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From NPR News, this is ALL THINGS CONSIDERED. I'm Robert Siegel.

AUDIE CORNISH, HOST:

And I'm Audie Cornish. You may have noticed that big, flightless birds are scattered across the continents of the southern hemisphere. Africa has the ostrich, Australia has the emu and South America has the rhea. Well, scientists used to think they knew how these crazy-looking birds got to where they are today. Now, as NPR's Nell Greenfieldboyce reports, they're astonished to learn just how wrong they were.

NELL GREENFIELDBOYCE, BYLINE: The ostrich is probably the most famous member of a group known as the ratites - primitive-looking, flightless birds. Evolutionary biologists have puzzled over them for more than a century.

ALAN COOPER: And the mystery was how did they turn up on all the southern continents, and why were they so large, these great big flightless things.

GREENFIELDBOYCE: Alan Cooper is director of the Australian Centre for Ancient DNA at the University of Adelaide. He says ratite birds, like the ostrich and the emu are just weird.

COOPER: They don't do any of the conventional bird things. They don't fly, and they're fairly unspectacular-looking in many ways, apart from the fact they're huge.

GREENFIELDBOYCE: Scientists used to think all of these large, flightless birds descended from a large, flightless ancestor that lived millions of years ago when the southern hemisphere had a giant supercontinent. That supercontinent broke up - Africa drifted away, then India and South America. Cooper says people thought that if these land masses moved...

COOPER: The ratite birds were sitting on board and were being separated by these continents moving and that that's how they came to be where they were. And because they're flightless, it seemed like a pretty good model.

GREENFIELDBOYCE: Pretty good, but also wrong - or at least that's what Cooper now thinks, because of a recent surprise discovery. He and some colleagues extracted DNA from the bones of an extinct bird called the elephant bird. It lived on the island of Madagascar until about 1,000 years ago. It looked like an ostrich, only about 30 percent bigger. It didn't fly, obviously. When Cooper's team compared its DNA to all the other birds in this flightless group, they found that its closest relative was the kiwi.

COOPER: The last one I would have predicted if I had to guess that it was going to be related to would be the kiwi.

GREENFIELDBOYCE: Cooper was shocked. The kiwi is a chicken-sized bird that lives in New Zealand, a place with no geographic connection to Madagascar.

COOPER: And you can't have a continent or route to get from Madagascar to New Zealand. It's just completely the opposite side of the world and there's never been a close relationship between these two places.

GREENFIELDBOYCE: So, how did a bird get from one place to the other? He was left with only one explanation.

COOPER: It has to be flying. You can't get from Madagascar to New Zealand any other way.

GREENFIELDBOYCE: He says small ancestral birds must have flown long distances, taking up residence in new places and then independently evolving into the big, flightless birds we see around the southern hemisphere today. All of this is reported in the journal Science.

ALLAN BAKER: No one would ever have expected it.

GREENFIELDBOYCE: Allan Baker is a researcher at the Royal Ontario Museum in Toronto. He used to believe that separating continents and not flying explained the history of these birds, but says this new evidence made him think again.

BAKER: You can't close your mind to this. And so when other evidence comes up that points this out, you know, you have another hypothesis that you're testing and it appears to be a much better explanation of what's going on. So, I think that's fabulous. It's how science progresses.

GREENFIELDBOYCE: One remaining question is why these birds around the world would all independently lose the ability to fly and get big. Cooper notes that this happened right after the extinction of the dinosaurs. Between that event and the eventual rise of mammals, there would be a time with no predators, when birds could let themselves become big and flightless, with no danger of being eaten. Nell Greenfieldboyce, NPR News.

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