Opossum Genome May Answer Human Questions

Human DNA isn't that different from what you find in other mammals. So how does it combine to form people in some cases and dogs or chimps in others? It's one of the big mysteries of biology. And now biologists are turning to the opossum for answers.

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Our DNA isn't terribly different from what you find in a chimpanzee - or a dog, for that matter. So how does that make us uniquely human? To help answer that question, biologists have turned to the opossum. Yes, the opossum.

NPR's Joe Palca explains.

JOE PALCA: Journalists are not supposed to offer opinions, but having met the man, I think it's safe to say that you wouldn't mistake geneticist Eric Lander for an opossum.

Professor ERIC LANDER (Biology, Massachusetts Institute of Technology; Geneticist, Broad Institute): No, probably not. Opossums are cute little things. They fit in the palm of your hand, and the a little snouts - they're adorable little things, but you wouldn't mistake me for an opossum.

PALCA: Lander is a geneticist at the Broad Institute in Cambridge, Massachusetts. He and his colleagues have just completed a preliminary sequence of all the DNA in the opossum genome. But they didn't do that because opossums are cute little animals that sit in the palm of your hand. They had more selfishly human reasons.

Prof. LANDER: If you really want to understand the human genome, the only way you can understand it is by comparing it to other things.

PALCA: To see what DNA sequence has change as you move from animals that evolved long ago to more modern creatures like - well, us. To start with, Lander and his colleagues made comparisons between us and mammals like mice and dogs and chimpanzees.

Prof. LANDER: But there's a whole another branch of mammals called marsupial mammals - most famously in Australia kangaroos and wallabies, but also all over South America with opossums. And we parted company from those marsupials 180 million years ago in evolutionary time, and they represent a different way to make mammals.

PALCA: By studying the differences between opossums and mammals that don't keep their young in a pouch, Lander and his colleagues found something surprising. As they report in the journal "Nature," it's not a whole new set of genes that makes humans so different from the opossum.

Prof. LANDER: The vast majority of innovation in the human genome has been not in the invention of new genes at all, but more than 95 percent of all the major innovations have been in the controls for genes, the regulations of when to turn them on or off.

It's not as if the symphony has been inventing lots of new notes. It's been figuring out what order to play them in.

PALCA: Lander says science has had spent the last several decades focusing on the DNA that spells out genes, and they are only just now coming to realize there are sections of DNA that don't spell out a gene but are critically important because they orchestrate the way genes work.

Data from the opossum genome revealed something else interesting. Genomes are full of something called transposons - these are bits of DNA that jump around to different places without any evident rhyme or reason. And they bring with them instructions to turn on or off any genes they happen to land next to.

Geneticist David Haussler of the University of California, Santa Cruz says depending on where they land, these control switches can change the kind of animal the genes produce.

Mr. DAVID HAUSSLER (Geneticist, University of California, Santa Cruz): Evolution is marvelously sensitive, and it's going to pick up on some of these copies that are right located near an important gene. And they're going to change the regulation of that gene, and that is going to be the material for the creativity of evolution.

PALCA: Haussler says you can think of these transposons as stirring the pot, giving evolution some options - which probably explains why an opossum and Eric Lander look so different.

Joe Palca, NPR News, Washington.

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