Within the Seychelles archipelago of the Indian Ocean is found the world's second-largest pristine coral atoll: Aldabra. Because of its geographical isolation and the difficulties accessing it, Aldabra has long been protected from human interference. Given this unique circumstance, animal populations on Aldabra have thrived, constrained solely by nature. Although Aldabra is over 400,000 years old, the islands were flooded most recently around 136,000 years ago, wiping out most terrestrial life. Nevertheless, Aldabra was swiftly recolonized by animals once the waters receded, giving credence to Dr Ian Malcolm, in Jurassic Park, who memorably said, ‘Life, uh, finds a way’. For scientists interested in natural history, the Aldabra atoll provides a wealth of well-preserved animal fossils, spanning the period from before to after the last inundation. Paleobiologists Julian Hume, from the Natural History Museum at Tring, UK, and David Martill, from the University of Portsmouth, UK, took advantage of this exceptional fossil record to show that flightlessness has evolved twice in the same bird species on two separate occasions on Aldabra.
The coral atoll is home to the only living flightless island bird species in the Indian Ocean, the Aldabra rail (Dryolimnas aldabranus), which lost its ability to fly thanks to the absence of predators allowing it to roam freely on the ground. The Aldabra rail is descended from the living white-throated rail (D. cuvieri), a migratory species found widely throughout the south-western Indian Ocean that settled on the atoll and lost its ability to fly after the atoll remerged from the sea. Interestingly, the fossil record shows that there was another flightless species of rail on Aldabra before the flood. Curious about the origins of this extinct flightless species, Hume and Martill hypothesized that it also evolved from the white-throated rail, which is notorious for colonizing isolated islands, and then quickly evolving flightlessness.
The researchers analyzed specimens of all three Dryolimnas species, and found remarkable similarities between the bones of the migratory white-throated rail and the extinct and modern flightless Aldabra rails. These three species share nearly identical structures at the bottom of their legs (where the foot bones meet), yet the flightless rails have much thicker leg bones that are better suited to running on land, by enhancing balance. Conversely, the researchers found that the long and slender shape of the white-throated rail's wing bones match its airborne lifestyle, whereas the short and thick wing bones of both the extinct and Aldabra flightless rails make them unfit for flight. The authors also reconstructed a timeline of the white-throated rail's evolutionary history on Aldabra – using specimens of all three species – to show a rapid transition from bones useful for flight to ones adapted to living on the ground. These findings not only show that the white-throated rail successfully colonized Aldabra twice but also that it evolved flightlessness at two distinct periods, essentially giving rise to the exact same bird.
The discovery by Hume and Martill adds an intriguing chapter to our understanding of evolutionary history, by providing the first example of recurring flightlessness in the same species of bird, and they join an extremely short list of rare examples of iterative (repeated) evolution. Furthermore, although this study illustrates the devastating effects of a sea-level rise, it also shows the resilience of nature and gives hope for how species will recover once human interference is reduced. Given sufficient time, animals adapt to and flourish in their environments.