

This is like the suburban mullet.

It's like business in the front and party in the back kind of thing going on.

Exactly.

OK. (chuckling) (engine starts) (Shane) Driving around Palm Springs, you can see just how deeply grass is rooted in the American psyche.

(Shane) I love the Christmas decorations.

That's such a Christmas atmosphere we got going on here.

Yes.

Just when you think about this desert landscape and the amount of lawn we're passing by now, it's a lot of grass.

It's a lot of damn grass, yeah.

In a country where we talk about productivity, it's so unproductive, is I think the irony of it all.

Yeah.

(birds chirping) (Shane) Grass is a signal.

Just having it tells the world and ourselves that we're part of the community.

(Marc) People think this is freedom of choice, but if they ever wanted to make different choices, they're forbidden to.

It takes a whole slew of federal, state regulatory frameworks to make this happen.

(sprinklers hissing) (Shane) No matter what was there before, we dig it up, chop it down, irrigate, fertilize, and replace it with, you guessed it--grass.

I--honestly, I can't even tell if that's real grass or not.

I think it's real grass.

That's incredibly uniform.

But what's interesting is that-- So many damn cul-de-sacs!

Literally everything is a cul-de-sac.

Everything is a cul-de-sac.

(chuckling) (Shane) Everywhere we go, humans reshape the landscape not just to suit our needs, but to match our ideas of what we want it to be, and grass is an idea that's so powerful, it compelled us to transform a desert into a lawn.

But turf isn't the only grass on the block.

My next guest needs no introduction.

It's in your snacks and on your dinner plates.

It's on your face and in your gas tank.

Hell, it's even in your drywall and spark plugs.

Say hello to our dear friend maize... known to many as corn.

But before it became a global phenomenon, maize was already reshaping human cultures across the Americas, particularly where it was first cultivated, here in Mexico.

Santiago and Dani love maize... almost as much as they love their corgis.

(slurping) (barking) (Santiago) Come with us.

At Maizajo, what we do every day is we make tortillas.

(Shane) The key ingredient in a tortilla is maize, but at Maizajo, it's not just any maize.

(Shane) Sourcing ancient varieties of maize from farmers all over Mexico, Maizajo crafts their tortillas using a millennia-old process.

(Dani) What this does, is it's gonna modify the protein chains in corn.

Why?

Because corn is not digestible for humans.

Same in and out is what you're saying.

(Santiago) Yeah.

OK. (laughs) (whirring) Let's go to make some tortillas.

(Dani) This whole corn process is really related to family.

The corn is very literally a reflection of who the Mexican people are.

We are made of corn.

(Shane) Over millennia and across the Americas, people adopted corn and wove it into their cultures, creating thousands of unique varieties in the process.

Just outside of Mexico City, you can actually see all that diversity in one place.

Isn't it cool?

(Shane) This is the place to be.

(woman) This is the most secure vault we have on the site.

OK.

I'm excited to see what's behind this vault.

Things that money can't buy.

(Man) Check this out (Shane) Meet Sarah Hearne.

She's a good baker and an even better geneticist, working at one of the world's largest seed banks.

(Shane) You have more security in here than I've seen in any biological building I think I've ever been in.

This is like CDC level.

It is like going into a bank vault.

(Shane) How many different varieties of maize are in this room?

There are around 28 and a half thousand different kinds of maize.

28 and a half thousand.

This is like being in a candy store for me.

They're like jelly beans.

Every different one is a flavor.

(Shane) There's black corn, blue corn, red, yellow, and white; small, medium, and large.

Experimentation by native peoples created tens of thousands of varieties of maize, each one adapted to a specific time, place, and purpose.

But in the last century, just a handful of commercial varieties have come to dominate global corn production.

What happened?

Corn is the first of The Replacements to replace...itself.

Abeer.

What's going on, man?

(soft accordion music) (Shane) This is Abeer Saha, food historian and open mic aficionado.

(singing) He studies the history of corn.

How did the rest of the world get cheated out of all the variety that I have here?

I think it was Robert Reid in the mid-19th century.

He planted two kinds of corn.

Through the magic of crossbreeding, what was produced was Reid's Yellow Dent.

(equipment humming) (Shane) Before Reid's Yellow Dent, American farmers were growing and selling hundreds of varieties of corn.

(train whistle) Then came the railroads.

Traders demanded corn that was more uniform to streamline mass transportation and set global market prices.

(machinery whirring) Reid's Yellow Dent became that standard.

(Abeer) At the 1893 Chicago World's Fair, Reid's Yellow Dent was recognized and given prizes and awards for its traits.

(Shane) Later, in response to the Dust Bowl and The Great Depression, the federal government tried to save farmers by subsidizing corn production.

In a historical heartbeat, this culture-sustaining crop became a government-backed commodity.

Then came World War II.

(Abeer) The war effort was not just about bombs and guns.

It was also about how to feed people.

Farmers began automating and mechanizing their production processes.

(Shane) After the war, companies that had grown rich making bombs grew even richer when they realized the same chemicals made powerful fertilizers.

(Abeer) Farmers began using fertilizers and pesticides, and every farm started to look more like a factory.

Then the government begins to look for ways to create outlets for this corn.

(Shane) One of those outlets: meat.

Farmers since the 19th century thought of livestock as corn that walks itself to market.

(Shane) Corn became a staple in our livestock's diet and the meat industry exploded, but we were still producing more corn than we could possibly use.

We had to find another outlet, and we did.

(Abeer) Somewhere close to 40% of most corn now goes into ethanol production.

(Shane) But meat and fuel are just the beginning.

Today, corn is in just about everything, and we keep finding new ways to use it, creating a vicious cycle of demand and surplus.

(splash) (Abeer) The amount that humans consume, according to some estimates, is probably less than 1% of all the corn produced in--in the United States.

(corn clattering) Corn is not just a plant.

Corn is a kernel of human existence.

Uh, tres tamales.

OK. (Shane) So, what does it matter if our way of life depends on just a few varieties of maize?

(pop) One of the potential consequences of having less diversity is you don't have as much room for maneuver in terms of adaptation.

(Shane) Maize varieties don't just differ in their taste and texture, but in their ability to survive extreme conditions and new diseases.

Those differences are encoded in their DNA.

On a fast-changing planet, that genetic variation is like money in the bank.

There's climate change, we have higher frequency of drought, higher frequency of heat stress.

Well, I work at the interface between the collection that we've seen and the active breeding programs to identify characteristics that will help future-proof maize growth.

(Shane) Sarah is scouring the seed bank for genes hidden in ancient and seldom-grown maize varieties that can make commercial varieties more productive and resilient.

For some, it makes the difference between having a harvest and food to eat versus having no food to eat.

I guess in that sense it really is a bank.

It's a bank.

It's a very important vault.

(Shane) For Sarah, ensuring corn can meet the demands of the present and the future means looking into its evolutionary history.

But that history is built on our cultural connections to maize.

Salud.

(Shane) Santiago used to work at this restaurant under his sensei, Chef Gerardo Vzquez Lugo, but now his company Maizajo supplies the tortillas and masa for the restaurant's unique dishes.

How did you two come to meet?

We had this event with the chef of this restaurant, and we met there, and we clicked.

(laughing) (Dani) Yeah.

(laughing) (Chef) You take one tortilla.

Put this roll made with beef stuffed with guacamole.

OK.

Enjoy it like a taco.

(Shane) Amazing.

Thank you.

Mmm.

I'm trying real hard not to curse.

This is delicious.

(Dani) These are our tortillas.

That's a hell of a tortilla.

(laughs) (laughs) (Shane) Representing.

I like that.

Yeah.

Do you think something is being lost, you know, as, you know, so much of the corn is being dominated by this one type of corn now?

(Dani) When we first met, I asked Santiago what he wanted, and he told me that he wanted to change the world.

And I told him that he was crazy, that how on Earth was he gonna do that?

And he told me, "The thing is, I already did.

I make people turn around and see corn for what it is."

(Santiago) Sometimes, like... That's beautiful.

That's beautiful.

Thank him.

I want to cry.

(laughing) (Shane) For thousands of years, people worshiped maize.

And today, we're still devoted to it, but that relationship doesn't feel sacred anymore except in places like this, where people are embracing its deep heritage to keep a piece of their culture alive.

But culture, it's a funny thing.

(bell rings; cheering) (crunching) The paradox of The Replacements is that they're so abundant, so ubiquitous, that they become invisible.

Even as they prop up our civilization and often define our cultures, they fade into the background.

Once that happens, it's easy to ignore the problems they create, and seeing an alternative can take a leap of imagination.

(man) The chicken is the single most successful species of animal in history.

And what's driven that is, people love the taste of chicken.

(man) Every day I'm hustlin', hustlin', hustle, hustlin' Hustlin', hustle, hustlin', hustlin', hustle... (Shane) This is Josh Tetrick.

His childhood dream was to play pro football, but life had different plans.

Our next story might sound like something out of a science-fiction novel, but you'll soon be able to buy meat that was grown in a lab.

(Shane) Today, he runs a food tech company with an ambitious goal... to replace the chicken.

I grew up in Birmingham, Alabama, eating a lot of chicken wings and chicken nuggets and chicken sandwiches at Burger King.

They go through a lot of chicken in the Deep South.

A lot of chicken in the Deep South.

I've probably eaten more chicken per pound than anyone who's listening to this.

But when you're eating a chicken sandwich, you're not thinking about everything that it took to get there.

You're not thinking about the individual chickens' lives.

You're not thinking about the fact that a third of our planet is dedicated to planting soy and corn to feed animals like chicken so we can eat them.

You're not thinking about zoonotic diseases... (Shane) Humans have loved chicken for millennia, but the problems Josh is describing are much newer.

(Josh) It really wasn't until between like 1910 and 1930 was there a concerted effort to farm the chicken at larger scales.

There's a woman in North Carolina who had a whole bunch of chickens outside, and then she realized, "Man, if I bring them inside, I can cram many more of them into a tiny space."

Step by step by step by step, we get to a world that looks pretty bizarre, to a world dominated by chickens.

There are--80 billion chickens are on the planet today.

So, more biomass of chickens than you and me and all the other human beings on the planet.

(Shane) To put that into perspective, more than 6 million chickens have been born and slaughtered since you started watching this episode.

It's something that, yeah, it tastes good, that satisfies all these sensory feelings that we have: taste, texture, flavor profile, all that.

Has a lot of protein, there's a lot of important nutrients in chicken.

But if you just double-click into it... it's broken.

And it doesn't have to be that way.

There's a different approach.

(man vocalizing) (man) Here comes the man... (Shane) 10 years ago, Josh began a mission to replace chickens.

But for his company, the egg actually came first.

All right.

(Josh) Yeah.

Yeah.

Dig in.

Let's--let's give this a go.

I want you to have a bite of the sandwich, too.

And an interesting thing about this-- Wait a minute.

(chuckling) That's not an egg.

That is an entirely plant-based egg.

I ain't mad at it.

That was good.

Yeah.

That's--that's one of the oddest experiences I think I've had in a while, because, like, my mouth is definitely telling me that this is an egg.

We had found this bean called the mung bean that actually has a storage protein inside of it that when you remove it, it actually scrambles or gels at a similar time and temperature as chicken egg protein.

And we're coming up on our 300 millionth egg sold, which sounds like a lot until you realize that 2 trillion eggs were laid last year.

2 trillion.

That's huge.

2 trillion eggs laid last year.

OK, so you still got some ground to catch up on.

Still got a bit of ground to catch up on, and it's become the fastest... (Shane) Josh's plant-based egg product is gaining popularity, but eggs are just one part of the chicken equation.

Most of the world's chickens are broiler chickens that live out about 45 days in overcrowded feedlots before they're slaughtered for their meat.

Josh's solution?

Make the chicken without the egg.

(Josh) The star of the show is real chicken skewered over rice, but that chicken didn't need to be slaughtered.

It's made in this entirely new way.

We call this cultivated chicken, and our hope is in the future, we don't call it cultivated chicken anymore.

We just call it chicken.

Uh, yeah, meat with less of the guilt.

That's right.

That's right.

I'm about to grub down.

OK. OK. (laughing) Tastes like chicken.

This is awesome.

I wouldn't be mad to add a little hot sauce, but it's good, though.

(chuckling) (Shane) To cultivate their meat, Josh's team of scientists starts with chicken cells.

They study real chickens' diets to identify the nutrients those cells need to thrive.

Then, inside a sterile, temperature-controlled vessel, the cells feed on those nutrients, divide, and grow into meat.

In 14 days, the meat, real chicken meat but made without the bird, is ready to harvest and eat.

You done good, science.

How much are we talking for like a pound of this chicken?

How much are we talking about?

It's in the--today, it's in the hundreds of dollars.

(Shane) According to Josh, once they get approval to sell cultivated chicken in the U.S., they'll scale up production and the cost will plummet.

Still, I can't help but wonder if it's worth all that money and effort.

One might say that it's easier to just stop eating chicken.

I have a lot of friends who say to me like, "Josh, what the heck are you doing?

Just tell people to stop eating chicken."

And if I thought that was more effective, I would be all over it.

Unfortunately, human beings, and I'm obviously included in that, are very imperfect.

I visited this chicken facility in Western Europe, as much as I know about chicken, you know, and they took me through all the, you know, not very appetizing steps.

And then at the very last step, there was a chicken nugget, and I wanted it.

(sizzling) It smelled really good.

Yeah.

And I was hungry.

It's as simple as that.

We're imperfect.

And culture and identity and sensory and habits drive so much of what we do, not pure reason.

Right, is this about offering alternative options?

This is about replacing.

This is about replacement.

We think this is the highest probability of replacing the current way of doing things.

Being an option is just not enough.

We want it to be so good, so tasty, so desirable, so low-cost that it would be silly to choose the conventional.

(clucking) (Shane) So, this story of chickens, what does that say about us as a species?

It says that we're really smart.

We're really creative.

When we want to get stuff done, like feed a lot of people quickly and cheaply, we can.

But it's also an incredibly sad story about what we can do when we don't pay attention.

(clucking) (Shane) That's the story of The Replacements: humans harnessing other species for our own interests-- for knowledge, for companionship, for status, or for sustenance.

(liquid sloshing) They say that a rising tide lifts all boats, but the tide of humanity has been more selective, pushing just a few species to dominate the globe and replace their wild counterparts.

The Replacements' journeys always start with a spark of human ingenuity, a new vision for what's possible.

Yet for all our creativity in solving the problems of the present, we rarely anticipate the consequences.

So, we live in a world depleted, homogenized by the rise of The Replacements.

(man) But there's gotta be a better way Better way, better way, yeah (Shane) Maybe Josh is right.

Maybe the only way forward, the only way to fix the problems we've created, is through science and technology.

I actually think those are the easy solutions, the ones that don't require us to change... but I hope we can.

I hope we can also remember that we share one fragile planet with millions of species that haven't yet proven themselves useful to us, that we can envision a future beyond the ingrained systems that got us here and that we can find ways to live with the species we use and love, but save space for all the others that make life on our planet so special.

(man) But there's gotta be a better way Better way, better way, yeah Human Footprint is available with PBS Passport and on Amazon Prime Video.