A new study from the University of Michigan has uncovered how the mass extinction event 66 million years ago, which wiped out non-avian dinosaurs, sparked rapid evolution in the early ancestors of birds.
The research, published in the journal Science Advances, reveals that the end-Cretaceous mass extinction led to significant changes in bird genomes, contributing to the incredible diversity of birds we see today.
The study examined the evolutionary paths of major bird groups and found “genomic fossils” in their DNA that mark crucial evolutionary steps.
These changes helped birds evolve into more than 10,000 species currently living.
Lead author Jake Berv, who conducted the study as a Michigan Life Sciences Fellow, explains, “By studying the DNA of living birds, we can detect patterns of genetic sequences that changed after one of the most important events in Earth’s history.
The signature of those events is imprinted in the genomes of the survivors, detectable millions of years later.”
A genome is made up of four nucleotide molecules—A, T, G, and C—whose order defines the “blueprint” of life. The DNA code can evolve, shifting the overall composition of these nucleotides across the genome. These changes are crucial for genetic variation, which drives an organism’s ability to evolve.
Researchers found that the mass extinction event caused shifts in nucleotide composition. These changes are linked to how birds develop as babies, their adult size, and their metabolism. For example, within 3 to 5 million years after the extinction, surviving bird lineages tended to develop smaller body sizes and different development patterns.
More species became “altricial,” meaning they hatched in a very underdeveloped state and needed their parents to feed them, unlike “precocial” birds like chickens, which hatch ready to fend for themselves.
“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,” says Berv, now a Schmidt AI in Science postdoctoral fellow.
One challenge in studying bird evolution is determining the relationships between major bird groups. Researchers have used genomic data to reconstruct the history of genetic changes, typically assuming that DNA composition remains constant over time.
However, Berv and Stephen Smith, a professor at U-M, developed a new software tool to track DNA composition changes over time and across different branches of the tree of life. This allowed them to identify shifts in DNA composition concentrated within 5 million years of the mass extinction.
Daniel Field, a professor of vertebrate paleontology at the University of Cambridge and co-author of the study, explains, “Mass extinction events can dramatically affect biodiversity, ecology, and organismal form. Our study shows that these events can also influence how genomes evolve.”
The researchers believe that by relaxing traditional assumptions in evolutionary biology, they can gain a more nuanced understanding of bird evolution. “This study illustrates that we have probably been missing something important by not looking at changes in DNA composition across the tree of life,” Smith says.
