Mass Extinction Left a Genetic Mark in Bird Genomes

A rock that struck Earth with immense force around 66 million years ago altered the trajectory of life on the planet.

The asteroid impact and the rise of birds

The worldwide cascade of upheaval brought an end to the giant creatures that once dominated the land. Non-avian dinosaurs vanished into the fossil record, yet their extinction created opportunities for other life forms to emerge and prosper.

Soon after the impact, the first ancestors of modern birds appeared. Scientists have now identified evidence of this disaster in bird genomes: striking genetic changes caused by the mass extinction event that enabled birds to diversify into the remarkably successful and varied animal class seen across the world today.

"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," says ornithologist Jake Berv of the University of Michigan.

"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."

When a vast asteroid crashed into the area that is now Mexico's Yucatán Peninsula, the consequences were catastrophic. This event, now called the Cretaceous–Paleogene extinction, eliminated an estimated 76 percent of all animal life on Earth.

That enormous loss quickly created an ecological gap, which was occupied by surviving life as it evolved, diversified and established itself in the remade world. Avian dinosaurs underwent major changes, eventually giving rise to the roughly 10,000 bird species found today.

How mass extinction changed bird DNA

As animals evolve, the make-up of DNA within their genomes can also change.

DNA's basic components, known as nucleotides, contain four bases: A, C, G and T. The proportions of these nucleotide bases in a genome may alter, leading to developmental changes in the animal.

Earlier studies of bird evolution had treated DNA composition as fixed and unable to change. With newly created software, the researchers could remove that constraint and instead use an approach that permitted shifts in nucleotide-base ratios.

Berv and his colleagues used the software to examine genomic differences among every major bird group. This enabled them to pinpoint changes in genome composition since the Cretaceous–Permian extinction.

Bird genomes in the five million years after impact

The team concentrated on the 5 million years directly after the asteroid strike. They found that the extinction event triggered several substantial changes in avian genomes within 3 to 5 million years.

In particular, the changes concerned adult bird size, metabolism and development as hatchlings.

For instance, adult birds became markedly smaller than avian dinosaurs. There was also a move towards smaller, less developed young, which hatched without feathers and needed an intensive period of parental care. Some modern birds, including ducklings and chickens, do not require such care. This characteristic is called precociality, and evidence suggests it was also present in avian dinosaurs.

"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," Berv says.

"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."

Until now, DNA compositional changes had not been examined closely in relation to mass extinction. Yet mass extinctions are known to transform the world dramatically, reshaping entire ecosystems and their relationships, as well as interactions among the organisms living within them.

The study suggests there are further layers of change still to be explored.

"Our study emphasizes that these extinction events can actually influence organismal biology even more profoundly – by altering important aspects of how genomes evolve," says palaeontologist Daniel Field of the University of Cambridge in the UK.

"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."

The research was published in Science Advances.