The following is a script of "The Smartest Dog in the World" which aired on October 5, 2014, and was rebroadcast on September 20, 2015. Anderson Cooper is the correspondent. Denise Schrier Cetta, producer.

Human beings have lived with dogs for thousands of years. You'd think that after all that time we'd have discovered all there is to know about them. But, as we first reported last fall, it turns out that until recently scientists didn't pay much attention to dogs. Dolphins have been studied for decades, apes and chimps as well, but dogs, with whom we share our lives, were never thought to be worthy of serious study. As a result, we know very little about what actually goes on inside dogs' brains. Do they really love us, or are dogs just licking us so they can get fed? How much of our language can they understand? Before you answer, we want you to meet Chaser, who's been called "the smartest dog in the world."

Eighty-six-year-old retired psychology professor John Pilley and his border collie Chaser are inseparable.

John Pilley: We are almost there. We are almost there. Can you speak? Speak? Speak!

Chaser: Woof!

John Pilley: Good girl. Good girl.

Anderson Cooper: Do you view Chaser as a family pet? As a friend? How do you see Chaser?

John Pilley: She's our child.

Anderson Cooper: She's your child?

John Pilley: She's our child, a member of the family. Oh yes. She comes first.

Many people think of their dogs as children, but John Pilley has been teaching her like a child as well. By assigning names to toys, Pilley has been helping Chaser learn words and simple sentences.

[John Pilley: Take KG.]

He's been teaching her up to five hours a day, five days a week for the past nine years.

John Pilley: My best metaphor is this is a two-year-old toddler.

Anderson Cooper: That's how you think about your dog, a two-year-old toddler?

John Pilley: Yeah, she has the capabilities of a two-year-old.

[John Pilley: Chicken, chicken, chicken. Where's chicken? Yes. Good girl.]

He's not kidding. Most two-year-old toddlers know about 300 words.

[John Pilley: Figure 8. Figure 8. Good girl. That's figure 8.]

Chaser's vocabulary is three times that.

[John Pilley: To tub.]

She's learned the names of more than a thousand toys. And all those toys add up.

[John Pilley: Wheel. Yes, bring it on.]

To show us Chaser's collection, Pilley's brought us to his back porch.

Anderson Cooper: So, these are all the toys in here?

John Pilley: Yes.

Anderson Cooper: Got a chicken in here. Is it all right if I dump them out?

John Pilley: Please do. Please do.

There are 800 cloth animals, 116 different balls and more than a hundred plastic toys. One thousand twenty-two toys in all. Each with a unique name.

Anderson Cooper: So Chaser could recognize the names of every one of these toys?

John Pilley: That's true, that's true.

To prove it, Pilley cataloged the toys and then, over the course of three years, gave Chaser hundreds of tests like this.

[John Pilley: Chaser, find circle, find circle.]

In every test, Chaser correctly identified 95 percent or more of the toys.

[John Pilley: Find circle Chase. Yeah.]

The results were published in a peer reviewed scientific journal, and a star was born.

[Fan: How are you? I'm so glad to see you.]

Chaser even landed a book deal. But John Pilley didn't stop with the names of toys.

[John Pilley: Nose, KG. Nose KG. Nose it. Nose it. Good girl.]

He's taught Chaser that nouns and verbs have different meanings.

[John Pilley: Paw it. Paw it.]

And can be combined in a variety of ways.

[John Pilley: Take wheel. Do it girl, do it. OK. Out. Out. Chase, take KG. Do it. Good girl. Good girl.]

Anderson Cooper: So she's actually understanding the difference between take, paw, putting her paw on something and putting her nose on something?

John Pilley: Right. And that's what we are demonstrating.

All this learning has been possible, Pilley says, because of a breakthrough Chaser had when she was just a puppy.

"There was no evidence until the last decade that dogs were capable of inferential reasoning."

Anderson Cooper: At a certain point she realized that objects have names?

John Pilley: Right. It was an insight that came to her.

Anderson Cooper: How could you tell that she'd suddenly had that insight?

John Pilley: Well, it was in the fifth month and she'd learned about 40 names. And the time necessary to work with her kept getting shorter and shorter.

Anderson Cooper: She was starting to learn words faster and faster?

John Pilley: Yes.

Brian Hare: It's the closest thing in animals we've seen to being like what young children do as they are learning words.

Brian Hare, an evolutionary anthropologist at Duke University, believes Chaser is the most important dog in the history of modern scientific research.

Brian Hare: This is very serious science. We're not talking about stupid pet tricks where people have spent, you know, hours trying to just train a dog to do the same thing over and over. What's neat about what Chaser's doing is Chaser is learning tons, literally thousands of new things by using the same ability that kids use when they learn lots of words.

He's talking about what researchers call social inference - a capability humans, like Hare's son, Luke, acquire around age one. To demonstrate the concept, Hare hides a ball under one of these two cups.

Brian Hare: Hey Lukey guy. Where is it? Can you get it? Can you get the ball?

Luke doesn't know which cup the ball is under, but when his father points, he makes an inference.

Anderson Cooper: Hey, nice job.

Brian Hare: You got it.

Anderson Cooper: So what does that show you?

Brian Hare: So when kids his age start understanding pointing, it's right when the foundations of what lead to language and culture start to develop.

It might look simple, but when Hare tried the same test with bonobos, great apes he studied for more than a decade, look what happened.

Brian Hare: Oh, you chose the wrong one.

Bonobos, our closest genetic relatives, can't do it. But Hare discovered dogs can.

Brian Hare: You ready? So I'm going to hide it in one of these two places.

This two-year-old Labrador named Seesu has no trouble understanding the meaning of pointing.

Anderson Cooper: Now she doesn't know for sure which place you've put it in?

Brian Hare: That's right. There is no way she could know. And I'm just going to tell her where it is. Okay Seesu. So that's really hard for a lot of animals and that's what is really special about dogs is they're really similar to even human toddlers.

Anderson Cooper: That's a level of thinking that people didn't really think dogs could do?

Brian Hare: Right. I mean, there was no evidence until the last decade that dogs were capable of inferential reasoning, absolutely not. So that's what's new, that's what shocking is that of all the species, it's dogs that are showing a couple of abilities that are really important that allow humans to develop culture and language.

It's not surprising that dogs share characteristics with humans; after all we've evolved alongside each other for more than 15,000 years. There are now some 80 million dogs in this country, more dogs than children. But for all the playing and petting, the companionship, we still know very little about their brains.

Anderson Cooper: Does your dog really love y... 05:05

Dr. Greg Berns, a physician and neuroscientist at Emory University, has studied the human brain for more than two decades, but three years ago questions he had about his own dog inspired him to start looking at the canine brain.

Dr. Greg Berns: It started out with the desire to know, really, what does my dog think of me? I love my dog, but do they reciprocate in any way? When they hear you come home, you know they start jumping around. Is it just because they expect you to feed them? Is this all just a scam by the dogs?

Anderson Cooper: Are dogs just big scammers?

Dr. Greg Berns: Yeah.

To try and answer that question, Dr. Berns is doing something scientists have had a difficult time with. He's conducting brain scans on dogs while they're awake and un-sedated. Inside the fMRI machine they're trained to stay completely still.

Anderson Cooper: What's around Tigger's head here?

Dr. Greg Berns: The scanner makes a lot of noise. It's quite loud. And because dogs' hearing is more sensitive than ours, we have to protect their hearing, just like ours. So we, we put earplugs and earmuffs and just wrap it all to just keep it in place.

[Trainer: OK. Now we can go up.]

Tigger certainly knows the drill. Once in the machine he lies down and doesn't move. These scans are giving Dr. Berns the first glimpse at how a dog's brain actually works.

Anderson Cooper: So these are slices of Tigger's brain that you're seeing?

Dr. Greg Berns: Yeah, exactly. So we're slicing from top to bottom. We analyze them later to see which parts increase in response to the different signals.

While in the scanner the dogs smell cotton swabs with different scents. First, the underarm sweat of a complete stranger. Next, the sweat of their owner.

As Dr. Berns expected, when the dogs sniffed the swabs the part of their brain associated with smell, an area right behind the nose, activated. It didn't matter what the scent was.

But it was when the dogs got a whiff of their owner's sweat that another area of the brain was stimulated - the caudate nucleus, or "reward center." Dr. Berns believes that means the dog is experiencing more than the good feeling that comes with a meal. It shows the dog is recognizing somebody extremely important to them. It's the same area in a human brain that activates when we listen to a favorite song or anticipate being with someone we love.

Anderson Cooper: So just by smelling the sweat of their owner, it triggers something in a much stronger way than it does with a stranger?

Dr. Greg Berns: Right. Which means that it's a positive feeling, a positive association.

Watch YouTube videos of dogs welcoming home returning service members and it's easy to see the bond between dogs and their owners.

Brian Hare says there's even more proof of that bond. It's found in our bloodstreams.

Brian Hare: We know that when dogs and humans make eye contact, that actually releases what's known as the love hormone, oxytocin, in both the dog and the human.

It turns out oxytocin, the same hormone that helps new mothers bond with their babies, is released in both dogs and humans when they play, touch or look into one another's eyes.

[Dog owner: Thank you very much.]

Brian Hare: What we know now is that when dogs are actually looking at you, they're essentially hugging you with their eyes.

Anderson Cooper: Really?

Brian Hare: Yes. And so it's not just that when a dog is making a lot of eye contact with you that they're just trying to get something from you. It actually probably is just really enjoyable for them because they get an oxytocin or they get an uptick in the love hormone too.

All these new discoveries about dogs have led Brian Hare to create a science-based website called Dognition, where owners can learn to play games to test their dog's brain power.

Anderson Cooper: So you're allowing people to do an intelligence test for their dogs?

Brian Hare: That's exactly right. And the idea though is that there's not one type of intelligence. We help you measure things like how your dog communicates, how empathic your dog is. Is your dog cunning? Is your dog actually capable of abstract thought like reasoning?

Anderson Cooper: So there are different kinds of intelligence for dogs just like with humans?

Brian Hare: Absolutely. And so just like some humans are good at English, and others are good at math, it's the same for dogs.

When Hare tested his own dog, a mixed breed named Tassie, he was surprised by what he learned.

Brian Hare: What I found out was that I had someone sleeping in my bed that I didn't even know.

Anderson Cooper: Really?

Brian Hare: And I didn't know my dog doesn't really rely on its working memory. So if I'm saying sit and stay, I no longer have to wonder why my dog wanders off. He like literally forgot.

Anderson Cooper: So you're dogs not the sharpest of dogs?

Brian Hare: He did great on communication. He's very communicative.

Anderson Cooper: So he can basically be a TV anchor?

Brian Hare: Yes.

[John Pilley: Fetch shirt. Fetch shirt. There we go.]

If you're wondering how Chaser did on Brian Hare's intelligence tests? She was off the charts on reasoning and memory. Not surprising perhaps considering Chaser is a border collie - dogs bred specifically for their ability to understand how farmers want their sheep herded.

Anderson Cooper: Is Chaser just like an Einstein of dogs?

Brian Hare: So that's really fun. Is Chaser somehow special? And I think the idea actually is that no. I mean, when Dr. Pilley chose Chaser, he just randomly took her out of a litter.

[John Pilley: Drop. Drop.]

Brian Hare: What's special is that he spent so much time playing these games to help her learn words, but are there lots of Chasers out there? Absolutely.

[John Pilley: On your mark, get set, go!]

Anderson Cooper: There's going to be a lot of people who see this and are jealous of your relationship with Chaser.

John Pilley: Well, start working with your dog more.

[John Pilley: Yeah, you're so sweet.]