NOVA

This planet is home to over a billion dogs.

So in the U.S.A., there's more than 75 million dogs alone.

They're everywhere now.

They are really special animals.

Pickles!

They're more than just great pets...

That's my girl.

Mm, love you, too.

Dogs have helped make humans better hunters.

Better farmers.

They protect people from danger.

And might even save them from themselves.

You can hug your dog.

The dogs change so many of them.

It's amazing.

What makes dogs so special?

Discoveries in genetics and behavioral science

are now starting to unlock their secrets.

The last decade in dog science

has been the most exciting there's ever been.

This is just the most amazing time

to be a dog scientist.

Where did they come from?

How did they evolve from wild beast

to lapdog?

Fox behavior looks like dog.

It's impossible not love them.

How can dogs look so different,

yet still be dogs?

What lies behind a dog's affection for humans?

Are dogs living with us because we feed them,

or are they living with us because they love us?

And could the answer

finally explain our species' enduring friendship?

I feel that we've cracked the puzzle of domestication.

"Dog Tales."

Right now, on "NOVA."

Humanity's best friend:

the dog.

They come in all shapes and sizes.

But whatever the breed,

a pooch can make you feel like a million bucks.

Hug.

Good girl.

They bring me so much

happiness and companionship.

I would dive in front of traffic for her.

I would do anything for her.

Dogs and humans.

We're the best duo in all of evolutionary history.

It's humanity's oldest friend.

The roots of our relationship

have long been a mystery.

But now, scientists are making extraordinary breakthroughs.

Y'all let me know when she's ready, and I'll start.

They're coming to the conclusion

that the bond between our species

runs even deeper than we thought.

And might explain how these once-wild animals

became humanity's perfect partner.

It's easy to love dogs.

That's my girl.

Mm, love you, too.

But how do they feel about humans?

It certainly looks like love,

but could they just be in it for the food?

Seems like an impossible question to answer.

We can't ask them, but could we read their minds?

Here in Atlanta, a unique experiment is underway.

Kaylin is visiting Emory University.

Good girl, all righty.

Good morning!

Hey!

Hey, Kaylin.

Good to see you.

She's part of a special project

run by Gregory Berns.

I know.

Okay, yeah, so are we going to do some work today?

He's attempting to resolve the biggest question about dogs.

The party line would say

that dogs, in some way, are scam artists.

That in fact the only reason that they live with us

is because we feed them and because we provide shelter.

She is ready.

So the core of this project is the questions

are dogs living with us because we feed them

or are they living with us because they love us?

And to me, that's all the difference in the world.

To understand just what a dog is thinking,

Gregory has trained them to lie in a brain-imaging MRI machine.

Good girl, okay.

You know, the traditional ways

to understand what dogs are thinking

is to look at their behavior.

But the problem with that

is that dogs will often do things we don't expect.

Brain imaging gets around these problems

by going directly to the brain, and, in essence,

bypassing behavior and go directly to the organ

that causes everything to happen.

The dogs in the study

are trained to relate toys to different outcomes.

When they see a pink car, the dogs learn to expect praise.

When it sees a blue knight,

it learns to expect some sausage.

By comparing those two conditions...

the anticipation, in particular...

to food versus human,

we can go in and look at the reward system of the brain

and see which one is higher or not,

or they equal, perhaps?

The level of pleasure the dog experiences

is determined by the amount of the neurotransmitter dopamine

the brain releases,

revealed on the scan by increased blood flow

to specific regions.

Good girl... stay.

So, Laurie,

if you want to fade out of view, and then we'll do the toys.

Gregory has conducted this experiment

on numerous dogs.

And the scans are a revelation.

So, these colored areas represent what dogs' brains do

when they're anticipating food.

It actually isn't showing very much happening,

which means that on average, the dogs probably

didn't care as much about the food as we thought.

But, when expecting praise...

The dopamine levels are regularly higher.

Now we actually do see quite a bit of activity.

What that speaks to is the fact that praise

is very interesting to the dogs.

Did they get one?

That was a good job!

The brain scans show unambiguously

that most dogs love praise just as much as food.

And some dogs love praise even more.

Good girl.

Who's a good girl?

You are.

These results show that a social bond

is as strong as the food that we're providing to the dogs,

and that it's intrinsically rewarding in and of itself.

And, to me, that's about as close as you can say

that a dog loves you.

They are really special animals.

Our relationship from a species-to-species level

just seems to be so amazing and enmeshed.

So some may ask,

"How did humans ever come to find

"an animal that seems perfectly fit for us?

How did they ever come to exist?"

The answer to this canine mystery

lies in the distant past,

with an animal not known for friendliness.

The wolf.

Folklore is filled with tales of these frightening predators.

Even today, a wolf's eerie howl can freeze blood.

Yet genetic research has revealed that all dogs

originally evolved from the gray wolf...

An animal that has roamed

the breadth of the Northern Hemisphere

for at least 300,000 years.

The gray wolf may look like a modern dog,

but it's a very different creature...

an apex predator...

And ruthless carnivore

with a bone-splintering bite.

So how did that...

Give rise to this?

At the Royal Belgian Institute of Natural Sciences,

paleontologist Mietje Germonpré

oversees a collection of Ice Age wolf bones.

So, I have here a skull of a typical wild wolf.

So, it's a big skull, it's from a big animal.

And what is very special is this long, elongated snout.

Now, I will show you a skull of a modern dog.

So, you see immediately

that the snout is much shorter and wider.

The relative proportions of the skulls of wolves and dogs

are markedly different.

But in 2008,

Mietje made an intriguing discovery.

She was studying a 30,000-year-old wolf skull

called the Goyet skull,

named after the cave it had been excavated from

in the 19th century.

At that time, the discoverers thought

it was the skull of a wolf.

But I was not very happy with that

because it looked different to me.

Mietje's careful measurements

backed up her hunch.

This was no ordinary wolf.

This skull is clearly smaller

than the skull of wild wolves.

The snout is wider and shorter.

More comparable to the skull of the modern dogs.

The shape of the wolf skull