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Over the next few weeks we'll be bringing you The Human Edge. That's our series exploring, bit by bit, how evolution brought Homo sapiens into being. In this first story, NPR science correspondent Joe Palca discovers that we humans owe just about all that we are to other species that lived long before the first ape existed.

JOE PALCA: It took him years of searching in the Canadian Arctic, but in 2004, Neil Shubin found the fossilized remains of what he thinks is one of our most important ancestors.

Now, the bones are in Shubin's lab at the University of Chicago and he shows them off like a proud parent.

Dr. NEIL SHUBIN (University of Chicago): This is a shoulder, a massive shoulder. You can hold it but you got to do really two hands though, 'cause it's (unintelligible).

PALCA: Looks like a dark chunk of rock to me, but what do I know? Shubin hands me another chunk.

Dr. SHUBIN: This is the elbow of this fish.

PALCA: Yes, he said fish.

Dr. SHUBIN: Fish.

PALCA: Fish.

Dr. SHUBIN: So, you're looking at a fish's elbow. Isn't that cool?

PALCA: Shubin says this ancient fish, which he named Tiktaalik, is one of our key ancestors because it has an elbow and a shoulder - two things animals would need when they made the transition from water to land. In Tiktaalik, you also see what would ultimately become legs for walking.

Dr. SHUBIN: You see a head that can move independently from the body 'cause it now has a neck.

PALCA: You see a wrist that can move back and forth. Not the same wrist as we have or the same neck or the same elbow.

Dr. SHUBIN: Everything that we have are versions of things that are seen in fish.

PALCA: There are things that we have that Tiktaalik doesn't.

Dr. SHUBIN: We have a big brain, and portions of those big brains are not seen in creatures like Tiktaalik. But the template, all the way down to the DNA that builds it, is already present in creatures like this.

PALCA: Inside this fish, Neil Shubin sees us.

Dr. SHUBIN: It's like peeling an onion. Layer, after layer, after layer is revealed to you. Like in a human body: the first layer is our primate history; the second layer is our mammal history, and on and on and on and on, 'til you get to the fundamental molecular and cellular machinery that makes our bodies, keeps are cells alive and so forth.

PALCA: Peel back enough layers of the onion and you may be surprised at what you get to.

(Soundbite of music)

PALCA: Not only are we related to an ancient fish fossil...

(Soundbite of music)

Unidentified Woman: (Singing) Budweiser beer, the king is second to none...

PALCA: In some ways, we're related to a glass of beer. Yeast is critical for making beer and many of the parts critical for making yeast are also critical for making us.

Dr. GAVIN SHERLOCK (Yeast Geneticist, Stanford University): About one-third of the yeast genes have a direct equivalent version that still exists in humans.

PALCA: That's Gavin Sherlock. He's a yeast geneticist at Stanford University. Turns out, we share a common ancestor with yeast - an ancestor more than a billion years old.

Sherlock says not only do many of the same genes still exist in humans and yeast; they're so similar that you can exchange one for the other.

Dr. SHERLOCK: There are several hundred examples where you can knock out a yeast gene, you put in the human equivalent, and it restores it back to normal.

PALCA: Sherlock says, think about it. We have a lot in common with yeast. Yeast consume sugars like we do, yeast make hormones like we do, yeast have sex well, not quite like we do - but sex.

And in case you're curious, here's how yeast do it - and don't worry, you don't have to ask the children to leave the room. The yeast equivalent of a boy will sidle up to the yeast equivalent of a girl.

Dr. SHERLOCK: And then when they get close enough they form what's called a schmoo.

PALCA: A schmoo.

Dr. SHERLOCK: And they schmoo towards each other, touch and then fuse. That's yeast sex for you.

PALCA: Sounds OK to me.

And by the way, sex isn't just fun and games you know. Sexual reproduction is critical for stirring the genetic pot, speeding the evolution of endless forms most beautiful; from fruit flies, to blue whales, to humans.

Now, yeast is a single-celled organism. We have trillions and trillions of cells in our bodies different kinds of cells, all fitting together. How did that happen?

Well, we can't see the answer to that question in Gavin Sherlock's lab in Stanford. We have to go somewhere else. Somewhere wetter.

(Soundbite of song, "Under the Sea")

Mr. SAMUEL WRIGHT (Singer): (Singing) Under the sea, under the sea...

PALCA: Long before the fish Tiktaalik was getting ready to leave the sea for dry land, cells were just starting to assemble themselves into a variety of simple forms. You can see a depiction of some of the evolutionary efforts at the Field Museum in Chicago. Neil Shubin won't let you miss them.

Dr. SHUBIN: This tiny little diorama here, which you would just walk by, is arguably one of the most important ones for understanding our bodies.

PALCA: Understanding our bodies?

Dr. SHUBIN: Yeah, you wouldn't know it by looking at it. I mean, what you see is like plastic fronds and jellyfish-like creatures in this primitive ocean. But it's here where single-celled creatures like bacteria and other microbes got together to make the first bodies.

PALCA: And as time goes on, more forms emerge. Again, we stop at a display that's easy to miss.

Dr. SHUBIN: This little smudge on the rock is real important, because this is the earliest known creature with our kind of body plan.

PALCA: It's an ancient worm.

Dr. SHUBIN: We have a left and a right, a front and a back, a top and a bottom. The whole coordinate axis of the body. And in fact, we believe, if you look at the sort of evolutionary history of these things, many of the genetic processes that make bodies like this and bodies like our own, arose over 500 million years ago.

PALCA: As Shubin and I walk through the exhibit, we see the results of tinkering with these genetic processes. As time goes by we come to the fish Tiktaalik then to dinosaurs, then to the first mammals, then to the first apes - evolutionary variations on a 500 million-year-old theme.

Finally, we come to a familiar-looking four-foot tall creature. This is Lucy, an Australopithecus, more apelike than modern humans, but getting there.

Dr. SHUBIN: I look at Lucy and I see an entire branch of the tree of life.

PALCA: Despite Lucy's proximity to humans, she's clearly not human. Australopithecus went extinct.

On the way to us, something changed, something more than just physical. Shubin points to a cabinet across the room.

Dr. SHUBIN: And that's so beautiful.

PALCA: Inside is a re-creation of a prehistoric human burial site.

Dr. SHUBIN: A ritual burial.

PALCA: There's the skeleton of a woman. She's been placed in the grave, surrounded by her jewelry.

Dr. SHUBIN: It's hard to look at this as a fossil anymore. You know, you look at this as a person who lived, and about how people loved this person enough to do this. And that's what changed.

PALCA: Shubin the scientist is now Shubin the human being, looking at one of his ancestors. He spends his professional life tracking how species changed and evolved. But in the end, it's not a bone or a muscle or a gene that made us human. It was something else.

Dr. SHUBIN: The physiology and genetics made that possible. That's the template that made all this happen, you know. But when was that spark, when was that moment? We don't know.

PALCA: That moment that gave us the evolutionary edge that led to what we are today the species that buries its dead, builds museums, explores outer space. Shubin says it's the culture we built with our bones and muscles and brains that makes our species unique.

Joe Palca, NPR News.

(Soundbite of music)

MONTAGNE: You can hear what our fishy relatives reveal about the origins of human shape and skin color this afternoon on ALL THINGS CONSIDERED. The Human Edge continues next week on MORNING EDITION.

(Soundbite of music)

MONTAGNE: This is NPR News.

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