Scientists have discovered a species of bird which was previously extinct is alive again thanks to an extremely rare process called "iterative evolution".

They found the birds – white-throated rails – which were wiped out 136,000 years ago in a flood on an isolated atoll have now re-emerged in the same place.

The birds had repeatedly evolved from the same ancestor on the the Aldabra group of islands in the Indian Ocean.

White-throated rails are a flightless bird native to that ocean and evolved after colonising the islands which are almost untouched by humans.

The birds which migrated to these islands became flightless both times that they colonised them, and the last surviving colony of this new species is still found on the atoll today.

It is the first time that scientists have seen iterative evolution in rails and is one of the most significant instances to be found in birds, according to the University of Portsmouth, which carried out the research with the Natural History Museum.

Iterative evolution refers to the repeated evolution of species from the same ancestor at different times. It is an example of how very similar evolutionary pressures result in the same evolutionary path for organisms.

Only about the size of a chicken, the original rail is indigenous to Madagascar. Groups have historically made many attempts at migration, many to Africa where they were eaten by predators, but some to islands in the Indian Ocean.

Because there are no predators across the Aldabra atoll where some of these groups landed, the birds evolved and lost the ability to fly – similar to the dodo.

This meant that a rare flood 136,000 years ago wiped out all of the plants and animals on the islands, including the flightless rails which were unable to escape.

When the sea-levels fell during the subsequent ice age, the white-throated rails from Madagascar recolonised the islands again.

Bones from 100,000 years ago recovered by the researchers show that the rails which landed on the atoll both before and after the ice age repeatedly became flightless.

The lead researcher in the study, which is published in the Zoological Journal of the Linnean Society, Dr Julian Hume, said the evidence was irrefutable.

Dr Hume, an avian palaeontologist and research associate at the Natural History Museum, said: 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.

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.

One of the paper’s co-authors, Professor David Martill, from the University of Portsmouth, 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.

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.

(c) Sky News 2019: Scientists discover bird that came back from the dead