Sounds Wild and Broken, page 5
Later, other members of the Orthopteran order left spectacular fossils. In the Triassic, the geologic period after the Permian, fossils of cricket-like wings possess stridulatory files and, perhaps, rudimentary “windows.” These windows, flat panes of membranous tissue, have no known function for flight and appear to be smaller versions of the wing windows in living crickets that focus and amplify sound, giving their chirps a clear tonal quality. These Triassic crickets likely sounded sweet, not ragged and raspy as the coarse file of Permostridulus surely did. The most well preserved of all Orthopteran fossil sound-making devices are the wings of a katydid from 165-million-year-old Jurassic rocks in Inner Mongolia. The fossil is so exquisitely well preserved that broad dark bands are still visible across the forewings. A sound-making ridge lies across each wing, close to the attachment point to the body, and comprises a row of just over one hundred small teeth. The spaces between these teeth gradually increase, as they do in many modern katydids. As wings scissor closed, they accelerate. Evenly spaced teeth would produce a sound of increasing pitch, like running an accelerating fingernail across a comb: brr-eee! But the increasing spacing between the teeth exactly compensates for this acceleration, yielding a pure-toned sound: eeee! It seems likely that teeth in the extinct species did the same.
The team of scientists who described this fossil, led by Jun-Jie Gua and Fernando Montealegre-Z, described the morphology of the wing and made a speculative re-creation of its sound. Comparing the dimensions of the fossil to those of living species with known sounds, they estimated that the katydid made sixteen millisecond pulses of just over six kilohertz. To human ears, these are brief taps of pure tone, with a high, bell-like timbre. Fossil plant remains from the same rocks as the katydid suggest that this singer’s home was an open woodland of ancient coniferous trees and giant ferns. The katydid’s sound frequency would have traveled especially well in this habitat, and so the song and its ecological context seem well matched. Unlike Permostridulus, this katydid was also likely heard by vertebrate animals. By this time, amphibians, dinosaurs, and early mammals could hear higher frequencies. Like many modern katydids, this ancient insect may have sung at night, reducing the risk from predators.
Insect wings first evolved as stubby extensions of the external skeleton. Studies of wing development in modern species suggest that this evolutionary feat was accomplished by a merger of the actions of genes that control body armor with those that build legs. We have no fossils of the first flap-like wings, but evolutionary trees built using the genes of living species strongly suggest that the first wings evolved between 400 million and 350 million years ago. These first wings probably slowed the jumping descent of plant-climbing insects, a behavior still seen today in the bristletails, cousins of modern insects. Many insects at the time grazed on plant spores that were held in capsules at the tips of branches. Gliding would have been a useful skill in these forests of fern- and conifer-like plants. Wings also allow easy access to food, rapid dispersal to new habitats, and more efficient searches for mates. The earliest fossil of a complete wing—veined, shaped with leading and trailing edges, and large enough to support flight—is 324 million years old. By about 300 million years ago, the fossil record contains dozens of winged insect species.
Insect wings also provide materials that readily make sound. Their flat, lightweight surfaces broadcast vibrations, animal versions of the papery interiors of electric loudspeakers. Flight muscles move with fast, repetitive motion and are well supplied with oxygen for sustained action. Any insect that developed a propensity to repeatedly rub wings without flying might make sound. Thickened or corrugated wing veins made the sound louder and more tonal.
Among animals like primitive crickets that lived in thick foliage or the jumble of debris on the ground, sound making was perhaps especially advantageous. Sound allows mates to find one another in the tangle of miniature jungles where sight lines are blocked.
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.
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Why did communicative sound take so long to evolve? Bacterial and single-celled life existed for three billion years with no known sonic signals. 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. No known fossil from this time has a rasp or other sound-making structure. The expert paleontologists whose advice I sought all said that they knew of no physical evidence of sound-making structures from animals until the first cricket- and cicada-like insects evolved. Of course, the fossil record is incomplete and some sound-making structures, such as the swim bladders of fish, leave little or no trace in rock, and so we hear imperfectly across these great stretches of time.
This long silence is a puzzle. Sound is an effective and inexpensive way to send signals. Soon after the Ediacaran, the time period when disk- and ribbonlike animals first evolved, animal bodies evolved skeletons and other structures that could easily make sound. 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.
After the Ediacaran, the number and variety of fossilized animals exploded in a geologic era known as the Cambrian. Starting about 540 million years ago, Cambrian oceans filled with diverse new animal forms, including the ancestors of the major groups we know today: arthropods, molluscs, annelid worms, and tadpole-like creatures that later evolved into the vertebrates. 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. Animals inherited the cilia of their single-celled ancestors, now attached to skin and spines, lodged into exoskeletons, and on the surfaces of organs within the body. 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. Arthropods such as crustaceans and the now-extinct trilobites had bodies cloaked in arrays of sensors. 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. Over time, animals lower down the food chain evolved more elaborate defenses—spines and thicker shells—and even took to burrowing in mud when it came time to molt, a behavior recorded in fossils of animals that died and were entombed as they shed their skeletons.
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. 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. In water, where sound creates fast and far-reaching molecular movements, the danger was especially acute. 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. 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 other eighty-five thousand species of molluscs and eighteen thousand species of annelid worms are, as far as we know, mute except for the slither and bubble of their body movements. The same is true for nematodes, flatworms, sponges, and jellyfish. This silence is not the result of anatomical deficiency. The plate-like doors to snail shells would make excellent rasps. Soft, muscular flesh can make sound too, as popping worms, fish swim bladders, and our own vocal folds demonstrate.
Only two branches of the animal family tree account for almost all the voices and songs of our contemporary world: the vertebrate animals—fish and their terrestrial descendants, including us—and the arthropods—crustaceans, insects, and their kin. 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.
Then, a revolution. 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.
That sound-making insects evolved after they gained the power of flight does not prove that a release from predation caused the evolution of the first animal calls and songs. 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.
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As I walk the road verge in southern France, I am struck by the vigor of the insect sound around me. At any one spot on the road, I hear a dozen grasshoppers purring. The air is a haze of blended chirps from uncountable crickets. Jean-Henri Fabre, the great French scientist and poet of insects, wrote of crickets in this region filling the air with their “monotonous symphonies” in the late nineteenth and early twentieth centuries.
This soundscape contrasts with that of the cultivated areas in the lowlands, away from the unruly woodlands and verges along this mountain road. In fields and along country roads in areas where agriculture is more industrialized, insect song is muted. There is little natural vegetation left in fields made tidy by herbicides and the vigorous application of the plow. Diverse native grasslands and forests have been transformed into monocultures of annual crops. Insecticide arrives both from the nozzles of farm machinery and in the wind and rain, the stirred-up vapors and dusts of now-banned chemicals from decades past.
A 2016 report synthesizing the knowledge of sixty experts in insect biology found that Europe’s grasshoppers, crickets, and their kin are in crisis. About 30 percent of species are threatened with extinction, and the majority of species for which we have good population data are in decline. In North America, grasshopper populations are dwindling even in areas away from the plow and the fog of insecticide. In two decades, grasshoppers decreased by 30 percent on the Konza Prairie in Kansas, a reduction that was associated with a sharp drop in nutrients—nitrogen and minerals—in prairie plants. Likely stimulated by extra carbon dioxide in the atmosphere, plants on the prairie have doubled their growth in twenty years, but nutrients in this rank vegetation have become diluted. The grasshoppers’ food is now more like bulky, savorless straw than nutritious salad.
Not only are crickets and grasshoppers in trouble, but so, too, are many other insects. A recent compilation of 160 long-term studies of the abundance of insects of all kinds—bees, ants, beetles, grasshoppers, flies, crickets, butterflies, caddis flies, dragonflies, and more—found an average rate of decline of more than 10 percent per decade for terrestrial insects, but the reverse trend for the few insects that live in fresh water. These insects are the foundation of most ecosystems on land. By biomass, insects outweigh all mammals and birds combined by over twenty times. By number of species, they are at least four hundred times more numerous. On land, the sonic diversity produced by hundreds of millions of years of evolution is being sharply cut back. In the growing silence of insects in forests and meadows, we hear the decline of the animals whose lives sustain the vitality of all terrestrial ecosystems.
This extinction of sensory diversity has many causes: technologies that deliver poisons; ever-rising carbon dioxide levels; economies that force the costs of production onto other people and other species, the “externalities” of business; and ever-expanding human appetites and numbers that shoulder out other species. All these social and economic factors exist in a culture of inattention and lack of appreciation. There is a connection between the anonymity of this fossil site in southern France—one of the great mileposts along life’s evolutionary epic—and the silencing of the living voices in the surrounding landscape. Our ears are directed inward, to the chatter of our own species. Introductions to the sounds of the thousands of species that live in our neighborhoods have no place in most school curricula. We generally regard human language and music as outside nature, disconnected from the voices of others. When a concert starts, we close the door to the outside world. Books and software that teach us “foreign” languages include only the voices of other humans. Public monuments to sound are rare and honor a handful of canonical human composers, not the sonic history of the living Earth. Permostridulus’s discovery passed without comment in the media.
Even within the community of environmentalist activism, we speak of crises in the lexicons of chemistry and statistics: concentrations of gases and estimates of extinction rates. These are essential ways of knowing and thereby healing the world, but they omit the lived experience of animal senses. Life is made not only of molecules and countable species but of relationships among living beings. These relationships—life-giving interconnections between the “self” and the “other”—are mediated through the senses. Diversity of sensory experience is a generative force, a catalyst for future biological innovation and expansion, not merely a product of evolution’s creativity.
The Permian period ended 252 million years ago in a spasm of extinction. In the seas, more than 90 percent of species went extinct. On land, both animal and plant diversity was reduced by more than half, including the loss of most of the insects and vertebrates whose fossils dominate the Salagou Formation. The causes of this global cataclysm are much debated, but they likely involved a combination of massive volcanic activity, global heating and deoxygenation of the oceans, and the release from ocean sediments of poisonous levels of hydrogen sulfide. We’re now in a rapid decline of our own making, albeit one that is, so far, much less severe than the decimation of the end of the Permian. One necessary part of our response to this swift decline must be to reawaken our senses and thereby human culture to the community of life.
