Scientists pinpoint growth of brain's cerebellum as key to evolution of bird flight

Johns Hopkins Medicine evolutionary biologists announce that they have integrated studies of dinosaur fossils with PET scans of contemporary pigeons to address a long-standing biological question: How did avian brain evolution lead to flight?

According to them, the explanation seems to be an adaptive increase in cerebellar size in certain extinct species. The area of the brain in charge of movement and motor control is called the cerebellum.

The study's results are released in the Proceedings of the Royal Society B, which was published on January 31.

Researchers have long assumed that the cerebellum plays a significant role in avian flying, but they haven't had any concrete proof. In order to determine its worth, the new study examined the braincases of extinct dinosaurs and the brain areas of living birds during flight using data from contemporary PET scan imaging of common pigeons.

Amy Balanoff, Ph.D., associate professor of functional anatomy and evolution at the Johns Hopkins University School of Medicine and primary author of the published paper, states that "powered flight among vertebrates is a rare event in evolutionary history."

According to Balanoff, only three types of vertebrates, or creatures with a backbone, have gained the ability to fly: birds, bats, and extinct pterosaurs, the feared acrobatic predators of the Mesozoic era, which ended over 65 million years ago.

The evolutionary tree of the three species shows little relationship between them, and it is still unknown what exactly allowed any of the three species to fly.

In addition to the physical adaptations for flight, such extended upper limbs, specific types of feathers, a streamlined body, and other characteristics, Balanoff and her colleagues planned studies to identify brain characteristics that prepared an animal for flight.

To do this, she compared the brain activity of contemporary pigeons before and after flight in collaboration with biomedical engineers at Stony Brook University in New York.

Using positron emission tomography, or PET, imaging scans—the same technique frequently employed on humans—the researchers compared the activity in 26 brain areas throughout the bird's 10 minutes of flight between perches and at rest. Eight birds were scanned on several days.

PET scans employ a glucose-like substance that may be traced to the areas of the brain where it is most absorbed by brain cells, suggesting higher activity and energy utilization. In a day or two, the tracker breaks down and is expelled from the body.

Among the 26 locations, the cerebellum exhibited statistically significant increases in activity levels in all eight birds throughout the transition from resting to flight. When compared to other brain regions, the cerebellum's increased level of activity varied by more than two standard statistical deviations overall.

Additionally, the network of brain cells known as the "optic flow pathways," which link the cerebellum and retina in the eye, showed signs of increased brain activity, according to the researchers. These circuits handle movement in the field of vision.

According to Balanoff, their results of increased activity in the optic flow pathways and cerebellum weren't all that surprising, given that the regions may be involved in flying. Their research was novel in that it connected the fossil record, which demonstrated how the brains of dinosaurs that resembled birds first developed the neural circumstances necessary for powered flight, to the cerebellar findings of brains capable of flight in current birds.

Balanoff achieved this by using a digital database of endocasts, which are molds made of the inside cavities of dinosaur skulls that, when filled, mimic the human brain. She recognized and linked some of the first maniraptoran dinosaur species—which predated the first instances of powered flight among extinct bird cousins, such as the winged dinosaur Archaeopteryx—to a significant rise in cerebellar volume.

Additionally, Balanoff and her colleagues discovered evidence in the endocasts of early maniraptorans' cerebellum of increased tissue folding, a sign of growing brain complexity.

The researchers issued a warning, noting that these are preliminary results and that variations in brain activity that occur during powered flight may also occur during other actions, including gliding. They also point out that other brain areas could be more active during difficult flight maneuvers, even though their testing featured simple flying with an easy flight route and no obstructions.

The next step for the study team is to identify the specific regions in the cerebellum and the neurological pathways connecting these structures that support a brain prepared for flight.

According to Gabriel Bever, Ph.D., an associate professor of functional anatomy and evolution at the Johns Hopkins University School of Medicine, there are scientific explanations for why the brain grows larger over the course of evolutionary history, including the need to navigate novel and varied environments, which paved the way for flight and other types of locomotion.

"At Johns Hopkins, the biomedical community has a wide-ranging set of tools and technology to help us understand evolutionary history and link our findings to fundamental research on how the brain works," he continues.