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MICHELE NORRIS, Host:

From NPR News, this is ALL THINGS CONSIDERED. I'm Michele Norris.

MELISSA BLOCK, Host:

And I'm Melissa Block.

It's time now for the latest installment in our series "The Human Edge," tracking how evolution made us into one of the most successful species on Earth. So far, we've talked about the physical changes that made us human. And today, we turn to the question of how we became human in our minds.

NPR: symbolic thinking.

ALIX SPIEGEL: Ever since Darwin came up with this whole idea of evolution, there's been one dominant picture of the moment that we truly became human. You see a hairy ape man with a heavy brow, hunched in profile. Then, bit by bit, his back uncurls and straightens until all of a sudden there is he, upright, truly a man.

Recently, I've been thinking about this picture because I've decided that we somehow ended up with the wrong one - that there's something much more fundamental to being human than our ability to stand upright. Think for one minute about the beginning of your day. If you're anything like me, it goes something like this:

(SOUNDBITE OF AN ALARM)

SPIEGEL: About two hours before you'd actually like to be conscious, the numbers on your alarm clock hit that magical combination, 6:15. You head to the shower and turn on the news.

DON GONYEA, Host:

This is MORNING EDITION from NPR News. I'm Don Gonyea, in for Steve Inskeep.

RENEE MONTAGNE, Host:

And I'm Renee Montagne...

SPIEGEL: All the talking fills your head with different pictures from faraway places, very nice. And then comes the problem of trying to dress yourself for work.

Too informal...

You leaf through the hangers in your closet, in search of something that might suggest competence, professionalism...

To schlumpy...

You go through a lot of hangers.

Whoops.

Finally, you find something. But as you're zipping yourself up, your 3-year- old comes in and decides that the closet is not, in fact, a closet - but a train. He's into trains.

Unidentified Child: We're going to Asia.

SPIEGEL: Finally, it's off to the corner store for coffee.

Okay, here's $2.

You have been awake for approximately two hours, and almost every moment of your day has been predicated not on your ability to stand upright, but on something else entirely: your completely underrated, chronically overlooked capacity for symbolic thought.

If you don't believe me, just ask Alison Brooks, an anthropologist at George Washington University.

ALISON BROOKS: Thinking symbolically is the foundation of everything we do. We live in a symbolic world. It is the most fundamental thing about us.

SPIEGEL: Okay, so I know what you're thinking: What, exactly, is symbolic thought? If you're fuzzy on this, don't feel bad. A couple weeks ago, I took a tape recorder around NPR to see if anyone here could cough up a decent definition.

Unidentified Woman #1: Symbolic thought - that's a hard one. Um...

NORRIS: I have no idea.

NORRIS: I have no idea.

Woman #1: Um...

(SOUNDBITE OF LAUGHTER)

SPIEGEL: So then I posed another question: Name five symbols. This was much easier.

BLOCK: A cross is a symbol of Christianity. Our flag is often a symbol for the United States.

NORRIS: I would say the peace sign is a symbol.

BLOCK: A wedding ring.

SPIEGEL: Wedding rings, flags, Christian crosses, peace signs. When we think about symbols, these are the sorts of things that come to mind, signs that act as a stand-in or shorthand for a whole set of ideas. But in fact, the symbols play a much larger role in our lives.

Let's return to one of the most basic parts of your day: getting dressed.

Too informal, too schlumpy...

Every piece of clothing you place on your body is a symbol. That leather motorcycle jacket or button-down polo communicates to the world who you are, what you believe in, and where you sit on the social ladder. And they do that instantly.

For example, Alison Brooks points out that her school, George Washington University, is located only 30 feet away from the office of the World Bank, so students and bankers are constantly walking the same streets.

BROOKS: However, you can easily tell - in a second - which ones are the students and which ones are the world bankers because the students dress like students, and the world bankers dress like bankers. They all wear suits or very formal clothing. Each of those different populations gets up in the morning, and puts on symbols of their status.

SPIEGEL: So that's getting dressed. Now, let's consider the next moment in your day.

Child: We're going to Asia.

SPIEGEL: Your 3-year-old has declared that he and the closet are going to Asia. Asia, like America, is a concept that depends on our ability to think symbolically. We can recognize it on a map; we can look up its GDP. But ultimately, America is an abstraction.

BROOKS: We're never going to meet all the other Americans on the planet. We're never going to meet all the other citizens of our state or city. But we create these communities in our minds.

SPIEGEL: The numbers on your alarm clock, symbols. The money you pass to the lady at the coffee shop, also a symbol - a symbol of the gold in Fort Knox, which itself is symbolic of something else: power.

And finally, there's the stuff you heard coming out of the radio during your shower: language. In order to have language, any language, you need to be able to think symbolically. Think of the word...

Unidentified Man #2: Cat.

SPIEGEL: The word...

Man #2: Cat.

SPIEGEL: ...sounds nothing like a cat sounds.

(SOUNDBITE OF A CAT)

SPIEGEL: And the written word, C-A-T, looks nothing like a cat. The word...

Man #2: Cat.

SPIEGEL: ...is completely arbitrary. Still, when someone says...

Man #2: Cat.

SPIEGEL: You see a cat in your mind's eye.

Language, Brooks says, is entirely composed of these arbitrary symbols.

BROOKS: As I'm talking to you, every sound that comes out of my mouth has some kind of arbitrary meaning assigned to it. I could just as well be talking to you in another language and making totally different sounds, and saying the same thing.

SPIEGEL: And the miracle is that these arbitrary sounds, these symbols, allow us to see what's going on in other people's minds, and also allow us to share what's going on in ours.

For example, Brooks says, think of the word bead.

BROOKS: You immediately have a picture in your mind of what I'm talking about. If I said glass beads, you immediately see something a little bit different, if I said gold beads. So what I have done, then, is to make you think in your mind of a thing that I have in my mind. And that is an extraordinary thing to have done.

SPIEGEL: And once we have this ability for symbolic thought and language, all kinds of things become possible. Through language, we can pass down what we've learned, organize larger and larger groups of people who can do more and more complex things - like build bridges and schools and computers and, you know, everything else in modern life.

BROOKS: We could not invent things. We couldn't build on the advances of the past. We couldn't hold our society together. I think it's impossible to imagine our modern existence without symbolic thought.

SPIEGEL: The question to answer, though, is when did we get like this? When is the first evidence that we had acquired this magical ability and were finally, mentally modern?

Museums are full of bones under glass, fossils that can tell us when we became physically modern. But how do you find a fossil of a symbolic thought?

Well, not very long ago, a man named Chris Henshilwood stumbled upon one possible answer to this question, which is how come I decided to call him up.

Hello, can you hear me?

CHRIS HENSHILWOOD: I can hear you.

SPIEGEL: Oh, that's great.

Chris Henshilwood is an academic in Norway now. But when he was a small boy, he would often visit his grandfather's farm on the western coast of South Africa. And it was there, one day in his youth, that Henshilwood discovered a cave half-obscured by a sand dune.

HENSHILWOOD: Most of the cave was, in fact closed, but there was an entrance on one side. You could climb into it. I can remember going into that cave, probably when I was - maybe 11 or 12 years old, and thinking it was very romantic.

SPIEGEL: Thirty years later, when Henshilwood was a new archeologist trying to make a name for himself, he went back to that cave and found, under a layer of sand that dated back 75,000 years, several dozen seashells.

HENSHILWOOD: They are the size, probably - or even smaller - than the nail on your pinkie. They're very tiny, little shells. And really, if you don't look at them carefully, they're rather insignificant.

SPIEGEL: In fact, Henshilwood thought they were left over from meals or something. But then Henshilwood put them under a microscope and noticed that each of these small, insignificant shells had a tiny hole in its lip. And that's when Henshilwood had his epiphany. These shells weren't simply shells; these shells, he realized, were beads.

HENSHILWOOD: The hole that had been made was in the same place. I could very clearly see, under the microscope, the wear that had been made by the string - or whatever had been used to string these beads together. And by the time I'd looked through 30, I was convinced, right, these are beads. These are the oldest beads yet discovered.

SPIEGEL: Today on my desk, there's a pile of papers four inches thick, on Henshilwood and his shell beads. Why are a few shell beads such a big deal? Because this handful of shells was an early version of the wedding band around your finger, or the golden cross around your neck. The beads were symbols, symbols indicating to the people of that community who this person was, what they believed, whether they were friend or enemy.

HENSHILWOOD: Absolutely. There was no doubt that if we had beads, we had evidence for symbolic thought.

SPIEGEL: So with all of this in mind, I'd like to make a proposal. It's time for a new iconic image of the moment that we truly became human. And here's the one that I suggest: A hairy ape man with a thick brow sits crouched, working a tiny hole into a small shell. He pushes a strip of animal hide through the hole and suddenly, you and I are born.

Alix Spiegel, NPR News, Washington.

BLOCK: And we invite you to participate in this story. Head to npr.org/science to submit your drawing or photograph of a new iconic image of evolution. We'll feature some of the best on our website.

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