Asteroid that wiped out dinosaurs may not have impacted bird evolution – 02/16/2024 – Science

66 million years ago, an asteroid hit the Gulf of Mexico. The catastrophe led to the extinction of up to three-quarters of all species on Earth, including dinosaurs like the Tyrannosaurus rex. But some feathered dinosaurs that flew survived and eventually evolved into the more than 10,000 species of birds that live 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 boost in bird evolution. The mass extinction of other animals may have eliminated a lot of competition for birds, giving them the chance to evolve into the remarkable diversity of species that fly around us today.

But a new study of 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 did not have a major effect on bird evolution.

“I imagine this is going to be controversial,” said Scott Edwards, an evolutionary biologist at Harvard University and one of the study’s authors. The research was published on Monday (12) in the scientific journal Proceedings of the National Academy of Sciences.

Dinosaurs developed primitive feathers at least 200 million years ago, not for flight but probably for thermal insulation or mating display. In a lineage of small bipedal dinosaurs, these feathers became more complex and eventually carried the creatures into the air like birds.

How feathers became wings for flight is still up for debate. But once birds evolved, they diversified into a variety of forms, many of which became extinct when the asteroid plunged Earth into a multi-year winter.

When looking for fossils of the main groups of birds alive today, scientists found almost none that formed before the asteroid impact. This striking absence has led to a theory that mass extinctions cleared the evolutionary landscape for birds, allowing them to explode into many new forms.

But the new study presented here 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.

Wu and his colleagues used the birds’ DNA to reconstruct a family tree that showed how the main groups were related. The earliest split created two lineages: one that includes today’s ostriches and emus, and another 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 accumulated.

The team included paleontologists who helped adjust the genetic estimates by examining the ages of 19 bird fossils. If a branch appeared to be more recent 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 the research was especially notable because of the fossil analysis. “They had a dream team of paleontologists,” he said.

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

Jacob Berv, an evolutionary biologist at the University of Michigan not involved in the study, said the research illustrated cutting-edge methods for analyzing large amounts of genetic data to reconstruct evolutionary history. But he does not agree with the study’s conclusion.

If the new study was correct, there should be fossils of all the major groups of birds alive long before the asteroid impact. But almost none were found.

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

Berv suspects that the correct story comes from the fossils and that most of the major bird groups arose after the asteroid impact. The problem with the new study, according to Berv, is that it assumes that the birds’ DNA accumulated mutations at a constant rate from one generation to the next.

But the devastation from the asteroid impact — causing forests to collapse and creating a shortage of prey — may have led to the deaths of larger birds, while smaller birds survived. Small birds take less time to reproduce and would produce many more generations — and many more mutations — compared to before the impact. If scientists ignore this kind of mutational acceleration, they will get the timing of evolution wrong.

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