Jul 21, 2019 | Updated: 09:46 AM EDT

A New Study Helps to Explain the Evolution of Flightless Birds

Apr 07, 2019 09:46 PM EDT

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Large flightless birds are scattered across all but one of the world's southern continents. Since Darwin's era, people have wondered: How are they related? Ostriches, emus, cassowaries, rheas, and kiwis can't fly. Unlike most birds, their flat breastbones lack the keel that anchors the strong pectoral muscles required for flight. Their puny wings can't possibly lift their heavy bodies off the ground. These flightless birds, called ratites, are clearly different from other avian species.

Darwin noticed, and he predicted that ratites were related to each other. His contemporary, Thomas Huxley, found another commonality among them: The arrangement of bones in the roofs of their mouths appeared more reptile-like than that of other birds. At about the same time, another biologist, Richard Owen, assembled the remains of a giant ostrich-like fossil skeleton, the first extinct moa known to the western world. But a pesky detail puzzled Huxley--small, ground-dwelling South American tinamous--didn't seem to fit neatly with the ratites or other birds.

Tinamous fly, albeit reluctantly. And they possess keeled sternums, suggesting that they evolved with flying birds. But their palate bones match the ratites. Where do they belong? Scientists have debated this question for 150 years. Now, a new study analyzing the largest molecular dataset to date, clarifies the tinamous' place on the evolutionary tree and offers clues about the origins of flightlessness. Scientists probed almost 1,500 DNA segments from tinamous, emus, ostriches, the extinct little bush moas, and others, which have generally showed tinamous on the outskirts of the ratite group, relying solely on morphological traits like skeletal details. Other investigations of limited genetic information suggested tinamous were evolutionarily tangled with the flightless birds. "Fundamentally, the recent debate is about molecular data versus morphology," says Allan Baker, lead author of the study. "We can't both be right."

Baker's study suggests that the tinamous evolved within ratites, not as a separate lineage. "And the DNA says absolutely that moas and tinamous are closely related," says Baker.

The tinamous' place on the evolutionary tree offers a glimpse into the origins of flightlessness. All ratites, including tinamous, probably trace their ancestry back to a flying relative, according to Baker. Tinamous retained their ability to fly, while the other lineages each lost flight independently. "It's very unlikely that tinamous would re-evolve flight from a flightless ancestor," says Baker.

The study upends an alternate, oft-cited story. Scientists speculated that the breakup of the supercontinent Pangaea's southern section split up a population of flightless ratite ancestors. The story conveniently explained how flightless birds dispersed across the oceans. But now it looks like each group invaded New Zealand separately. The new evidence doesn't align with the timing of Pangaea's split more than 100 million years ago. The ratites evolved into separate lineages between 90 and 70 million years ago, and the tinamous and moas diverged about 45 million years ago, according to the study. "We can't rule out that the birds flew to each continent," says Baker, and then independently evolved their flightless features.

The debate about these birds has been contentious, Baker says. "But I think this study will put it to rest."

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