The massive meteor strike that wiped out the dinosaurs 65 million years ago may have sparked a rapid evolution of bird species over just a few million years. The few bird lineages that survived the extinction bottleneck gave rise to stunning diversity, resulting in the more than 10,000 species alive today.
Now, a new study published in Nature provides the clearest look yet at those early evolutionary branches as birds emerged as a dominant life form on the planet—relationships that have stumped scientists since the dawn of paleontology.
“This question of understanding the deepest relationships in the bird family tree has plagued scientists for decades,” says Jacob Berv, a Cornell Lab of Ornithology graduate student and an author of the study. “Some people call it the most difficult problem in dinosaur systematics.”
Birds are the only living descendants of dinosaurs. They evolved from a group called the theropod dinosaurs that included bipedal carnivores such as Tyrannosaurus rex and Velociraptor. “After the great dinosaur extinction, birds began to rapidly evolve new forms—and that’s the era that our study manages to reconstruct in great detail,” Berv says. “It’s revolutionizing our ability to understand avian evolution.”
The researchers paid particular attention to a taxonomic group known as the Neoaves, which contains about 90% of all bird species—everything except game birds, waterfowl, tinamous, and flightless birds such as the Ostrich and kiwis.
Within this group they found unexpected relationships. Virtually all landbirds diverged early on from a group that includes the vultures and hawks, raising the possibility that terrestrial birds evolved from raptor-like ancestors. The researchers also found that owls are closely related to toucans and hornbills, and that falcons are closely related to parrots and songbirds. The study also confirmed that the nocturnal nightjars are closely related to hummingbirds.
And one species, the prehistoric-looking Hoatzin of South America, traces its lineage back nearly 64 million years. It’s the oldest bird lineage that leads to a single living species. The Hoatzin is a curious bird—it’s the only species that feeds by fermenting leaves in its crop and esophagus. Its relationship to other birds has been long debated by evolutionary biologists.
“This is a very exciting time in evolutionary ornithology,” says coauthor Richard Prum of Yale University. “In just a few short years, we will complete the phylogeny of birds. There will always be a few branches to argue about, but the tree is taking shape rapidly.”
Compared to other recent studies that have attempted to clarify evolutionary relationships among bird families, the study authors analyzed genetic markers for a much larger number of species (198 birds, 2 alligators). The large species sampling was enabled by a new technique developed by authors Alan and Emily Lemmon, of Florida State University, that efficiently targeted a few hundred key locations on each species’ genome—DNA markers strategically chosen for their ability to reflect early evolutionary changes.
The researchers turned to the fossil record to calibrate the timescale of birds’ evolution, by matching points on the evolutionary tree to similar forms in fossils whose ages were already known. This approach led to the finding that birds may have arisen only about 70–80 million years ago, more recently than has been reported in previous studies.
“The closest relatives of modern birds suffered a major mass extinction at the end of the Cretaceous,” says Daniel Field, a study author and graduate student at Yale. “It seems that only a handful of modern avian lineages survived, and those survivors rapidly evolved into the incredible diversity of birds we see today.”
The study also finds that it may have been extremely rare for early bird species to evolve transitions between terrestrial and aquatic lifestyles. Rather than multiple lineages evolving independently to live near water, the researchers conclude that nearly all waterbirds, including loons, grebes, penguins, pelicans, gulls, and others, share a single common ancestor, and that the switch between habitats may have happened only a few times in bird evolutionary history.
“The fact that adapting to an aquatic environment appears to have been a rare occurrence in the history of bird life is consistent with the story from dinosaurs in general,” Field says. “It seems that birds may have inherited a strong preference for terrestrial habits from their dinosaurian ancestors.”
“Bird enthusiasts will love to learn that cuckoos are closely related to bustards,” Prum adds, “and that the hummingbirds and swifts that are now active during the day actually evolved from nightjars, which are totally nocturnal. It appears that the ancestors of the highly colorful and visual-foraging hummingbirds were predominantly nocturnal for 10 million years.”
So why does it matter which species evolved before or after another or whether one species is closely related to another?
“Living birds have a very long and complex history,” explains Berv. “Any attempt to understand their biology at a broad scale requires an understanding of this deep historical context. It’s critical to every area of bird biology. How they act, where they live, what they look like, how they communicate—it’s all linked to how they evolved in relation to each other.”
“The most exciting thing is that we can now study the mechanisms and patterns of avian evolution in greater detail,” agrees Prum. “We used genetic tools, but the study is about how the entire evolution of birds unfolded.”
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