Human ballads, bird songs, and croaky frog serenades are all controlled by a common brain circuit that first appeared in animals hundreds of millions of years ago, a new paper reveals.
The finding is based on a study of toadfish and their close relatives midshipman fish, which both use their air-filled swim bladders to grunt, growl, and hum to attract mates and defend territories. (See photos of toadfish and midshipman fish.)
A group of rhythmically firing nerve cells in the fishes' brains determines the contraction rate of muscles attached to their swim bladders and controls the pitch and duration of the calls.
Andrew Bass of Cornell University and colleagues mapped this neuronal network in larval toadfish and midshipman fish. The team found that the circuit for vocalizing develops across a specific brain region that includes the base of the hindbrain and the upper spinal cord.
This is precisely the same pattern of brain development seen in other vocalizing vertebrates, including birds, amphibians, and primates such as humans.
The last time all of these creatures shared a common ancestor was more than 400 million years ago, when the evolutionary line that led to toadfish split from the line that eventually led to land vertebrates.
Because the vocalization circuit is found in different groups of animals belonging to both of these lineages, scientists reason that the brain region must have evolved before the lineages split.
"This one circuit in the brain is very ancient, and it's been retained throughout the course of evolution by all animals that vocalize," Bass said.
(Watch video of a male midshipman fish grunting to defend its nest.)
Bass and colleagues speculate that the primitive fish in which the brain circuit first appeared may have been able to communicate vocally.
But it's also possible that the circuit first developed to serve an entirely different purpose and was recruited for vocalization much later in evolutionary history.
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