The Silencing Power Of Predation

After the long silence of Earth’s first 3.5 billion years, insects gave the terrestrial world its first songs. Ancient forests of fern, cycad, club moss, and conifer were brightened with sounds that would be familiar to our ears. When we hear crickets chirping from the mulch in a city park, in a mountain meadow, or along a rural road, we are transported to the first days of song on Earth. Although all these cells could sense water motions and vibrations, none reached out to others with sound. The first three hundred million years of animal evolution, too, seem to have lacked any communicative signals. This long silence is a puzzle. Sound is an effective and inexpensive way to send signals. These bodies surely made incidental noises as they crawled across the ocean floor, swam, and chewed. Yet as far as we know, the early oceans had no communicative sound. Perhaps the right mutations did not happen, depriving evolution of raw material? This seems unlikely, given that evolution in the early days of animal diversification had enough creative power to produce all the known branches of the animal kingdom, equipped with sophisticated eyes, jointed limbs, and complex nervous systems. We cannot know for sure, but it is likely that the waiting ears of predators put a brake on evolution’s sonic creativity, one that would only be eased when animals got swift and nimble enough to escape the maws of listening enemies.

Shadows and Tall Trees

After the Ediacaran, the number and variety of fossilized animals exploded in a geologic era known as the Cambrian. The first skeletons, jointed limbs, complex mouthparts, nervous systems, eyes, heads, and brains all appear in the fossil record in the space of about thirty million years. Cambrian oceans were full of listeners. The animal kingdom thus came into being with a preexisting sensitivity to the motions of water, including sound. All the early animals of the oceans sensed pressure waves and vibration in water. The first predatory cephalopods and, later, jawed fish added to the dangers. Early cephalopods detected vibrations and motions of water through sensors on their skin and with statocysts, organs in their heads lined with sensitive hairs. Ancient fish sensed vibrations through their lateral line system and early rudiments of inner ears. The fossil record reveals a pattern of increasing peril in the ocean, especially in the Ordovician, Silurian, and Devonian, the geologic eras after the Cambrian. Many fossils of shells and other prey show the marks of predatory attacks. To make a sound in the early oceans, then, was to reveal one’s presence to a community of predatory arthropods, fish, and molluscs. No aquatic animal can entirely avoid making some sounds as it moves and feeds.

A Matter Of Trust

No doubt many perished when paddling and chewing revealed their locations. The penalty for early attempts at sonic communication would likely have been death. Sound making was likely also dangerous for the first land animals. Fossilized footprints of small arthropods walking on land date to 488 million years ago. These colonists may have grazed on terrestrial algae and worms, or perhaps ventured onto land in search of sand in which to deposit their eggs, much as horseshoe crabs do today. Predaceous scorpions and spiders were on land 430 million years ago. By 400 million years ago, the land was inhabited by mites, millipedes, centipedes, daddy longlegs, scorpions, spider relatives, and the ancestors of insects. All these creatures could, through sensors in their legs, detect vibratory motions in soil or plants. The early animal communities in the ocean and on land, then, seem to have been hostile places for sound making. But even on land, the fact that many of the early colonists were predatory scorpions and spiders likely created a high cost for sound making. If the first animals in the oceans and land had been only vegetarians, the sonic diversity of the world might have bloomed much earlier. But this is not only a tale from long ago.

There's Nothing Like A Social Disease

A survey of living animals lends support to the idea that predation is a powerful silencer. To this day, animals whose lives are sedentary or slow and whose bodies lack weaponry are voiceless. Among worms and snails, for example, only a couple of species are known to make sound. A marine worm that lives enclosed in glass sponges in deep waters off the coast of Japan makes popping sounds when it fights, drawing water into its mouth then expelling the fluid with a snap. The sharp strands of the worms’ glassy home protect the fighters from passing predators. A land snail of tropical forests in Brazil makes quiet squeaks as it oozes bright, likely poisonous mucus when attacked by predators, the equivalent, perhaps of the warning buzz of disturbed bees. The same is true for nematodes, flatworms, sponges, and jellyfish. This silence is not the result of anatomical deficiency. Soft, muscular flesh can make sound too, as popping worms, fish swim bladders, and our own vocal folds demonstrate. Both are often swift and weaponized. Sound required a measure of fearless verve from its first animal makers. The first half a billion years or more of Earth’s sonic history comprised the voices of wind, water, and rock. Then came three billion years of hum from bacteria and the slosh, skitter, and chomp of early animals, a time with many incidental sounds of life but no known communicative voices. A long silence from the living world. Terrestrial insects evolved wings. This likely broke the silencing power of predation. Wings on a tiny insect enabled escape. The costs of sound making plunged, allowing sonic communication to gain a foothold. Cause and effect are hard to infer across such spans of time. If predation did act as a silencer, we can make a prediction, though. If examples of sound making are found in the fossil record from creatures older than Permostridulus, they will be from fierce, fast, or heavily protected animals. Perhaps an early insect with powerful hind legs or wings, an ancient prototype of a grasshopper. In the water, we’d expect sound from predaceous trilobites or crustaceans, and fish well suited to rapid escape or bristling with defensive spines.