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New Research Sheds Light on the Age-Old Question: How Did Birds Evolve To Fly?

Digital reconstructions of endocasts of a woodpecker, Melanerpes aurifrons (top), and a troodontid dinosaur, Zanabazar junior (bottom). The blue area is the cerebellum. Credit: Amy Balanoff

Evolutionary biologists at Johns Hopkins Medicine report they have combined PET scans of modern pigeons along with studies of dinosaur fossils to help answer an enduring question in biology: How did the brains of birds evolve to enable them to fly?

The answer, they say, appears to be an adaptive increase in the size of the cerebellum in some fossil vertebratesVertebrates are animals that have a backbone and include mammals, birds, reptiles, amphibians, and fish. They have a more advanced nervous system than invertebrates, allowing them greater control over their movements and behaviors, and they are able to move and support their body weight using their spine. Vertebrates are found in many habitats and play important roles in the ecosystem as predators, prey, and scavengers.” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]” tabindex=”0″ role=”link”>vertebrates. The cerebellum is a brain region responsible for movement and motor control.

The research findings were recently published in the journal Proceedings of the Royal Society B.

The Importance of the Cerebellum in Flight

Scientists have long thought that the cerebellum should be important in bird flight, but they lacked direct evidence. To pinpoint its value, the new research combined modern PET scan imaging data of ordinary pigeons with the fossil record, examining brain regions of birds during flight and braincases of ancient dinosaurs.

“Powered flight among vertebrates is a rare event in evolutionary history,” says Amy Balanoff, Ph.D., assistant professor of functional anatomy and evolution at the Johns Hopkins University School of Medicine and first author of the published research.

In fact, Balanoff says, just three groups of vertebrates, or animals with a backbone, evolved to fly: extinct pterosaurs, the terrors of the sky during the MesozoicThe Mesozoic Era is an interval of geological time from about 252 to 66 million years ago. It is also called the Age of Reptiles and the Age of Conifers and is subdivided into three major periods: the Triassic, Jurassic, and Cretaceous.” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]” tabindex=”0″ role=”link”>Mesozoic period, which ended over 65 million years ago, bats, and birds.

The three speciesA species is a group of living organisms that share a set of common characteristics and are able to breed and produce fertile offspring. The concept of a species is important in biology as it is used to classify and organize the diversity of life. There are different ways to define a species, but the most widely accepted one is the biological species concept, which defines a species as a group of organisms that can interbreed and produce viable offspring in nature. This definition is widely used in evolutionary biology and ecology to identify and classify living organisms.” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]” tabindex=”0″ role=”link”>species are not closely related to the evolutionary tree, and the key factors or factors that enabled flight in all three have remained unclear.

Besides the outward physical adaptations for flight, such as long upper limbs, certain kinds of feathers, a streamlined body, and other features, Balanoff and her colleagues designed research to find features that created a flight-ready brain.

To do so, she worked with biomedical engineers at Stony Brook University in New York to compare the brain activity of modern pigeons before and after flight.

Methodology and Findings

The researchers performed positron emission tomography, or PET, imaging scans, the same technology commonly used on humans, to compare activity in 26 regions of the brain when the bird was at rest and immediately after it flew for 10 minutes from one perch to another. They scanned eight birds on different days.

PET scans use a compound similar to glucose that can be tracked to where it’s most absorbed by brain cells, indicating increased use of energy and thus activity. The tracker degrades and gets excreted from the body within a day or two.

Of the 26 regions, one area — the cerebellum — had statistically significant increases in activity levels between resting and flying in all eight birds. Overall, the level of activity increase in the cerebellum differed by more than two standard statistical deviations, compared with other areas of the brain.

The researchers also detected increased brain activity in the so-called optic flow pathways, a network of brain cells that connect the retina in the eye to the cerebellum. These pathways process movement across the visual field.

Balanoff says their findings of activity increase in the cerebellum and optic flow pathways weren’t necessarily surprising, since the areas have been hypothesized to play a role in flight.

What was new in their research was linking the cerebellum findings of flight-enabled brains in modern birds to the fossil record that showed how the brains of birdlike dinosaurs began to develop brain conditions for powered flight.

To do so, Balanoff used a digitized database of endocasts, or molds of the internal space of dinosaur skulls, which when filled, resemble the brain.

Balanoff identified and traced a sizable increase in cerebellum volume to some of the earliest species of maniraptoran dinosaurs, which preceded the first appearances of powered flight among ancient bird relatives, including Archaeopteryx, a winged dinosaur.

Linking Modern Birds to Dinosaur Ancestors

Balanoff and her team also found evidence in the endocasts of an increase in tissue folding in the cerebellum of early maniraptorans, an indication of increasing brain complexity.

The researchers cautioned that these are early findings, and brain activity changes during powered flight could also occur during other behaviors, such as gliding. They also note that their tests involved straightforward flying, without obstacles and with an easy flight path, and other brain regions may be more active during complex flight maneuvers.

The research team plans next to pinpoint precise areas in the cerebellum that enable a flight-ready brain and the neural connections between these structures.

Scientific theories for why the brain gets bigger throughout evolutionary history include the need to traverse new and different landscapes, setting the stage for flight and other locomotive styles, says Gabriel Bever, Ph.D., associate professor of functional anatomy and evolution at the Johns Hopkins University School of Medicine.

“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 adds.

Reference: “Quantitative functional imaging of the pigeon brain: implications for the evolution of avian powered flight” by Amy Balanoff, Elizabeth Ferrer, Lemise Saleh, Paul M. Gignac, M. Eugenia L. Gold, Jesús Marugán-Lobón, Mark Norell, David Ouellette, Michael Salerno, Akinobu Watanabe, Shouyi Wei, Gabriel Bever and Paul Vaska, 31 January 2024, Proceedings of the Royal Society B.
DOI: 10.1098/rspb.2023.2172

In addition to Balanoff and Bever, other authors of the study are Elizabeth Ferrer of the American Museum of Natural History and Samuel Merritt University; Lemise Saleh and Paul Vaska of Stony Brook University; Paul Gignac of the American Museum of Natural History and University of Arizona, M. Eugenia Gold of the American Museum of Natural History and Suffolk University; Jesús Marugán-Lobón of the Autonomous University of Madrid; Mark Norell of the American Museum of Natural History; David Ouellette of Weill Cornell Medical College; Michael Salerno of the University of Pennsylvania; Akinobu Watanabe of the American Museum of Natural History, New York Institute of Technology College of Osteopathic Medicine, and Natural History Museum of London; and Shouyi Wei of the New York Proton Center.

Funding for the research was provided by the National Science Foundation.

Source: SciTechDaily