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.
Provided by
Johns Hopkins University School of Medicine
