The Aldabra rail, a bird species found on the Aldabra atoll off the southeast coast of Africa, has an astonishing evolutionary history. Although initially unremarkable in appearance, this bird has a unique claim to fame – it has evolved to be flightless not once, but twice. Being the only living flightless bird in the Indian Ocean, the Aldabra rail has captured the attention of researchers due to its remarkable evolutionary journey.
In a 2019 study published in the Zoological Journal of the Linnean Society, researchers examined the fossil record of rails in Aldabra. Through their analysis, they discovered evidence of a flightless rail subspecies that lived on the atoll before it became submerged beneath the waves 136,000 years ago. This inundation led to the extinction of the flightless rail subspecies. However, when the atoll resurfaced around 118,000 years ago, the white-throated rail, a species capable of flight, recolonized the area and began its journey towards evolving into a flightless bird once again. Fossil evidence revealed that the leg bones of the rails dating back to around 100,000 years ago were heavier and more robust, indicating a transition towards flightlessness.
The ability to evolve flightlessness appears to be a beneficial trait for the Aldabra rail in its environment. Since these birds lay their eggs on the ground, having strong legs to run around immediately after hatching is crucial for their survival. This unique evolutionary journey led the Aldabra rail to essentially evolve twice, a rare process known as “iterative evolution.” In this process, a species goes extinct, and another species evolves the same traits, becoming nearly identical to the extinct one. The remarkable aspect of this case is that it involved the same ancestral bird evolving to become flightless twice.
Julian Hume, a paleontologist at the Natural History Museum, London and lead author of the study, expressed his awe at this discovery, stating, “There is no other case that I can find of this happening, where you have a record of the same species of bird becoming flightless twice. It wasn’t as if it were two different species colonizing and becoming flightless. This was the very same ancestral bird.”
