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Theory of Flightless Birds Shot Down
Ostriches, emus, kiwis and other winged non-flyers might seem to be birds of a feather, sharing similar evolutionary origins, but the story could turn out to be much weirder, with perhaps numerous flying ancestors. This group of birds, called ratites, has been viewed by biologists as part of a larger group (paleognaths) of mostly extinct birds that are key to understanding the early evolution of birds. All living ratites are found in the Southern Hemisphere and share features associated with flightlessness, such as small or absent keels on their breastbones; smaller, simpler and fewer wing bones; bigger leg bones; and structures in their feathers that don't help with aerodynamics. So for these reasons and others, many scientists previously figured the ratites, a group that also includes rheas and cassowaries, all shared a common ancestor that was flightless. However, a new analysis shows they do not share this single hypothetical flightless ancestor. Rather they probably evolved from more than one different airborne ancestor. Ostriches are special The research, led by John Harshman of the Field Museum of Natural History in Chicago, Edward L. Braun of the University of Florida and Michael J. Braun of the Smithsonian's National Museum of Natural History, was based on sequencing genetic material sampled from the tissues of many different bird species and using the gene sequences to determine how the birds relate to one another. Nearly all of the ratites, except for ostriches, which the analysis revealed as a class unto themselves, are actually more closely related to tinamous (a chubby, ground-dwelling bird with a stubby tail, that nonetheless can fly and lives in Central and South American) than they are to ostriches, says UF zoologist Edward L. Braun. The analysis also placed tinamous in a sub-group that includes rheas, cassowaries, emus and kiwis but excludes ostriches. "We think the most likely hypothesis is that the tinamous, rheas, cassowaries, emus, and kiwis had an ancestor that could fly, and that flight was lost at least three times (once in ostriches, once in rheas, and once in the emu-cassowary-kiwi group)," Braun said.
Julia Clarke, a paleontologist at North Carolina State University, was not involved in the new research, and said the results are surprising, the analyses are well done and the dataset used for them is the largest one available. "It is unquestionable that any future analysis will need to address this dataset," she said, "but the results are in conflict with nearly every previous hypothesis of relationships within paleognath birds."
