About 66 million years ago, at the end of the Cretaceous period, a 10 km-wide asteroid struck Earth near what is now the town of Chicxulub in Mexico. The impact wiped out about 75% of Earth's animal and plant species, including groups such as non-avian dinosaurs and ammonites. A new study identifies key changes in bird genomes caused by the end-Cretaceous mass extinction that ultimately contributed to the remarkable diversity of modern birds.

“By studying the DNA of modern birds, we can detect patterns in gene sequences that changed shortly after one of the most significant events in Earth's history,” said Dr Jake Barb, from the University of Michigan.

“The signatures of these events appear to be imprinted in the genomes of survivors in ways that are detectable tens of millions of years later.”

An organism's genome is made up of four nucleotide molecules designated by the letters A, T, G, and C. The order of these nucleotides in the genome defines the blueprint of life.

The DNA code can evolve in ways that change the overall composition of DNA nucleotides across the genome.

These changes in composition are important in determining what genetic variations are possible and contribute to an organism's evolutionary potential, or ability to evolve.

Dr. Belf and his colleagues found that the mass extinction caused a change in nucleotide composition.

The researchers also found that these changes appear to be related to the birds' development as young birds, their adult size, and their metabolism.

For example, in the approximately 3 to 5 million years following the mass extinction at the end of the Cretaceous period, surviving bird lineages tended to decrease in body size.

The development of hatchlings has also changed, with more species becoming 'altricial'.

“This means that when they hatch they are still in a fetal state and need to be fed by their parents, and it may take several weeks for them to fledge,” Dr Barb said.

“Birds that are ready to fend for themselves immediately after hatching, like chickens and turkeys, are called 'precocious.'”

“We found that adult body size and pre-hatching developmental patterns are two important traits of bird biology that we can link to the genetic changes we are detecting.”

“One of the most important challenges in evolutionary biology and ornithology is unraveling the relationships between the major bird groups. The structure of the extant bird phylogenetic tree is difficult to determine.”

Over the past 15 years, researchers have been trying to solve this problem by applying increasingly large genomic datasets.

So far, they have used genomic data to study the evolution of bird genomes using statistical models based on strong assumptions.

These traditional models allow researchers to reconstruct the history of genetic change, but they typically assume that the makeup of DNA, i.e. the proportions of A, T, G and C nucleotides, remains constant throughout evolutionary history.

The study authors developed software tools to more closely track DNA composition over time and across different branches of the tree of life.

This tool allowed us to relax the assumption that DNA's composition is constant.

“This allows us to vary our models of DNA evolution across the evolutionary tree and identify places where there may have been changes in DNA makeup,” said Professor Steven Smith, from the University of Michigan.

“In this new study, these changes were clustered within about five million years of the end-Cretaceous mass extinction,” Dr Belff added.

This approach also allowed the team to estimate which bird traits are most closely associated with changes in DNA composition.

“This is an important type of genetic change that we think is associated with mass extinctions,” Dr Barb said.

“To our knowledge, changes in DNA composition have never before been so clearly linked to the end-Cretaceous mass extinction.”

“We know that mass extinctions can have dramatic effects on biodiversity, ecosystems and the forms of life,” said Professor Daniel Field, from the University of Cambridge.

“Our study highlights that these extinction events can have even larger effects on organismal biology by altering key aspects of genome evolution.”

“This study improves our understanding of the dramatic biological impact of mass extinction events and highlights that the mass extinction that wiped out the giant dinosaurs was one of the most biologically consequential events in the entire history of the Earth.”

By relaxing typical assumptions used in evolutionary biology, the researchers are developing more nuanced insights into the sequence of events in birds' early history.

“We haven't typically thought of changes in DNA configurations and models across the tree of life as changes that indicate something interesting happened at a particular time and place,” Prof Smith said.

“This study shows that we've probably missed something.”

of study Published in the journal Scientific advances.

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Jacob S. Belf others2024. Genomic and life-history evolution link bird diversification and the end-Cretaceous mass extinction. Scientific advances 10(31); doi: 10.1126/sciadv.adp0114

This article is a version of a press release provided by the University of Michigan.