A bird that previously went extinct rose from the dead after it evolved all over again, scientists have found.
The last surviving flightless species of bird in the Indian Ocean, a type of rail, has actually been around before, the research found. It came back through a process called "iterative evolution", which saw it emerge twice over, the researchers found.
It means that on two separate occasions – tens of thousands of years apart – a species of rail was able to colonise an atoll called Aldabra. In both cases it eventually became flightless, and those birds from the latter time can still be found on the island now.
Iterative evolution happens when the same or similar structures evolve out of the same common ancestor, but at different times – meaning that the animal actually comes about twice over, completely separately.
This is the first time it has been seen in rails, and one of the most significant ever seen in a bird of any kind.
White-throated rails are roughly the size of a chicken. They come from Madagascar, but repeatedly colonise other isolated islands, growing in number and then heading out of the island where they began.
Many of those that left to go north or south either died or were eaten. But some of the ones that headed eastwards went to live on the other ocean islands in the area, which includes Aldabra.
Aldabra does not have predators, and so the rails gradually lost the ability to fly. But then the island completely disappeared when it was covered by the sea, and the rail was wiped out, along with everything else on the island.
But after that event, some 100,000 years ago, the sea levels fell again and the atoll was once again taken over by flightless rails. By comparing the bones of those after and the ones before, researchers found that the evolution happened twice over a few thousand years ago.
"These unique fossils provide irrefutable evidence that a member of the rail family colonised the atoll, most likely from Madagascar, and became flightless independently on each occasion," said lead researcher Dr Julian Hume, avian paleontologist and Research Associate at the Natural History Museum.
"Fossil evidence presented here is unique for rails, and epitomises the ability of these birds to successfully colonise isolated islands and evolve flightlessness on multiple occasions."
There is no better evidence of such a process happening to a bird, the researchers said.
"We know of no other example in rails, or of birds in general, that demonstrates this phenomenon so evidently. Only on Aldabra, which has the oldest palaeontological record of any oceanic island within the Indian Ocean region, is fossil evidence available that demonstrates the effects of changing sea levels on extinction and recolonisation events," said co-author Professor David Martill, from the School of Earth and Environmental Sciences at the University of Portsmouth.
"Conditions were such on Aldabra, the most important being the absence of terrestrial predators and competing mammals, that a rail was able to evolve flightlessness independently on each occasion."