Sixty-six million years ago, an asteroid slammed into the Gulf of Mexico. The catastrophe led to the extinction of as many as three-quarters of all species on Earth, including dinosaurs like Tyrannosaurus rex. But some flying feathered dinosaurs survived, and eventually evolved into the more than 10,000 species of birds living today, including hummingbirds, condors, parrots and owls.
Based on the fossil record, paleontologists have long argued that the asteroid’s impact was followed by a big pulse of bird evolution. The mass extinction of other animals may have eliminated a lot of competition for the birds, giving them the chance to evolve into the remarkable diversity of species that fly around us today.
But a new study on the DNA of 124 bird species challenges that idea. An international team of scientists found that birds began diversifying tens of millions of years before the fateful collision, suggesting that the asteroid had no major effect on bird evolution.
“I imagine this will ruffle a few feathers,” said Scott Edwards, an evolutionary biologist at Harvard and one of the study’s authors. The research was published on Monday in the Proceedings of the National Academy of Sciences.
Dinosaurs evolved primitive feathers at least 200 million years ago, not for flight but most likely for insulation or a mating display. In one lineage of small bipedal dinosaurs, those feathers became more complex and then ultimately took the creatures into the air as birds. How feathers turned into wings for flight is still debated. But once birds evolved, they diversified into a variety of forms, many of which became extinct when the asteroid plunged the Earth into a yearslong winter.
When searching for fossils of the major groups of birds alive today, scientists have found almost none that formed before the asteroid hit. That striking absence has led to a theory that the mass extinctions cleared the evolutionary arena for birds, allowing them to explode into many new forms.
But the new study came to a very different conclusion.
“We found that this catastrophe didn’t have impact on modern birds,” said Shaoyuan Wu, an evolutionary biologist at Jiangsu Normal University in Xuzhou, China.
Dr. Wu and his colleagues used the birds’ DNA to reconstruct a family tree that showed how the major groups were related. The oldest split created two lineages, one that includes today’s ostriches and emus, and the other with the rest of all living birds.
The scientists then estimated when the branches split into new lineages by comparing the mutations that accumulated along the branches. The older the split between two branches, the more mutations each lineage built up.
The team included paleontologists who helped to fine-tune the genetic estimates by examining the age of 19 fossils of birds. If a branch appeared to be newer than a fossil that belonged to it, they adjusted the computer model that estimated the pace of bird evolution.
Michael Pittman, a paleontologist at the Chinese University of Hong Kong who was not involved in the new study, said that it was particularly noteworthy because of the fossil analysis. “They had a dream team of paleontologists,” he said.
The study found that living birds shared a common ancestor that lived 130 million years ago. New branches of its family tree steadily split off throughout the Cretaceous Period and afterward at a fairly steady pace, both before and after the asteroid impact. Dr. Wu said that this steady trend might have been fueled by the growing diversity of flowering plants and insects during the same period.
Jacob Berv, an evolutionary biologist at the University of Michigan who was not involved in the study, said that it illustrated state-of-the-art methods for crunching huge amounts of genetic data to reconstruct evolutionary history. But he did not agree with its conclusion.
If the new study was right, there should be fossils of all major groups of living birds from well before the asteroid impact. But almost none have been found.
“The signal from the fossil record is not ambiguous,” Dr. Berv said.
Dr. Berv suspects that the correct story comes from the fossils, and that most major groups of birds emerged after the asteroid impact. The potential problem with the new study, he said, is that it assumes that the bird DNA accumulated mutations at a steady rate from one generation to the next.
But the devastation of the asteroid’s impact — causing forests to collapse and creating shortages of prey — might have led to the deaths of bigger birds, while smaller birds survived. Small birds take less time to reproduce, and they would produce many more generations — and many more mutations — than birds did before the impact. If scientists ignore this kind of mutational overdrive, they will get the timing of evolution wrong.
Still, Dr. Berv acknowledged that scientists are just starting to develop methods to that could allow them to better estimate the rate of evolution and integrate it with other evidence like DNA and fossils. “I suspect that will reconcile some of the debates,” he said.
