An asteroid wiped out the dinosaurs. Did it help the birds thrive? | Trending Viral hub


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

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

but a new study on the DNA of 124 species of birds challenges that idea. An international team of scientists found that birds began to diversify 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 will ruffle some 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 developed primitive feathers at least 200 million years ago, not for flight, but probably for insulation or display during mating. In a lineage of small bipedal dinosaurs, those feathers became more complex and eventually carried the creatures into the air like birds. As feathers turned into wings for the flight is still debated. But once birds evolved, they diversified into a variety of forms, many of which became extinct when the asteroid plunged Earth into a year-long winter.

By searching for fossils of the major groups of birds living today, scientists have found almost none which was formed before the asteroid impact. That surprising absence has led to the theory that mass extinctions cleared the evolutionary field 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 that showed how the main groups were related. The oldest split created two lineages, 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 lineage accumulated.

The team included paleontologists who helped refine the genetic estimates by examining the age of 19 bird fossils. If a branch appeared to be newer than a fossil that belonged to it, they adjusted the computer model that estimated the rate of evolution of the birds.

Michael Pittman, a paleontologist at the Chinese University of Hong Kong who was not involved in the new study, said it was particularly noteworthy for its 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 their family tree split off constantly throughout the Cretaceous Period and thereafter at a fairly constant rate, both before and after the asteroid impact. Dr Wu said this steady trend could have been driven by the increasing 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 illustrated cutting-edge methods for processing huge amounts of genetic data to reconstruct evolutionary history. But he did not agree with his 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 have been found.

“The signal from the fossil record is unambiguous,” Dr. Berv said.

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

But the devastation of the asteroid impact, which caused forests to collapse and created a shortage of prey, could have led to the deaths of larger birds, while smaller ones survived. Small birds take less time to reproduce and would produce many more generations (and many more mutations) than birds before the impact. If scientists ignore this type of mutational overdrive, they will misunderstand the timing of evolution.

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


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