The world is home to a fascinating group of large, flightless birds, including ostriches, emus, and rheas, which inhabit six different landmasses separated by vast oceans. The question of how these birds managed to reach such isolated locations without the ability to fly has puzzled scientists for years. One prevailing theory suggests that the ancestors of these birds, known as paleognaths, were able to walk to their current locations during the era of the supercontinent Pangaea, which existed approximately 320 to 195 million years ago. As Pangaea fragmented, these birds would have already been in place. However, this theory faces challenges, as Pangaea had already split into separate continents well before the emergence of the last common ancestor of paleognaths.
Recent genetic studies indicate that the last common ancestor of paleognaths lived about 79.6 million years ago, and the various lineages we recognize today began to diverge between 70 and 62 million years ago. To explore this evolutionary puzzle further, Klara Widrig, a vertebrate zoologist at the Smithsonian National Museum of Natural History, and her team analyzed a specimen of the ancient paleognath known as Lithornis promiscuous. This species lived approximately 59 to 56 million years ago and is notable for being the oldest fossil of a palaeognath found in exceptional condition.
While it is uncertain whether Lithornis directly descended into modern paleognaths, it serves as the best representation of what their ancestor may have looked like. Previous studies on related species, such as Calciavis grandei, suggested that they might have had some flight capabilities, but the extent of this ability remained unclear. In their recent study, published in the journal Biology Letters, Widrig and her colleagues conducted a thorough analysis comparing the shape of the sternum, or breastbone, of Lithornis promiscuous with those of contemporary birds.
The sternum plays a crucial role in the mechanics of flight, as it serves as the attachment point for the large pectoral muscles responsible for flapping. Widrig noted that the shape of the sternum in Lithornis indicated it could have supported various styles of aerobic flapping, suggesting that it was capable of long flights. Interestingly, the breastbone's structure closely resembled that of living birds known for their long-distance migration, such as great egrets and herons.
According to Peter Hosner, curator of birds at the Natural History Museum of Denmark, this finding implies that ancient paleognaths may have flown to distant landmasses, where they subsequently evolved into the large, flightless birds we recognize today. This phenomenon is an example of convergent evolution, where unrelated species develop similar traits independently due to similar environmental pressures.
Today, there are approximately 60 species of living paleognaths, which include around 45 species of tinamou (capable of short bursts of flight), five species of kiwi, one emu species, three cassowary species, two ostrich species, and one or two species of rhea. Widrig explains that for a bird to become flightless, two criteria must be met: it must be able to obtain food on the ground and exist in an environment free from predators that necessitate flight.
In modern contexts, flightless birds typically evolved in predator-free island environments, much like the dodo (Raphus cucullatus). However, following the Cretaceous-Paleogene extinction event 66 million years ago, the world underwent significant changes. With the eradication of nonavian dinosaurs and the absence of mammalian predators, ground-feeding birds had unprecedented opportunities to evolve into flightless species. Widrig notes that flight is energetically demanding, and without the need to escape predators, these birds could thrive without flight.
As larger predators emerged over time, these flightless birds adapted by evolving characteristics such as size and speed. For instance, the cassowary became large and intimidating, while the ostrich evolved into a swift runner. However, these adaptations occurred independently, with each species evolving to suit its specific environment. As Widrig aptly puts it, “It’s not as if they got on a conference call with each other to plan their evolutionary paths.”
