DNA Snippet Sets Human and Chimp Brains Apart

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The human brain definitely differs from the brains of our primate relatives. But how did we get such big brains? A paper in the journal Nature says part of the answer may lie in a snippet of DNA buried deep in the human genome.


From NPR News, this is ALL THINGS CONSIDERED. I'm Melissa Block.

It doesn't take a scientist to know that humans are different from chimpanzees. It does take a scientist to figure out way. Now an international scientific team reports in the journal Nature it's found something that may be an important piece of the explanation.

NPR's Joe Palca has the story.

JOE PALCA reporting:

There are three billion DNA letters or base pairs, as scientists call them, in the human genome. Only a small fraction of these letters are genes that make proteins. For decades scientists looking for genetic differences between humans and chimps have been focusing on protein coding genes because proteins do the work inside cells.

Ms. SOPHIE SOLOMA(ph) (University of California Santa Cruz): We felt that by limiting ourselves to looking at protein coding regions we would be casting our net too narrowly.

PALCA: Sophie Soloma is a research biologist at the University of California Santa Cruz. Soloma says to broaden the search, she and her colleagues asked a simple question. If you line up the DNA from several different species, are there regions that are similar between chickens, rats, mice and chimpanzee, but very different in humans. The team found 35,000 candidates but Soloma says one stood out.

Ms. SOLOMA: It's a region of the genome of 118 base pairs. It happens to lie on chromosome 20, and there are 18 differences between the human and chimpanzee sequence.

PALCA: For comparison there are only two differences between chimpanzees and chickens in the same region. It sound became clear this region didn't code for a protein. Instead it coded for something that told other genes what to do.

What else did you know about this region at the time?

Ms. SOLOMA: Well, to start, not much. That's the challenge.

PALCA: There were clues that it might, might have something to do with genes in the brain and maybe, just maybe this little section of DNA might explain why human brains are so much larger than chimp brains. The next step was to find someone who could see what this DNA was doing in the developing brain. So she sweet talked a colleague in Belgium into taking on the project.

Ms. SOLOMA: I said, you know, if you're interested in new critical development genes, I have a candidate for you.

PALCA: He said he'd look into it, and Soloma gave him some biological probes that would allow him to see if her DNA snippet was doing anything as the fetal brain was developing.

Ms. SOLOMA: And I didn't hear anything for a long time and I just sort of thought, well, that was that. Either it didn't work or he didn't want to do the experiment. But about I think it was three or four months later, I got this email saying, Dear Sophie. Remember those probes you gave me.

PALCA: Something very interesting was going on, he wrote. The probes showed Soloma's DNA seemed to be active at the very time the brain was developing. Possibly orchestrating other genes so they'd make a big, human-sized brain. For geneticist Mary Claire King, Soloma's result is extremely gratifying. King is now at the University of Washington, but in the early ‘70s she was a graduate student at the University of California Berkley campus.

Ms. MARY CLAIRE KING (Geneticist): My PhD dissertation was a demonstration that humans and chimpanzees differ by only one percent in the sequence of genes that code for protein.

PALCA: So Kind suspected something else had to be going on to account for the obvious differences between them, something that didn't code for proteins but perhaps controlled other genes, exactly what Soloma seems to have found 30 years later.

But there's bound to be more to the story of why chimps and humans went their separate ways. Tom Curran is a researcher at Children's Hospital of Philadelphia, where he studies how genes affect the brain.

Mr. TOM CURRAN (Children's Hospital of Philadelphia): As always in biology your ideas at the beginning are fairly naïve, and once you do a few experimental tests, you'll find those ideas. And sometimes you go off in a completely different direction.

PALCA: With luck, human scientists' big brains will eventually point the way to the right direction.

Joe Palca NPR News Washington.

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