An asteroid wiped out dinosaurs. Did it help birds flourish?

Sixty-six million years ago, an asteroid crashed into the Gulf of Mexico. The catastrophe led to the extinction of as many as three-quarters of all species on Earth, including dinosaurs such as Tyrannosaurus rex. But some flying feathered dinosaurs survived and eventually evolved into the more than 10,000 species of birds alive today, including hummingbirds, condors, parrots and owls.

Based on the fossil record, paleontologists have long argued that the asteroid impact was followed by a major wave of bird evolution. The mass extinction of other animals may have eliminated much competition for the birds, giving them the opportunity to evolve into the remarkable diversity of species that fly around us today.

But one new study about 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, indicating that the asteroid did not have a 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 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 as a mating display. In one lineage of small bipedal dinosaurs, those feathers became more complex, eventually taking the creatures into the air like birds. There is still debate about how feathers turned into wings for flight. But once birds evolved, they diversified into a variety of forms, of which there are many became extinct when the asteroid plunged Earth into a years-long winter.

In searching for fossils of the major groups of birds alive today, scientists have found these almost none that formed before the asteroid hit. That conspicuous absence has led to a theory that the mass extinction 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 had no 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 showing how the major groups were related. The oldest split created two genera, one including today's ostriches and emus, and the other including 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 line accrued.

The team included paleontologists who helped refine the genetic estimates by examining the ages of 19 bird fossils. If a branch turned out to be newer than the 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 research, said it was particularly notable 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 branched off steadily throughout the Cretaceous and at a fairly steady pace thereafter, both before and after the asteroid impact. Dr. Wu said this steady trend may have been fueled by the growing diversity of flowering plants and insects over the same period.

Jacob Berv, an evolutionary biologist at the University of Michigan who was not involved in the study, said it illustrates state-of-the-art methods for processing vast amounts of genetic data to reconstruct evolutionary history. But he disagreed 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 hardly anyone has been found.

“The signal from the fossil record is not ambiguous,” said Dr. Berv.

Dr. Berv suspects that the correct story comes from the fossils, and that most major groups of birds emerged after the asteroid impact. The problem with the new study, he said, is that it assumes the bird's DNA accumulated mutations at a steady pace from generation to generation.

But the devastation of the asteroid impact – which collapsed forests and caused a shortage of prey – could have led to the deaths of larger 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 before the impact. If scientists ignore this kind of mutational overdrive, they will misinterpret the timing of evolution.

Still, Dr. acknowledged. Berv that scientists are just beginning to develop methods that can better estimate the rate of evolution and integrate it with other evidence such as DNA and fossils. “I suspect this will settle some debates,” he said.