A couple of years ago a little girl up in Seattle began feeding the neighborhood crows in her backyard. After a couple of weeks they began returning the favor by leaving things on the feeding tray: coins, rings, washers, buttons. After a few years she's gotten several drawers of trinkets.
“Birdbrain” may not be much of an insult: Humans and songbirds share genetic changes affecting parts of the brain related to singing and speaking, new research shows. The finding may help scientists better understand how human language evolved, as well as unravel the causes of speech impairments.
Neurobiologist Erich Jarvis of Duke University Medical Center in Durham, N.C., and colleagues discovered roughly 80 genes that turn on and off in similar ways in the brains of humans and songbirds such as zebra finches and parakeets. This gene activity, which occurs in brain regions involved in the ability to imitate sounds and to speak and sing, is not present in birds that can’t learn songs or mimic sounds. Jarvis described the work February 15 at the annual meeting of the American Association for the Advancement of Science.
Songbirds are good models for language because the birds are born not knowing the songs they will sing as adults. Like human infants learning a specific language, the birds have to observe and imitate others to pick up the tunes they croon. The ancestors of humans and songbirds split some 300 million years ago, suggesting the two groups independently acquired a similar capacity for song.
With the new results and other recent research, Jarvis said, “I feel more comfortable that we can link structures in songbird brains to analogous structures in human brains due to convergent evolution.”
Jarvis’ team analyzed tissue from throughout the brains of three humans, measuring the amount of particular molecules made by a given gene to determine how active it is. They compared the results with brain tissue from bird species capable of vocal imitation and song learning — such as songbirds, hummingbirds and parrots — as well as birds that don’t, such as doves and quails.
The vocal-learning birds and humans share a distinct pattern of activity in roughly 40 genes in analogous regions called Area X in birds and the anterior striatum at the base of the forebrain in humans. These structures are involved in imitation.
The team also found similar patterns of activity in a different set of about 40 genes in regions involved in speech and song production. For birds that was in the robust nucleus of the acropallium, or RA nucleus, and for humans, the laryngeal motor cortex. Previous studies have found connections between the laryngeal motor cortex, which is located in a part of the brain that controls voluntary movement, and brainstem nerve cells that control muscles of the larynx, the organ that produces sound. Similar connections have been found in the analogous regions of bird brains.
“It’s one of the most fundamental neural connections that leads to the evolution of human language,” Jarvis said. In the future, he plans to investigate how the activity of the 80 genes influences these connections and other brain circuitry related to speaking and singing.
The new work also has practical implications, said Simon Fisher of the Max Planck Institute for Psycholinguistics in the Netherlands. It’s “exciting stuff,” he said, because scientists could combine data on gene activity in the brain with studies that have decoded the entire genetic instruction book of people with various speech disorders to pinpoint how these problems arise.