About 66 million years ago, at the end of the Cretaceous period, a 10-km-wide asteroid crashed into Earth near the site of the town of Chicxulub in what is now Mexico. The impact eradicated roughly 75% of the animal and plant species on Earth, including whole groups like non-avian dinosaurs and ammonites. New research has identified important changes in birds’ genomes sparked by this end-Cretaceous mass extinction, ultimately contributing to the incredible diversity of living birds.
This painting depicts an asteroid slamming into tropical, shallow seas of the sulfur-rich Yucatan Peninsula in what is today southeast Mexico. The aftermath of this immense asteroid collision, which occurred approximately 65 million years ago, is believed to have caused the extinction of the dinosaurs and many other species on Earth. Shown in this painting are pterodactyls, flying reptiles with wingspans of up to 50 feet, gliding above low tropical clouds. Image credit: Donald E. Davis / NASA.
“By studying the DNA of living birds, we can try to detect patterns of genetic sequences that changed just after one of the most important events in Earth’s history,” said University of Michigan’s Dr. Jake Berv.
“The signature of those events seems to have imprinted into the genomes of the survivors in a way that we can detect tens of millions of years later.”
A living organism’s genome comprises four nucleotide molecules, referred to by the letters A, T, G and C. The order of these nucleotides in a genome defines the blueprint of life.
The DNA code can sometimes evolve in a way that shifts the overall composition of DNA nucleotides across the whole genome.
These compositional changes are crucial in determining what kind of genetic variation is possible, contributing to an organism’s evolutionary potential, or its ability to evolve.
Dr. Berv and colleagues found that the mass extinction event sparked shifts in nucleotide composition.
They also found that these shifts seem to be connected to the way birds develop as babies, their adult size and their metabolism.
For example, within approximately 3 million to 5 million years of the end-Cretaceous mass extinction, surviving bird lineages tended to develop smaller body sizes.
They also changed how they developed as hatchlings, with more species becoming ‘altricial.’
“This means they are still very embryonic when they hatch, need their parents to feed them, and can take weeks to fledge,” Dr. Berv said.
“Birds that hatch ready to fend for themselves, like chickens and turkeys, are called ‘precocial’.”
“We found that adult body size and patterns of pre-hatching development are two important features of bird biology we can link to the genetic changes we’re detecting.”
“One of the most significant challenges in evolutionary biology and ornithology is teasing out the relationships between major bird groups — it’s difficult to determine the structure of the tree of life for living birds.”
Over the past 15 years, researchers have been applying increasingly large genomic data sets to try to solve the problem.
Previously, they used genomic data to study the evolution of birds’ genomes using statistical models that make strong assumptions.
These traditional models allow researchers to reconstruct the history of genetic changes, but they typically assume that the composition of DNA, its proportion of A, T, G and C nucleotides, does not change across evolutionary history.
The study authors developed a software tool to more closely track DNA composition over time and across different branches of the tree of life.
With this tool, they were able to relax the assumption that the composition of DNA remains constant.
“This allowed the model of DNA evolution to vary across the evolutionary tree and identify places where there was likely a shift in DNA composition,” said University of Michigan’s Professor Stephen Smith.
“For this new research, these shifts were concentrated in time, within about 5 million years of the end-Cretaceous mass extinction,” Dr. Berv added.
This approach also allowed the team to estimate which bird traits were most closely associated with these shifts in DNA composition.
“This is an important type of genetic change that we think we can link to the mass extinction event,” Dr. Berv said.
“As far as we know, changes in DNA composition have not been previously associated with the end-Cretaceous mass extinction in such a clear way.”
“We know that mass extinction events can dramatically affect biodiversity, ecology and organismal form,” said University of Cambridge’s Professor Daniel Field.
“Our study emphasizes that these extinction events can actually influence organismal biology even more profoundly — by altering important aspects of how genomes evolve.”
“This work furthers our understanding of the dramatic biological impacts of mass extinction events and highlights that the mass extinction that wiped out the giant dinosaurs was one of the most biologically impactful events in the entire history of our planet.”
By relaxing the typical assumptions used in evolutionary biology, the researchers are building more nuanced insight into the sequence of events that occurred in the early history of birds.
“We have typically not looked at the change in DNA composition and model across the tree of life as a change that something interesting has happened at a particular point of time and place,” Professor Smith said.
“This study illustrates that we have probably been missing something.”
The study was published in the journal Science Advances.
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Jacob S. Berv et al. 2024. Genome and life-history evolution link bird diversification to the end-Cretaceous mass extinction. Science Advances 10 (31); doi: 10.1126/sciadv.adp0114
This article is a version of a press-release provided by the University of Michigan.
