There are thousands of species of birds living on Earth today–anywhere from around 10,000 to as many as 18,000, depending on how you define “species.” The fossil record suggests that almost all this feathered, flying diversity emerged after the last major extinction 66 million years ago when an asteroid rocked the planet and drove the non-avian dinosaurs extinct. Only a few birds–chicken, ostrich, and duck-like creatures–made it across the extinction line, giving rise to the dizzying array of modern birds, from songbirds to corvids to parrots.
Yet it’s been a long-standing biological mystery how that rapid evolution unfolded. New research sheds light on this proliferation of birds. Scientists report patterns of DNA change across the bird tree of life associated with the sudden, fiery end of the Cretaceous period in a study published July 31 in the journal Science Advances. According to lead study author Jacob Berv, an evolutionary biologist at the University of Michigan, it’s the first published evidence to date of major shifts in avian genomes directly resulting from the extinction event.
“We’re using new statistical models to detect this particular kind of pattern in the sequences that we haven’t been able to detect before,” Berv tells Popular Science. “It’s allowing us to associate major changes in bird genomes with this mass extinction event in a direct and very clear way, which we haven’t previously been able to do,” he adds. And these genetic signatures aren’t random, per the research. The observed DNA shifts are associated with changes in body size and parental care that may have been key to birds’ success and diversification, according to a secondary analysis by Berv and his colleagues.
The extinction marking the end of the Cretaceous and the beginning of the Paleogene period has left obvious signs behind. For one, there’s the 6.2-mile-wide Chicxulub crater in Mexico. Then, there’s the K-Pg boundary in the geologic record, a distinct layer of iridium-rich rock. Of course, there’s also the conspicuous absence of Tyrannosaurus rex wandering around. (Don’t be too sad for the dinosaurs, though; birds–after all–are dinos). Now, researchers have uncovered one more piece to the puzzle: how influential a single big asteroid has been for Earth.
“I think, in general, we’ve underestimated the extent to which these extinction events shape modern diversity,” says Nick Longrich, a paleontologist and evolutionary biologist at the University of Bath in England. Longrich wasn’t involved in Berv’s research but has previously studied the birds that didn’t cross the Cretaceous-Paleogene line. “We tend to focus on everyday, little mutations–what we call microevolution–but you’ll look back over huge periods and see that these very extreme, rare events have been massively important in driving the evolution of life on Earth.” With the new study, “what’s interesting is we can detect it,” he adds.
The scientists used a computer model to analyze partial genome samples from 198 modern bird species collected from museum specimens across the whole avian clade. Combined with information from the fossil record about the timing of certain lineage’s emergence, Berv and his colleagues were able to reconstruct a history of evolutionary transitions and uncover “genomic fossils.”
Similar past analyses have used models built around assumptions, such as the ratio of A’s, T’s, G’s, and C’s, which is relatively stable throughout evolution, and substitutions only occur randomly. In reality, though, T’s are much more likely to change to C’s than to A’s, says Sonal Singhal, a study co-author and an evolutionary geneticist at California State University, Dominguez Hills. She notes that the new study deployed a different approach using a model that didn’t include these standard assumptions and instead accounted for the most likely changes in DNA composition.
With this improved model, they identified 17 major molecular composition shifts in their dataset. Fifteen were clustered within 5 million years of the K-Pg extinction, and 12 were directly linked to divergences between avian lineages. “It’s very clear that these major changes in [DNA] composition are happening in a very short interval,” says Berv.