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You know, the words you're hearing right now are really just arbitrary sounds. Yet, after some decoding by your brain, these sounds makes sense, which is because humans have evolved a brain with an extraordinary knack for language. In our series, The Human Edge, NPR's looking at evolutionary changes that helped us become the earth's dominant species.

And today, NPR's Jon Hamilton explores the power of language and the uncertainty about where it comes from.

JON HAMILTON: As Jeff Elman why language is such a big deal and you get a simple answer.

Professor JEFF ELMAN (Cognitive Science, University of California San Diego): Because it's changed the face of the Earth. I mean, the Earth would not be the way it is if humankind didn't have the ability to communicate, to organize itself, to pass knowledge down from generation to generation. We'd be living in troops of very smart baboons.

HAMILTON: Elman, a cognitive scientist at the University of California, San Diego. Says language let's us cooperate in groups of millions instead of dozens. It also lets us share the complex ideas produced by our brains, and it's flexible in ways you don't find in the communication systems of other species.

Take bees. Elman says they have a remarkable ability to communicate precisely where to find pollen.

Mr. ELMAN: It's a very elaborate system. It works very well - but that's all it does. They can't talk about politics. They can't talk about, you know, who's having an affair with what other bee - and these are things that we can do.

HAMILTON: There's no single module in our brain. Instead, language seems to come from lots of different circuits. And many of those circuits also exist in other species.

Some birds can imitate human speech. Some monkeys use specific calls to tell one another whether a predator is a leopard, a snake or an eagle. And Elman says dogs read our gestures and tone of voice.

Mr. ELMAN: You could take a little bit from a dog, a little bit from a monkey, a little bit from a parrot and half a dozen other species, and you put it together and what you get is exactly the right ingredients for, you know, making language possible.

HAMILTON: What's unclear is how and when language first appeared. David Armstrong spent decades pondering that question at Gallaudet University. He says the problem is that, unlike physical changes, mental ones don't leave behind any fossils.

Professor DAVID ARMSTRONG (Gallaudet University): We have no way of knowing exactly when or how people began to speak, or in the case of sign language, when they began to sign or to gesture in a way that was complex enough for us to consider it to have been language.

HAMILTON: Language means not having just a label for, say, an ax, but being able to convey an idea like, The ax works better if you hold it this way. And there are several competing hypotheses about how language emerged.

Mr. ARMSTRONG: Gesture, visible gesture, especially gesture involving the hands, may very well have been the earliest form of complex human communication.

HAMILTON: Armstrong says evidence from fossils supports that idea.

Mr. ARMSTRONG: Relatively early on, we see the modern human hand pretty much in its current configuration.

HAMILTON: That was a couple of million years ago, not long after our ape-like ancestors stopped walking on their knuckles. But things like the modern vocal tract seemed to have arrived much later. And the modern version of a gene called FOXP2, which is important for speech and language, didn't appear until perhaps 100,000 years ago.

So, Armstrong says, early human ancestors probably used gestures to communicate.

Mr. ARMSTRONG: Articulate speech of the sort that we employ, would have been probably difficult for these animals.

HAMILTON: Also, Armstrong says, some kind of sign language would have suited the early human lifestyle.

Mr. ARMSTRONG: Clearly, one of the times that visual signing is advantageous, and especially to, say, hunter-gatherers, is when they're hunting. And you find sign systems that are used by hunter-gatherers to communicate about their collective activity, to communicate about the sort of animal that they might be pursuing.

HAMILTON: Without making sounds that could alert their prey.

Armstrong thinks these gestures eventually became associated with sounds, which got more sophisticated as the human vocal tract evolved.

Even now, there are close links between the brain centers involved in speech and those involved in sign language. Of course, once spoken language appeared, Armstrong says, it would have given our ancestors a huge advantage. You no longer had to put down your tools or your baby in order to communicate.

And then, of course:

Mr. ARMSTRONG: Up until the invention of the incandescent light bulb, people spent about half their time in the dark.

HAMILTON: Another idea about the origin of language is that it came from song.

Unidentified group of children: (Singing) A, B, C, D, E, F, G, H, I, J, K, L...

HAMILTON: Ani Patel of The Neurosciences Institute in San Diego, says that idea just feels right to a lot of people.

Mr. ANI PATEL (The Neurosciences Institute): We feel music just taps into this kind of pre-cognitive archaic part of ourselves. I reaches so deep inside us that must have come first and be there before we had this complicated articulate language that we use to do abstract thinking.

HAMILTON: Patel says even Charles Darwin thought language might have had its roots in musical expression.

Mr. PATEL: He talked about our ancestors - both males and females - singing songs - love songs to each other, essentially, without words before we could speak articulate language.

HAMILTON: And just like with language, you can see aspects of musical ability in other species. Some monkeys can recognize dissonant chords, songbirds use complicated patterns of pitch and rhythm, and a few parrots can even dance to a beat.

What's more, modern humans still combine music and speech in ways that seem innate. For example, when a parent speaks to a baby.

Mr. PATEL: It's this kind of lilting intonation. Aren't you a cute little guy. And, you know, they slow down, there is a lot of rhythm, a lot of exaggerated pitch contours, and people have speculated that this kind of way of communicating with infants might have been one of the important roots to language in our species.

HAMILTON: Patel says it's also worth noting that the brain processes music and language in a similar fashion. But he says just finding a connection between music and language, doesn't prove that music came first. Patel himself favors and explanation of the origin of language that doesn't start with gesture or music.

Instead, it comes from a behavior you see in another smart mammal with a very long life span.

Mr. PATEL: Killer whales have dialects where different pods have different characteristic calls that seem to be important in establishing identity. And our ancestors, living in small groups where affiliation and identity was important, it seems interesting to think about them having some kind of vocalizations that could have helped them identify each other.

HAMILTON: Patel says that over time, these social calls could have become words - a bit like the ones you're hearing right now.

Jon Hamilton, NPR News.

INSKEEP: And look where we've gone since then. We have more and more words. We have languages, grammar, dictionaries, crossword puzzles, other kinds of puzzles, which brings us to these two words: Robert Krulwich. Hi, Robert.

ROBERT KRULWICH: Hi there.

We know that words have so many flavors and so many connotations at this point, that for the fun of it, we thought we'd design a visual puzzle. So, if you go to NPR.org/Science, you will see a series of gorgeous scenes from every day life. Each picture supports or contains a hidden word. We've hidden about nine words in there, and the question is: can you guess what they are?

INSKEEP: I know that I can't 'cause I was never any good at these kinds of puzzles.

KRULWICH: Well, I don't know. You should give it a shot.

INSKEEP: Well, give it a shot - NPR.org/Science.

(Soundbite of music)

LINDA WERTHEIMER, host:

This is NPR News.

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