Modern bird brains reveal evolutionary history of flight, dating back to the dinosaurs
A team of scientists, including one from the University of Arizona, has pinpointed birds' cerebellums as key to the evolution of flight.
Evolutionary biologists 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 appears to be an adaptive increase in the size of the cerebellum in some fossil vertebrates. The cerebellum is a region at the back of the bird brain that is responsible for movement and motor control.
"We found that when birds transition from rest to flight, circuits in the cerebellum are activated more than in any other part of the brain," said study co-author Paul Gignac, an associate professor in the University of Arizona College of Medicine – Tucson, studying neuroanatomy and evolution. He is also a research associate for the American Museum of Natural History.
"We then looked at the skull corresponding to this region in dinosaur and bird fossils to track when the cerebellum enlarged," Gignac said. "The first pulse of enlargement occurred before dinosaurs took wing, which shows that avian flight uses ancient and well-conserved neural relays, but with uniquely elevated levels of activity."
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. PET scans show how organs and tissues are working.
"Powered flight among vertebrates is a rare event in evolutionary history," said lead author Amy Balanoff, from the Johns Hopkins University School of Medicine.
In fact, just three groups of vertebrates, or animals with a backbone, evolved to fly: extinct pterosaurs – the terrors of the sky during the Mesozoic period, which ended over 65 million years ago – bats and birds, said Balanoff. The three flying groups are not closely related on the evolutionary tree, and the key 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, the team designed research to find features that created a flight-ready brain.
To do so, the team included biomedical engineers at Stony Brook University in New York to compare the brain activity of modern pigeons before and after flight.
The researchers performed PET scans 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 significantly, 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 connects the retina in the eye to the cerebellum. These pathways process movement across the visual field.
Balanoff said the team's 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, the team used a digitized database of endocasts, or molds of the internal space of dinosaur skulls, which, when filled, resemble the brain.
They then 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.
The researchers led by Balanoff 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, said co-author Gabriel Bever of the Johns Hopkins University School of Medicine.
Other study authors include Elizabeth Ferrer of the American Museum of Natural History and Samuel Merritt University; Lemise Saleh and Paul Vaska of Stony Brook University; 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.
This research was funded by the National Science Foundation.
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