A
flightless bird that became extinct when its home island became flooded by the
sea "came back to life" when a similar species evolved in the same
location, scientists have discovered. Researchers from the University of
Portsmouth and the Natural History Museum found that a species of rail successfully
colonised an isolated atoll called Aldabra in the Indian Ocean on two occasions
separated by tens of thousands of years.
And
on both occasions, the white-throated rail - indigenous to Madagascar - evolved
completely independently to become flightless. The last surviving colony of the
flightless rails is still found on the island. A University of Portsmouth
spokesman said: "This is the first time that iterative evolution - the
repeated evolution of similar or parallel structures from the same ancestor but
at different times - has been seen in rails and one of the most significant in
bird records."
He
explained that the rail species are persistent colonisers that would migrate
from Madagascar during frequent population explosions. One group colonised the
Aldabra atoll and because of the lack of predators, like the dodo of Mauritius,
they evolved in a way that they lost the ability to fly.
He
explained: "Aldabra disappeared when it was completely covered by the sea
during a major inundation event around 136,000 years ago, wiping out all fauna
and flora including the flightless rail.
"The
researchers studied fossil evidence from 100,000 years ago when the sea-levels
fell during the subsequent ice age and the atoll was recolonised by flightless
rails. The researchers compared the bones of a fossilised rail from before the
inundation event with bones from a rail after the inundation event. They found
that the wing bone showed an advanced state of flightlessness and the ankle
bones showed distinct properties that it was evolving toward flightlessness. This
means that one species from Madagascar gave rise to two different species of
flightless rail on Aldabra in the space of a few thousand years."
Dr
Julian Hume, avian paleontologist 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."
Co-author
of the study published in the Zoological Journal of the Linnean Society,
Professor David Martill - from the School of Earth and Environmental Sciences
at 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."