How A Bird Became Flightless Through Evolution — Twice

Jun 11, 2019
Originally published on June 12, 2019 10:10 am
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MARY LOUISE KELLY, HOST:

To another story now. In the 1980s, researchers excavated fossils on a small atoll in the Indian Ocean. The rocks then sat at the Smithsonian Museum of Natural History here in Washington for years, until recently, when a paleontologist processed eight tons of them. When his team dissolved the limestone, they found rare evidence of evolution repeating itself. Irina Zhorov has more.

IRINA ZHOROV, BYLINE: The rail is a chicken-sized bird with long legs and an extended beak. Rail subspecies live all over the world, including on Aldabra, some 600 miles north of Madagascar. Paleontologist Julian Hume at the Natural History Museum in London says the Aldabra rail's ancestors probably flew up from Madagascar more than 100,000 years ago. When they landed on the island, the bird changed.

JULIAN HUME: When a rail chick hatches, it's a pair of legs with a little ball of a body on top. And it's able to run really fast. It can feed itself and so on. And the last thing to actually develop as that rail matures are the wings.

ZHOROV: Flying takes energy. Aldabra had food on the ground and no predators, so the birds ditched it. Today Aldabra rails are completely flightless and cute.

(SOUNDBITE OF BIRD SCREECHING)

HUME: Rails have, like, a rasping, wheezy noise.

(SOUNDBITE OF BIRD SCREECHING)

HUME: It's been described as dropping marbles down on tin roofs.

ZHOROV: When Hume started going through the Aldabra fossils, he found old rail bones. The material came from two sites on opposite sides of the island chain.

HUME: They separated not only by distance on Aldabra but also by age.

ZHOROV: Two of the bones are at least 136,000 years old. Another is about 20,000 years younger. There's just one problem.

HUME: Aldabra, in between the two deposits, had gone completely underwater, so everything disappeared due to a sea level rise.

ZHOROV: That means rails from Madagascar came to Aldabra and evolved flightlessness. They died out when the island was underwater. Then more rails flew over from Madagascar and again evolved flightlessness. Hume and co-author David Martill published their findings recently in the Zoological Journal Of The Linnean Society.

JULIA CLARKE: What's striking in this paper is potentially that they have this strong bound that indicates that these could not possibly be the same instance of flight loss.

ZHOROV: Julia Clarke is a paleontologist at the University of Texas at Austin. She says evolution can perform the same trick lots of times, but we often don't see clear evidence of such repeat evolution in the fossil record.

CLARKE: I sometimes describe studying the fossil record as you're looking through glasses. And these glasses are somehow dirty, so your field of vision is incomplete. And you're trying to look into great distances. And so necessarily, we don't get the full picture.

ZHOROV: The study is also interesting because it gives researchers a timeline, says Hanneke Meijer with the University of Bergen.

HANNEKE MEIJER: We've seen flightlessness in rails very often on many different islands all over the world, but there's always been a question about, how quickly does this happen? How quickly after a bird arrives on islands - a flying bird - how quickly afterwards does it start to lose its ability to fly?

ZHOROV: She says it happened fast.

MEIJER: Even though geologically, this is still 10-, 20,000 years apart, in the fossil record, that is still relatively quickly.

ZHOROV: Julian Hume is going to Aldabra next year for the first time and is looking forward to seeing the living Aldabra rails.

HUME: It just will pass me as if I don't exist. And I've just spent all these years working on where it came from and what it was doing, you know? (Laughter).

ZHOROV: For NPR News, I'm Irina Zhorov. Transcript provided by NPR, Copyright NPR.