Around 136,000 years ago, the Aldabra atoll in the Indian Ocean was inundated by a major flood that wiped out all the terrestrial animals that lived there—among them a species of flightless bird called the Aldabra rail. Tens of thousands of years later, sea levels fell back, once again making life possible on the atoll. And, according to a new study, the once-extinct Aldabra rail came back.
Writing in the Zoological Journal of the Linnean Society, Julian Hume of the Natural History Museum at Tring in the U.K. and David Martill of the University of Portsmouth explain that this feat of resurrection was made possible by “iterative evolution”—a rare process that involves the evolution of “similar or parallel structures” from the same ancestral lineage, but at different times. Or, as Sophie Lewis of CBS News, puts it, iterative evolution means that “species can re-emerge over and over, despite past iterations going extinct.”
The Aldabra rail is a subspecies of the white-throated rail (Dryolimnas cuvieri), which is indigenous to islands in the southwestern Indian Ocean. The birds are “persistent colonizers,” according to the University of Portsmouth; they are known to build up on large land bodies and subsequently depart en masse, possibly triggered by overcrowding and a lack of food.
“Something sets them off and they fly in all directions,” Hume tells Josh Davis of the Natural History Museum. “It can happen every fifty years or every hundred years. People still don't really understand it, but if the birds are lucky some of them will land on an island.”
At some point in the distant past, rails landed on Aldabra. There were no predators on the atoll, rendering the birds’ ability to fly unnecessary—so they lost it. And in the wake of the inundation event, the process happened again: Rails arrived on Aldabra and, faced with a lack of predation, once again lost their flight.
“In 20,000 years or less, the rails were evolving flightlessness again,” Hume tells Gizmodo’s Ryan F. Mandelbaum. “Evolution can be incredibly quick if the conditions are right.”
The researchers were able to piece together this evolutionary puzzle by studying fossil evidence from before and after the atoll was flooded. More specifically, two humeri dating to at least 136,000 years ago were compared to another rail leg bone found in a deposit that is around 100,000 years old. The researchers also looked at modern rail specimens—some originating from birds that could fly, and some from Aldabran birds that could not, according to Mandelbaum.
They found that the pre-flood specimens are very similar to the bones of the flightless rails that exist on Aldabra today. And the leg bone belonging to a rail that lived on Aldabra in the immediate post-flood period suggests that the bird was in the process of losing its flight—or, in other words, that virtually the same subspecies was evolving on Aldabra for the second time.
“[F]rom that one bone we can see that it is already becoming more robust when compared to the flying rail, showing that the bird is getting heavier and so losing its ability to fly,” Hume says.
The study authors say their findings offer “irrefutable evidence that Dryolimnas subsequently recolonized Aldabra after inundation and became flightless for a second time.” It is very rare to find such patent signs of iterative evolution in the avian fossil record, and unheard of for the rail family, according to the researchers.
Today, the flightless rails that exist on various islands are vulnerable to predation by introduced predators like cats and rats. The Aldabra rail is, in fact, the only flightless rail that still survives in the Indian Ocean. But the new study shows how quickly evolution works to favor flightlessness in this bird species—provided that conditions are right.