Birds are among the most diverse and successful groups of animals on Earth, with over 10,000 living species and a rich fossil record. But how did they acquire the ability to fly, a feat that only two other groups of vertebrates have achieved?
A new study by evolutionary biologists at Johns Hopkins University School of Medicine reveals that the answer may lie in the growth of a brain region called the cerebellum, which is responsible for movement and motor control.
The Cerebellum's Role in Flight
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The cerebellum is a small but complex structure located at the back of the brain, below the cerebral hemispheres. It is involved in coordinating voluntary movements, such as walking, running, and balancing, as well as learning new motor skills.
The cerebellum also receives sensory input from the eyes, ears, and muscles, and adjusts the output of the motor cortex, the part of the brain that controls muscle activity.
The researchers hypothesized that the cerebellum should be important for bird flight, as flying requires precise and rapid coordination of the wings, tail, and body.
To test this idea, they used positron emission tomography (PET) scans to measure the brain activity of modern pigeons before and after flight. PET scans use a radioactive tracer that is absorbed by active brain cells, indicating increased energy use and activity.
The results showed that the cerebellum was the only brain region that had a statistically significant increase in activity levels between resting and flying in all eight birds that were scanned.
To trace the origin of the cerebellum's role in flight, the researchers also examined the fossil record of dinosaurs and birds. They used digital reconstructions of endocasts, which are internal casts of the braincase, to estimate the size and shape of the brain regions of extinct animals.
They compared the endocasts of a woodpecker, a modern bird, and a troodontid dinosaur, a close relative of birds that lived in the Late Cretaceous period, about 70 million years ago.
The researchers found that the cerebellum was much larger and more prominent in both the woodpecker and the troodontid dinosaur than in other dinosaurs and reptiles.
This indicates that the cerebellum underwent a significant adaptive increase in size in some of the earliest ancestors of birds, before the evolution of powered flight.
The researchers suggest that this enlargement of the cerebellum may have been driven by the need for more refined motor control and sensory integration, as these animals developed longer arms, feathers, and other features that enabled them to glide and eventually fly.
The study not only sheds light on the evolution of bird flight, but also opens new avenues for exploring the relationship between brain development and physical capabilities in other animals.
By combining modern imaging techniques and fossil evidence, the researchers hope to unravel the mysteries of how the brain and the body co-evolve to produce remarkable adaptations and innovations.
