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Modern birds evolved earlier than previously assumed, long before the mass extinction of the dinosaurs, an event that appears to have had a limited impact on avian evolution.
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When did modern birds first appear on Earth? Scientists assumed that modern birds appeared around the time dinosaurs became extinct after an asteroid crashed into Earth 66 million years ago. Although this collision caused the extinction of three out of every four species alive on the planet at the time, birds as a group did not become extinct. In fact, paleontologists have long argued that the asteroid impact triggered a huge boost in bird evolution, presumably because it eliminated much of the competition for birds, giving them the opportunity to evolve into the remarkable diversity of species that we see today.
But a new study by an international team of researchers reports that modern birds began to diversify tens of millions of years ago. before the asteroid impact, suggesting that the asteroid impact did not have a major effect on the evolution of birds. This discovery, based on extensive mining and analysis of genomic data collected from 124 bird species representing the majority of modern bird diversity, reports that birds go back much further than previously thought.
The researchers came to this conclusion after analyzing the birds’ DNA to reconstruct a high-resolution bird family tree that shows how major groups of birds are related. The lead author of the study, evolutionary biologist Shaoyuan Wu in Georgia Institute of Technologyand their collaborators determined that the common ancestor of all modern birds lived approximately 130 million years ago (Figure 2).
Dr. Wu and his collaborators collected and analyzed genome-scale data from 118 species representing all 35 orders of neobirds to estimate how long ago these lineages diverged based on the number of genetic mutations that accumulated in each branch: the older was the division, the more mutations were found. . Dr. Wu and his collaborators examined 25,460 genetic loci in four types of DNA, which they used to construct their family tree.
Dr. Wu and his collaborators then refined their findings by comparing them to the estimated ages of 19 bird fossils: If a branch was estimated to be younger than a particular fossil belonging to it, the team adjusted their computer model to estimate the bird growth rate. evolution to match fossil evidence.
Using these methods, Dr. Wu and his collaborators discovered that the earliest split in the avian tree created two lineages, one that gave rise to ratites (present-day ostriches and emus, the paleognaths) and the other that comprises all the others. modern birds, the neoaves. They found that the two main branches of neobirds split very early in their evolutionary history, and that one branch eventually gave rise to land birds and the other to waterfowl. Dr. Wu and his collaborators also discovered that the appearance of birds coincides with a global warming event that occurred approximately 55 million years ago and that appears to have triggered the evolution of modern seabirds. In fact, this report indicates that the radiation of modern birds occurred at the same time that flowering plants and other creatures first appeared, and may have accelerated avian evolution.
Because it is based on the constant accumulation of mutations, this study raises a big and important question: Was the mutation rate constant? How can we be sure of this? This is an important question considering that an asteroid would have wiped out the larger birds but left the smaller ones relatively intact. Smaller birds produce more generations in a shorter period of time than larger birds, so mutations would accumulate faster after the asteroid impact, and this could alter the precision of our estimate of the pace of avian evolution. Right now, scientists are developing techniques to more accurately estimate the rate at which genetic mutations occur, so we can better match genomic data to existing fossil evidence.
Fountain:
Shaoyuan Wu, Frank E. Rheindt, Jin Zhang, Jiajia Wang, Lei Zhang, Cheng Quan, Zhiheng Li, Min Wang, Feixiang Wu, Yanhua Qu, Scott V. Edwards, Zhonghe Zhou and Liang Liu (2024). Genomes, fossils, and the simultaneous emergence of birds and modern flowering plants in the Late Cretaceous, proceedings of the National Academy of Sciences 121(8):e2319696121 | doi:10.1073/pnas.2319696121
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