We live on a planet thrumming with the diverse voices of communicative animals. But it was not always this way. For more than 90 percent of Earth’s history, it seems, no animals sang or cried. No creatures called when the seas first filled with complex animal life or when reefs first rose. The land’s primeval forests contained no singing insects or vertebrates. These ancient times had sound—wind, waves, thunder, geologic murmurs, and the splash, scrabble, and crunch of moving, feeding animals—but hundreds of millions of years of animal evolution unfolded in communicative silence.
Who were the first singers? And what do their lives tell us about the evolution of sonic communication? Here I will focus, as in my recently published book, Sounds Wild and Broken, on a cricket-like insect called Permostridulus, the oldest known singing land animal for which we have physical evidence of sound-making anatomy. The qualifier “known” is important because it is possible that other singers were present even earlier, but either left no trace or have yet to be unearthed by paleontologists.
As its name indicates, Permostridulus lived in the Permian, about 270 million years ago, in what was then the hot, dry interior of the ancient supercontinent Pangaea. The landscape was bare and windblown, although scrubby ferns and conifers grew alongside streams and lakes, wet places that provided habitat for animals. The mud also caught and preserved the bodies of dead insects, especially their papery wings. It was from such fossilized remains that paleontologists led by Olivier Béthoux at the Muséum National d’Histoire Naturelle in Paris described the only known specimens of Permostridulus.
The wings of this long-extinct insect not only revealed evidence of sound-making, but suggested how and why communicative sound evolved. Near the attachment point of the wing, some veins were thickened and raised. A prominent central vein was buttressed by side veins, creating a curved, corrugated ridge just a couple of millimeters long on a wing half the length of my thumb. Such a structure had no function in supporting the wing membrane. Instead, it was very likely a stridulating device, analogous to the ridges used for sound production on the wings of modern crickets. When the insects rubbed their wings together, the raised central vein would have scraped over the base of the other wing, making a chirping sound.
The bumps on the stridulating ridge of Permostridulus were uneven compared to the finely-wrought, regularly spaced ridges of modern singing insects and even some Jurassic species. But despite their crudeness, when rubbed together the wings would have rasped. By measuring the size and spacing of the bumps and comparing them to those of modern insects, I have created a speculative reconstruction of the sound of a rasping individual and of a chorus. We do not know how fast the insect moved its wings or whether the wings had resonant properties, and so this recreation may not be entirely accurate. We can say, though, that the unevenness of the bumps would have yielded a rougher, less tonal sound than those of modern stridulating insects.
The purpose of Permostridulus’s sound-making is unknown. The sound may have attracted mates or repelled rivals, just as the chirping of crickets does today. Alternatively, the rasping sound may have served to startle predators, buying time for escape. Such vibratory startle responses are common today among animals as diverse as lobsters, crickets, and pillbugs.
In providing physical evidence of when and in what form communicative sound-making appeared, the discovery of Permostridulus suggests how communicative sounds evolved after the long silence of early land animals. In general, wings likely unlocked communicative sound in two ways. First, flight allowed rapid escape from listening predators, reducing the ecological risks of singing. After all, predation is likely one reason that sound-making took so long to evolve. To this day, it is animals with wings or jumping legs that are most vocal. The slow and defenseless are typically silent. The evolution of insect wings, therefore, probably lowered predation risk and paved the way for the evolution of sonic communication. Second, wings served as the sound makers themselves.
Indeed, although wings allowed flight, they soon made sound production possible, too. Papery wing surfaces and pulsing wing muscles easily pump out sound waves, like loudspeakers driven by vibrating motors. This innovative duality occurred repeatedly in the subsequent evolution of sound making in other animals: the vertebrate larynx evolved as an anti-choking valve that later doubled as a sound-making organ; the jumping hind legs of grasshoppers became strumming devices; and the dextrous mammalian throat and mouth, originally used for suckling milk, became a sophisticated tool for shaping sound.
When we hear crickets chirping from the bushes in a city park or katydids singing from trees in late summer, we hear descendants and relatives of the first land animals to escape the listening ears of predators and use their wings to sing.