Modern birds evolved earlier than previously assumed — much earlier than the dinosaurs’ mass extinction — an event that seems 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 that the dinosaurs went extinct after an asteroid crashed into Earth 66 million years ago. Even though this collision caused the extinction of three out of four species alive on the planet at the time, birds as a group did not go extinct. In fact, palaeontologists have long argued that the asteroid impact triggered a large pulse of bird evolution presumably because it eliminated a lot of 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 diversifying tens of millions of years before the asteroid strike, suggesting that the asteroid strike had no major effect on bird evolution. This discovery, based on extensive mining and analysis of genomic data collected from 124 species of birds representing most of modern bird diversity, reports that birds date back much further than previously thought.
The researchers came to this conclusion after they analyzed bird DNA to reconstruct a high-resolution avian family tree that shows how the major groups of birds are related. Lead author of the study, evolutionary biologist Shaoyuan Wu at Georgia Institute of Technology, and collaborators determined that the common ancestor to all modern birds lived approximately 130 million years ago (Figure 2).
Dr Wu and collaborators collected and analyzed genome-scale data from 118 species representing all 35 orders of neoaves to estimate how long ago these lineages diverged based on the number of genetic mutations that accumulated on each branch: the older the split, the more mutations present. Dr Wu and collaborators examined 25,460 genetic loci across four classes of DNA and used the data collected to build their family tree.
Dr Wu and 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 that belonged to it, the team adjusted their computer model so it estimated the rate of avian evolution to coincide with the fossil evidence.
Using these methods, Dr Wu and collaborators found that the oldest split in the avian tree created two lineages, one that gave rise to the palaeognaths — today’s ostriches, emus and other ratites — and the other comprising all other modern birds, the neognathes. They found that the two main branches of neoaves then split very early in their evolutionary history, with one branch eventually giving rise to land birds and the other to waterbirds. Dr Wu and collaborators also found that the appearance of birds coincides with a global warming event that occurred roughly 55 million years ago that appears to have occurred at the same time that flowering plants and other creatures first appeared, and may have been sped up the process of avian evolution.
Because this study is based on the steady accumulation of mutations, it raises a big question: was the rate of mutations steady? How can we be certain of this? This is an important question when one considers that an asteroid would likely have wiped out bigger birds but left smaller birds relatively untouched. Smaller birds produce more generations in a shorter time frame than larger birds, so mutations would accumulate faster after the asteroid impact, and this could alter our precise estimation of the rate of avian evolution. Currently, this is the best method available but scientists are developing new techniques for more accurately estimating the rate at which genetic mutations occur so we can better match the genomic data to existing fossil evidence.
Source:
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 concurrent rise of modern birds and 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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