DNA Part May Play Role In Limb Formation

Scientists are becoming increasingly aware of the importance of what used to be called "junk" DNA. Yale researchers have found a region of DNA that appears to be crucial in making a human limb. Although it's not a gene in the traditional sense of the word, it is inherited.

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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. The human thumb is one of nature's great accomplishments. Beyond its usefulness for hitchhiking, our opposable thumbs allow us to grasp and manipulate tools in ways no other animal can. Now scientists are reporting new information that helps explain how the human thumb came to be. NPR's Joe Palca has our story.

JOE PALCA: Geneticist James Noonan of Yale University is trying to understand what makes humans human - why we can speak and other animals can't, why we can walk upright on two legs, why we can snap our fingers. To do that, he combs through the billions of letters of DNA that make up the human genome.

Professor JAMES NOONAN (Genetics, Yale University): We're proceeding from the idea that there are sequences in the genome that distinguish humans from other species. And we really don't know what those are or where they are.

PALCA: To find these hidden sequences, Noonan has an interesting strategy. He doesn't just look at the DNA in humans. He looks through the DNA in a variety of species: mice, chickens, dogs, chimpanzees.

Prof. NOONAN: We asked: Are there any where it's clear that the sequence has been evolutionarily stable for a long period of evolutionary time, but now in humans is different?

PALCA: You can think of it this way: Let's say there's a sequence of DNA in, say, a mouse that sounds like this…

(Soundbite of song, "Twinkle, Twinkle, Little Star")

PALCA: Then you look the same region of DNA in a chicken and a dog, even a chimpanzee, it sounds the same.

(Soundbite of song, "Twinkle, Twinkle, Little Star")

PALCA: But if you look at the sequence in humans, it's changed, mutated. It's still the same tune, but more complex.

(Soundbite of song, "Twinkle, Twinkle, Little Star")

PALCA: Geneticist Noonan searched for these DNA sequences that were different in humans than other animals and found one. As he reports in the journal Science, at first he had no idea what this sequence did. It didn't look like a traditional gene. It looked like what's called a regulatory element, something that controls when and where genes switch on and off.

So he took his human DNA sequence and injected into a mouse egg to see which cells it became active in as the mouse embryo developed. To his delight, it affected cells near the developing limbs, specifically to the place where a mouse thumb would be if a mouse had a real thumb. Noonan says this sequence, unique to humans, appears to control a critical aspect of hand development.

Prof. NOONAN: That's why we're so excited about this result, because we think maybe this sequence is one of those components that changed our evolution and is driving those sorts of developmental changes in the hand that differentiate us from other species.

PALCA: That's an extremely reasonable hypothesis, at least according to Brian Richmond. Richmond is an anthropologist at George Washington University.

Professor BRIAN RICHMOND (Anthropologist, George Washington University): Most changes in human evolution don't involve brand new structures that come out of nowhere. Rather, almost all of the ways that humans are different from other animals and other primates is mainly due to changes in the rate, timing and duration of growth.

PALCA: Richmond says the bones, tendons and muscles that make up a hand in humans and chimpanzees are basically the same. It's how fast they grow and when they start and stop growing that makes all the difference.

Prof. RICHMOND: That seems to be how we grow fingers that are shorter and grow a thumb that is long compared to an ape's.

PALCA: So it makes sense that the new DNA sequence Noonan found would control when genes are turning on and off in a developing limb.

Greg Gray of Duke University says Noonan's technique for finding hidden by important pieces of DNA should reveal a lot about evolution.

Dr. GREG GRAY (Duke University): The fact that we don't always know exactly what those pieces do doesn't mean they can't tell us something about our history, and that's what's really interesting.

PALCA: Because everybody likes to know what makes humans special.

Joe Palca, NPR News, Washington.

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