An asteroid wiped out the dinosaurs. Did you help birds thrive?

Sixty-six million years ago, an asteroid collided with the Gulf of Mexico. The disaster led to the extinction of up to three-quarters of all species on Earth, including dinosaurs such as Tyrannosaurus rex. But some feathered, flying dinosaurs survived, eventually evolving 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 impact was followed by a major pulse of bird evolution. The mass extinction of other animals may have removed much of the competition for birds, giving them the opportunity to evolve into the wonderful diversity of species that fly around us today.

But the New study on the DNA of 124 bird species challenges this idea. An international team of scientists found that birds began diversifying tens of millions of years before the fateful impact, suggesting that the asteroid had no significant impact on bird evolution.

“I imagine this will ruffle some feathers,” said Scott Edwards, an evolutionary biologist at Harvard University and one of the study's authors. The research was published Monday in the Proceedings of the National Academy of Sciences.

Dinosaurs developed primitive feathers at least 200 million years ago, not for flight but more likely for insulation or mating displays. In one lineage of small bipedal dinosaurs, these feathers became more complex, and eventually took the creatures into the air as birds. How feathers are transformed into wings for flight is still a matter of debate. But once birds evolved, they diversified into a variety of forms, many of them became extinct When the asteroid plunged the Earth into a winter that lasted for years.

When searching for fossils of the main groups of birds living today, scientists have found Almost nothing Formed before the asteroid hit. This striking absence has led to the theory that mass extinctions set the evolutionary stage for birds, allowing them to explode into many new forms.

But the new study reached a completely different conclusion.

“We found that this disaster had no impact on modern birds,” said Xiaoyuan Wu, an evolutionary biologist at Jiangsu Normal University in Xuzhou, China.

Dr. Wu and his colleagues used bird DNA to reconstruct a family tree that showed how major groups were related. The earliest split created two lineages, one comprising the present-day ostriches and emus, and the other comprising the rest of the living birds.

The scientists then estimated when the branches split into new lineages by comparing the mutations that accumulated along the branches. The longer the split between two branches, the more mutations accumulate in each lineage.

The team included paleontologists who helped fine-tune the genetic estimates by examining 19-year-old bird fossils. If the branch appears to be younger than the fossil it belongs to, they adjust the computer model that estimates the pace of bird evolution.

It's particularly noteworthy because of the fossil analysis, said Michael Bateman, a paleontologist at the Chinese University of Hong Kong who was not involved in the new study. “They had a dream team of paleontologists,” he said.

The study found that living birds share a common ancestor who lived 130 million years ago. New branches of its family tree split steadily throughout the Cretaceous and thereafter at a fairly steady pace, both before and after the asteroid impact. This steady trend may have been driven by the increasing diversity of flowering plants and insects during the same period, Dr. Wu said.

Jacob Perf, an evolutionary biologist at the University of Michigan who was not involved in the study, said the study demonstrates state-of-the-art methods for processing huge amounts of genetic data to reconstruct evolutionary history. But he did not agree with his conclusion.

If the new study is correct, there should be fossils of all major groups of living birds long 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 fossils, and that most major groups of birds appeared after the asteroid impact. A potential problem with the new study, he said, is that it assumes that bird DNA accumulates mutations at a constant rate from one generation to the next.

But the devastation caused by the asteroid's impact – which caused forests to collapse and created a shortage of prey – may have killed off larger birds, while smaller ones survived. Young birds take less time to reproduce, and will produce many more generations – and many more mutations – than birds before the collision. If scientists ignore this type of hypermutation, they will miss the timing of evolution.

However, Dr Berv admitted that scientists are only beginning to develop methods that could allow them to better estimate the rate of evolution and combine it with other evidence such as DNA and fossils. “I think that will reconcile some of the discussions,” he added.