A new analysis of a 56-million-year-old fossil has revealed that the ancient relatives of today’s large, flightless birds like ostriches and emus were strong long-distance flyers. This discovery fundamentally reshapes scientific understanding of how these birds, known as ratites, came to be distributed across the Southern Hemisphere, from Africa to Australia and South America. The findings suggest that their ancestors flew to these continents and independently lost the ability to fly, rather than drifting apart on separating landmasses as a single, flightless ancestral group.
The research, published in the journal Biology Letters, centers on the fossilized breastbone of an extinct group of birds called lithornithids. By examining the anatomy of this fossil and comparing it to modern birds, scientists determined that these early ancestors possessed the necessary musculature and skeletal structure for sustained, powerful flight. This evidence resolves a long-standing evolutionary puzzle about the global distribution of their flightless descendants, indicating that multiple lineages separately evolved to be ground-dwelling after their airborne ancestors had already colonized different parts of the world. The study argues that the loss of flight occurred after the extinction of non-avian dinosaurs removed the primary predatory pressure that made flying a necessity for survival.
Resolving an Evolutionary Paradox
The global spread of large flightless birds has long puzzled biologists. The group, known scientifically as Palaeognathae, includes the ostriches of Africa, rheas of South America, emus and cassowaries of Australia, and the kiwis of New Zealand. For decades, the prevailing theory was that a common, flightless ancestor to all these birds existed on the ancient supercontinent of Gondwana. As the landmass broke apart over 200 million years ago, it was believed that these birds were passively carried on the drifting continents, evolving into the distinct species seen today.
However, this explanation began to fray as new evidence emerged. Genetic studies revealed that the evolutionary timelines did not match the geological ones; the different ratite lineages diverged from each other long after the continents had already separated. Furthermore, the existence of the tinamous, a flying member of the Palaeognathae group in Central and South America, complicated the narrative. The new research provides a robust solution to this paradox, demonstrating that their shared ancestors were not flightless but were instead capable world travelers who crossed vast oceans through the air.
Evidence from Ancient Anatomy
The key evidence for this revised history comes from a well-preserved fossil breastbone, or sternum, belonging to a lithornithid, an ancient relative of modern palaeognaths. A team of researchers led by Klara Widrig of the Smithsonian Institution performed a detailed quantitative analysis of the fossil, focusing on the features related to flight capability. The size and shape of the sternum, particularly the keel (a ridge of bone to which flight muscles attach), were compared to those of modern birds with known flight styles.
The analysis showed that the lithornithid sternum was well-developed, comparable to that of modern birds capable of continuous, flapping flight, such as pigeons. This structure would have supported large, powerful pectoral muscles, enabling these ancient birds to cover long distances over open water. This anatomical evidence directly contradicts the idea that the ancestors of ratites were ground-bound. Instead, it paints a picture of a widespread, flying avian group that successfully established populations on multiple continents before the trait of flight was lost.
A Story of Convergent Evolution
The finding that ratite ancestors could fly leads to a compelling conclusion: flightlessness evolved independently in each lineage. After arriving in new environments, different groups of these birds separately adapted to a terrestrial lifestyle. This phenomenon, where unrelated species evolve similar traits, is known as convergent or parallel evolution. For ostriches in Africa, emus in Australia, and the now-extinct moas of New Zealand, the evolutionary path led to gigantism and a loss of flight.
Scientists suggest that the disappearance of dinosaurs created a unique ecological opportunity. With the primary ground-based predators gone, the evolutionary pressure to escape into the air diminished significantly. In these new, safer environments, staying on the ground may have offered advantages, such as conserving energy and allowing for an increase in body size to deter smaller predators. Over millions of years, the wings that had carried their ancestors across oceans became smaller and repurposed for other functions, like balance during high-speed running in ostriches.
Rethinking the Ratite Family Tree
This new perspective reorganizes the evolutionary history of the Palaeognathae. Rather than stemming from a single flightless ancestor, the group is now understood to have originated from a flying ancestor, with some descendants, like the tinamous, retaining a limited ability to fly while others abandoned it completely. This model explains why genetic data shows that some flightless birds, like the kiwi, are more closely related to the flying tinamous than to other flightless birds like the ostrich.
The study underscores that evolution is not a one-way street toward greater complexity. Flight, while a remarkable adaptation, is energetically expensive to maintain. When environmental conditions change and the advantages of flight are outweighed by the benefits of a terrestrial existence, it can be lost. The story of the ratites is a powerful example of this process, demonstrating how a single group of flying birds gave rise to a diverse array of ground-dwelling giants that came to dominate ecosystems across the Southern Hemisphere. This dispersal through the skies, followed by a return to the land, marks a significant new chapter in our understanding of avian evolution.