Thanks to an obscure evolutionary phenomenon, a small flightless, chicken-sized bird was able to bring itself back from extinction.
A new study from the University of Portsmouth and Natural History Museum in London revealed that the White-throated Rail — also known as the Aldabra Rail — has gone extinct and returned back from the dead over the course of tens of thousands of years.
The species’s ancestors were native to Madagascar, but, due to population growth, large flocks would often emigrate from the island. The flocks who flew to the north and south drowned in the ocean. Predators ate the groups who travelled west and landed in Africa. But those who ventured east were the luckiest, colonizing a few isolated ocean islands, including Aldabra. Aldabra, a ring-shaped atoll in the Indian Ocean, became their home. Since there were no real predators to fly away from on the atoll, the rail’s continued evolution left them flightless. Unfortunately, this meant they were unable to escape when sea level rose and inundated Aldabra. The atoll flooded and disappeared, wiping out all resident flora and fauna on the island, including the White-throated Rail.
But sea levels fell during an ice age several thousand years later. And somehow, the rail’s ancestors from Madagascar appeared again and recolonized the atoll. “Only on Aldabra, which has the oldest paleontological 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 recolonization events,” said study co-author David Martill, of the University of Portsmouth.
Researchers studied two sets of fossils from the species: two pre-flood wing bones dating back 136,000 years compared to a post-flood leg bone around 100,000 years old. They also examined specimens of modern rails, of both the birds that could fly and those that could not. They found that the pre-flood bones were incredibly similar to the bones of the flightless rails that currently exist in that they showed an advanced state of flightlessness. The ankles in the leg bone from the post-flood fossil showed distinct evidence that it was evolving toward flightlessness. Meaning, the original species from Madagascar evolved into the same species of flightless rail on Aldabra after the extinction event, within a span of only a few thousand years. The White-throated Rail is still alive and flourishing on the atoll today.
“These unique fossils provide irrefutable evidence that a member of the rail family colonized the atoll, most likely from Madagascar, and became flightless independently on each occasion,” said the lead researcher for the study, Dr. Julian Hume, avian paleontologist and Research Associate at the Natural History Museum. “Fossil evidence presented here is unique for rails, and epitomizes the ability of these birds to successfully colonize isolated islands and evolve flightlessness on multiple occasions.”
This process of extinction and recolonization follows an extremely rare phenomenon called “iterative evolution.” Iterative evolution is the repeated evolution of a species from the same ancestor at different times in history. This is the first time iterative evolution has occurred in rails and one of the only known and the most significant instance of iterative evolution in any bird population.
Header image: The Aldabra atoll. Photo by UNESCO.
Also published on Medium.