Vocal communication originated over 400 million years ago

The use of vocalizations as a resource for communication is common among several groups of vertebrates: singing birds, croacking frogs, or barking dogs are some well-known examples. These vocalizations play a fundamental role in parental care, mate attraction and various other behaviors. Despite its importance, little is known about when and at what stage in the evolutionary history of vertebrates this behavior first appeared. Comparative analyses can provide insights into the evolutionary origin of acoustic communication, but they are often plagued by missing information from key groups that have not been broadly studied.

Acoustic abilities are widespread in land vertebrates

An international research team led by the University of Zurich (UZH) has therefore focused on species that have never been accessed before. Their study includes evidence for 53 species of four major clades of land vertebrates -- turtles, tuataras, caecilians and lungfishes -- in the form of vocal recordings and contextual behavioral information accompanying sound production. "This, along with a broad literature-based dataset including 1800 different species covering the entire spectrum shows that vocal communication is not only widespread in land vertebrates, but also evidence acoustic abilities in several groups previously considered non-vocal," says first author Gabriel Jorgewich-Cohen, PhD student at the Paleontological Institute and Museum of UZH. Many turtles, for example, which were thought to be mute are in fact showing broad and complex acoustic repertoires.

Last common ancestor lived about 407 million years ago

To investigate the evolutionary origins of acoustic communication in vertebrates, the researchers combined relevant data on the vocalization abilities of species like lizards, snakes, salamanders, amphibians, and dipnoi with phylogenetic trait reconstruction methods. Combined with data of well-known acoustic clades like mammals, birds, and frogs, the researchers were able to map vocal communication in the vertebrate tree of life. "We were able to reconstruct acoustic communication as a shared trait among these animals, which is at least as old as their last common ancestor that lived approximately 407 million years before present," explains Marcelo Sánchez, who led the study.

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Novel brainstem circuit gives rise to the rhythms of vocalization

The vocal sounds of humans -- laughing, crying, and the babbling of babies -- have the same rhythmic quality as the sounds made by many mammals, songbirds, and even some species of fish. Researchers at UC San Francisco have discovered that a small cluster of neurons in the brain stem not only regulates tempo but also coordinates vocalization with breathing.

"Just to laugh or shout, the body has to coordinate about 100 different muscles in a rhythmic pattern within a single breath," said Kevin Yackle, MD, PhD, a Sandler Faculty Fellow and senior author of the study published online Jan. 7, 2022, in Neuron. "We discovered the neurons that, when switched on, give us this unconscious ability."

It's been widely understood that many animals, including humans, have innate control of breathing -- you don't have to use your brain to do it. Yackle and his team suspected that the same control exists for innate vocalizations.

To confirm the existence of this brain circuit, Yackle and his team studied the sounds made by baby mice when they were separated from their mothers. The babies' cries had a recognizable pattern associated with specific muscle movements. The researchers then determined which cells in the brain stem were responsible for this rhythm, which turned out to be a previously unknown circuit that appears to control the breath and coordinate the muscles needed to produce the vocal sounds.

Identifying this system will enable scientists to ask new questions about how we speak and why some people have trouble doing it.

A New Perspective on Speech Pathologies

"It could be that when we're learning to speak, we're learning to either bypass this system or to directly control it," Yackle said. "Altered wiring in this pattern-generating system could cause speech pathologies."

Yackle and his team are intrigued by studies suggesting that early in their lives, children with autism innately produce different kinds of sounds than other children do. Some children with autism are either unable to speak or have difficulty producing and understanding the tones and rhythms of speech.

Yackle and his team are intrigued by studies suggesting that early in their lives, children with autism innately produce different kinds of sounds than other children do. Some children with autism are either unable to speak or have difficulty producing and understanding the tones and rhythms of speech.

Such speech pathologies are often viewed as an inability to learn how to talk, said Yackle. He is flipping that perspective on its head, asking whether the speech problems are less about learning and more about the brain circuit controlling speech.

"A child may be hearing and learning language, but if the system that allows them to vocalize is different in some way, they just may not be able to coordinate the breath and movements to make the sounds," he said. "If we know how that system is different, it might change how we'd teach some people to speak."

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