Speech is a crowning achievement of human evolution , the skill that separates us from every other animal. So, it would stand to reason that evolving this capability required some enormous leap in brain complexity. A study published today in Nature suggests otherwise.
Alston's singing mouse (Scotinomys teguina), a small rodent from the cloud forests of Central America, produces loud, elaborate songs humans can hear across a room. These mice can sing solo but often perform rapid-fire duets with split-second timing. Among all mammals, it's one of the closest parallels to the turn-taking of human conversations.
A team at Cold Spring Harbor Laboratory (CSHL) wanted to know what changed inside this animal's brain to make singing possible. The answer was surprisingly simple. The singing mouse didn't evolve a bigger brain, new brain regions, or new categories of neural connections. Instead, evolution roughly tripled the number of neurons that connect the brain's mouth-movement control center with just two target regions. One is the cortex that controls hearing. The other is a midbrain structure that controls vocalizations for a variety of species, including humans. The rest of the brain wiring is essentially identical to that of an ordinary lab mouse!
Emily Isko , a grad student in the Banerjee lab, traced the differences using a molecular barcoding technique developed at CSHL by Professor Anthony Zador . This allowed the team to map thousands of individual cells across the whole brain. "When you look at singing mice and lab mice side by side, their brains are almost indistinguishable," Isko said. "The differences only show up when you trace where individual neurons send signals."
"You might expect that evolving a whole new means of vocal communication would require a significant reorganization of brain circuitry," said Associate Professor Arkarup Banerjee . "Instead, we found a couple of targeted changes to existing wiring patterns. Our approach gives the field a playbook. To understand how new behaviors evolve, find closely related species with big behavioral differences and start by mapping the wiring at high-resolution."
The findings have implications far beyond mice. At some point since humans split from chimps millions of years ago, our higher brain regions gained enough control over vocalization to produce speech. The singing mouse's two amplified brain regions are central to human vocal circuits. Additionally, brain-imaging research has identified stronger connections between similar motor and auditory areas in humans than in other primates. The singing mouse may be replaying a version of the evolutionary trick that put our ancestors on the road to language.
"The fact that these changes are relatively simple and targeted raises an exciting possibility," said Zador. "If only a few specific wiring changes separate singing mice from lab mice, we might be able to engineer those changes ourselves. Could we make a lab mouse sing?"
Could tomorrow's pop stars see competition in unexpected places? Maybe not, but the discovery could someday provide new tools for speech therapy while addressing a question central to the human experience. How did language emerge?
