Sounds Wild and Broken, page 6
Paying attention to sound offers us a pleasing and instructive invitation to this reawakening. Because so much of our human communication is aural, our ears and minds are primed to listen and understand. Sound is, of course, a complement to the other riches of life’s community: the aromas of soil and trees; the colors of birds, fish, and arthropods; the varied shapes and motions of plants and animals; and the textures and tastes of plants in the hand and mouth. Our curiosity, care, and love are evoked by all these senses. But sound’s special qualities—unlike light, it passes through barriers; unlike aroma and touch, it carries far—make listening an especially important, joyful, and sometimes heartbreaking practice in this time of crisis.
I sit down on a slab of bloody stone and close my eyes. Cricket music glows in the air around me. I smile, astonished.
Flowers, Oceans, Milk
We live surrounded by the many gifts of flowers. Their aromas, colors, and varied forms are a delight for the senses, of course. But their fruits, roots, and foliage also, and less obviously, give us the vitality and diversity of the living world as we know it. Except for the products of the ocean, almost every bite of human food comes from a flowering plant. Wheat and rice are the starchy products of wind-pollinated flowers. Pressed fruits give us olive, canola, and palm oil. The flesh of domesticated animals is made from grass, corn, and other flowering plants. Leafy greens, sugar, spices, coffee, and tea, too, all come from flowering plants.
What is true for the human diet is also true for nonagricultural ecosystems. Prairies, tropical forests, deserts, salt marshes, and deciduous woodlands are populated primarily by flowering plants. Only in the chill of the boreal forests or the dry soils of subtropical pine woods do the flowering plants’ cousins, the pines and their relatives, take over. On the tundra and on mountain peaks, lichens and mosses dominate, but even there, flowering plants can be common and provide the principal source of food for many nectar-supping insects and seed-eating vertebrates.
Might flowers have also given us some of the diverse sounds of Earth? This seems an improbable link. Yet voiceless greenery yielded much of the modern acoustic exuberance of animals. The first stages of Earth’s sonic evolution were a slow burn: 1 billion years of wind and water, 3 billion years of bacterial hum and quiet animal motion, and 100 million years of chirping crickets. Then, between 150 million and 100 million years ago, Earth’s terrestrial sounds flared into the stunning variety we know today. The trigger for this explosion was likely the evolution of flowers. Literally, a flourishing of sound.
This was not the only time that plants lifted the acoustic vibrancy of the world. The first plants to reach up with trunks and branches—mostly ancient relatives of ferns and club mosses—precipitated the evolution of insect flight and, later, the sound making of wings. The first forests therefore offered a leg up for sound. The first flowers offered not structural support but energy and ecological richness. Compared with the fine dust of fern spores or the seeds of conifers, flowers and fruits are a bonanza for animals, rich in sugars, oils, and proteins.
This abundance created new ecological connections among plants and pollinating and seed-dispersing animals. Co-evolution between animals and flowering plants fed the diversification of both groups, a creative reciprocity. This action was fueled, in part, by new underground symbioses. Flowering plants united their roots with communities of soil bacteria, to the benefit of both. Roots protected and nurtured the bacteria within root nodules. Bacteria helped plants by giving them biologically usable nitrogen, the chemical foundation of all proteins and DNA. Nitrogen is in short supply in most ecosystems and so the union of roots and bacteria gave flowering plants an edge over their competition. Animals were the indirect beneficiaries of this below-ground revolution because well-fertilized plants produce abundant foliage and fruit.
Flowers, fruit, new ecological connections, and enriched soil: the origin of flowering plants transformed the terrestrial world and spurred animal evolution.
Studies of the DNA of modern plants suggest that the first flowering plants appeared in the Triassic, 200 million years ago. Flowering plants then slowly diversified through the Jurassic and exploded in diversity in the Cretaceous from about 130 million years ago. The below-ground partnership with nitrogen-harvesting bacteria started about 100 million years ago and presaged further surges in diversification. It is from this fanning out of plant lineages in the Cretaceous that we have the first unambiguous flower fossils.
For life on land, the Cretaceous period—from 145 to 66 million years ago—saw the remaking of ecosystems. Habitats where formerly only conifers, ferns, and their relatives grew were invaded by flowering plants that soon became the most common species, even in forests where giant ferns were still abundant as overstory trees. This time span, barely 3 percent of the entire timeline of life on Earth, also saw the origins or the diversification of many animal groups, including most of the animals that sing in modern ecosystems. Biologists refer to this period as the “terrestrial revolution,” a burst of creativity unrivaled since the great Ediacaran and Cambrian evolutionary explosions of the early oceans. It was also a time of revolutionary expansion in sound making.
Insect diversity, especially, rapidly grew in concert with the rise of the flowering plants. Family trees of katydids, grasshoppers, moths, flies, beetles, ants, bees, and wasps, reconstructed from fossils and DNA, splay out in profusion coincident with the appearance and rise of flowering plants. This flourishing changed the sound of Earth, lifting old voices to prominence and catalyzing the origin of new groups of singing insects. For many of these singers, evolutionary history resembles a river flowing into a delta. A long single channel suddenly fans into a bush of rivulets that then further ramify. The channel is the ancestral lineage and the fan is the explosion of animal diversity that followed the ascendance of flowering plants on Earth.
Nighttime insect choruses worldwide are dominated by katydids (also called bush crickets), a group that now numbers over seven thousand species. Katydids sing by drawing a plectrum at the base of one wing over a ridged file on the other. The date of origin of the group is contested, with some DNA studies suggesting 155 million years, others closer to 100 million years. These first modern katydids descended from a lineage of ancestral crickets that stretches all the way back to Permostridulus’s time, at the dawn of cricket evolution nearly 300 million years ago. This long ancestry then burst into new forms starting after 100 million years ago, followed by another expansion in diversity after the asteroid impact and mass extinction 66 million years ago. Katydids are mostly foliage eaters. Many look just like the leaves of their flowering plant hosts, with emerald bodies and elegant leaf-like wings. A few feed on conifers and some species prey on other insects, but most are entirely dependent on flowering plants.
Crickets and their kin sang long before the advent of flowering plants, and their sounds were likely the principal element of the animal soundscape from 300 to 150 million years ago. These ancient sounds, too, received a boost when flowers evolved. The “true crickets,” the Gryllidae, that now sing from meadows, forests, and lawns worldwide appeared 100 million years ago in the midst of the diversification of flowering plants.
Grasshoppers are a much later addition to Earth’s soundscape. Unlike the wing-rubbing katydids and crickets, grasshoppers sing by drawing their hind legs over ridges in the abdomen. This sound-making talent has independently evolved at least ten times within the grasshopper clan, perhaps a result of the evolution of hind legs whose great length folds them right next to the abdomen, a pre-adaptation to song. Although the grasshopper branch of the insect family tree broke away from its cricket cousins 350 million years ago, it was not until the Cretaceous, when flowering plants became abundant, that they started to sing. Grasshoppers then continued to diversify and add new singing members of their family alongside the ongoing expansion of flowering plants.
The buzz, whine, and shriek of over three thousand modern cicada species come from the tymbal organ on the side of their abdomens. Within the organ, muscles pop fine corrugation back and forth, sometimes hundreds of times each second, yielding a crackling sound that is then filtered and amplified by resonant chambers in the abdomen. In warm climates worldwide, the unique structure of the tymbal defines the soundscapes of hot afternoons. The cicada clans that we hear today diversified after the rise of flowering plants, starting one hundred million years ago. But the lineage of sound-making cicada ancestors is much older, dating to at least three hundred million years ago. A descendant of this ancestral group still lives among the moss-covered branches of Antarctic beech forests in Queensland, Australia. The sound of this “moss bug,” a sonic living fossil, travels as vibrations through vegetation, a repeated low buzz transmitted through the insect’s legs. The ancestral lineages gave rise to the modern cicadas and moss bugs, but also to the spittlebugs, planthoppers, and treehoppers, a clan of over forty thousand species that feed like ticks on plants, using piercing mouthparts to draw nutritious juices from within. Almost all these species make sounds inaudible to us, usually by transmitting vibrations to the leaves or twigs on which they live. From ancient roots, the modern representatives of these groups ramified into their present diversity following the expansion of flowering plants.
When we hear crickets, katydids, grasshoppers, and cicadas—the insect species that dominate the range audible to human ears in many habitats— we receive plant energies turned to sound by insects. This relationship is both of the moment, fueled by the plants’ sugars and amino acids, and ancient, the result of the stimulus that flowering plants gave for the evolutionary diversification of these insect groups.
Diversity in other major insect groups was also boosted by the rise of flowering plants. Ancestors of moths and butterflies lived three hundred million years ago, feeding on nonflowering plants. Their nectar-supping proboscis appeared in the Triassic and when flowers became common, the diversity of the group shot up, largely in synchrony with the expansion of host plants that provided nutritious foliage for larvae and nectar-rich flowers for adults. Tiny drumlike ears evolved at least nine different times among moths, mostly around one hundred million years ago, organs that are variously located on the abdomen, thorax, or proboscis, depending on the type of moth. These ears hear into the ultrasonic range and likely first evolved to avoid attacks from predaceous insects and birds. Such excellent hearing opened a new avenue for courtship, and many moths sing by softly rubbing their wings together, giving swishing, whispery songs too high for human ears to detect. But, unlike ours, moth ears can detect these sounds. Electrodes inserted into nerves running from these ears show that they can pick up sound up to sixty kilohertz, far above the twenty kilohertz that is the maximum for humans. When echolocating bats evolved, fifty million years ago, ultrasonic-sensitive ears also allowed moths to detect and evade the bats’ sonar blasts. The tiger moths went further and evolved bumps in their exoskeletons that, when buckled, release ultrasonic clicks. These sounds startle and jam the echolocating signals of hunting bats, and also signal the distastefulness of poisonous tiger moth species. This aerial sonic warfare is founded on the flowering plants that both feed the moths in the present day and stimulated the flaring of their species diversity long ago.
Before flowering plants evolved, the soundscape of the terrestrial world comprised only a few insect voices, the crickets, stoneflies, and perhaps ancestors of the cicadas and treehoppers. By the late Cretaceous, the insect chorus was like that of our time, a diverse mix of katydids, crickets, grasshoppers, and cicadas. The Cretaceous climate was hot, what geologists call a “greenhouse world” of high carbon dioxide, and the land was cloaked with lush forests, even close to the poles. Likely this was the first time in the long history of Earth that the air thrummed and pounded worldwide with the communicative sounds of life. Like modern rain forests, late Cretaceous forests were animated night and day by the crepitations, drones, buzzes, shimmers, bleats, and whines of singing insects. Earth, finally, was wrapped in song.
Birds were part of this chorus, but not in the way that we hear them today. Modern birds vocalize with a unique organ, the syrinx. Buried deep in their chests, at the Y-shaped confluence of bronchi and trachea, membranes and lips attached to modified cartilage rings impart sound to flowing air. In many species, the sounds of this syrinx are refined by a dozen or more muscles, each smaller than a grain of rice. The fossil record is incomplete, but it seems that this syrinx evolved late in the history of birds.
The first birds took to the air in the Jurassic, right at the time when DNA evidence suggests that the major lineages of flowering plants were splitting from one another. These birds were mostly predatory, feeding on the new diversity of insect prey, a boon partly built on the ecological productivity of flowering plants. Birds then flourished in the Cretaceous, diversifying in the ancient forests and colonizing waters with diving, fish-hunting species. The dominant birds in forests in those days were the “enantiornithes” (named for the “opposite” articulation of the shoulder). Most were small and nimble, looking somewhat like modern jays and sparrows, with feathers and wings like those of modern birds, and feet adapted to perching in trees. They were good fliers and it seems from their beaks that they lived on diverse foods like insects, small vertebrates, and fruit. A few species resembled woodpeckers and others foraged for small invertebrates on muddy shores. These parallels with modern birds end with a closer look. Their beaks had teeth. Their wings were clawed. This was a parallel universe of bird evolution, one now entirely extinct. There is no fossil evidence that any of these species possessed a syrinx. Are the known fossils too degraded and incomplete to reveal such a delicate structure? Or did this diverse lineage of birds, a sister group to modern birds, break away and take its own path before the origin of the syrinx? If so, they may have hissed and grumbled with their throats, as do many other reptiles, but produced nothing like the complex, tonal and harmonic sounds that we now associate with birds.
This early bird diversity was almost entirely erased by an asteroid impact at the end of the Cretaceous, 66 million years ago, which not only snuffed out all the nonbird dinosaurs, but also decimated the birds. The asteroid struck at the northern tip of what is now the Yucatán Peninsula in Mexico, leaving a crater 20 kilometers deep and over 150 kilometers wide. This crater is now buried by younger sediments, but geologists have mapped its extent using rock samples and magnetic analogies. The impact caused a mega-tsunami, sent out a pressure wave strong enough to deform rock hundreds of kilometers away, and ignited fires worldwide. The ejected vapor and rock, along with smoke from the blazes, fogged the atmosphere with dust, sulfates, and soot, bringing on a dark, cold “impact winter” that lasted at least two years. The world’s forests were mostly destroyed. In their place, ferns, mosses, and weedy flowering plants grew back. Forest-dwelling and larger bird species, especially, were scythed down. The great branching tree of Cretaceous bird diversity was cut to just a few twigs.
It was from shortly before the asteroid calamity that we have the first fossil evidence of a syrinx. The fossil is of a relative to living ducks and geese, named Vegavis iaai for Vega Island in Antarctica from which it was disentombed. Vegavis’s syrinx looks like that of modern waterfowl, but less complex than songbirds’. It could honk, but not warble. The close familial relationship of Vegavis to living birds demonstrates that the syrinx was very likely present in the ancestors of modern birds. The few species that made it through the end-Cretaceous bird-apocalypse arrived in the post-asteroid world equipped with an ability to sing. The diverse soundscapes of birdsong around the world today are built from this legacy as survivors expanded their ranges and split into new species.
It is likely, therefore, that bird sounds as we know them only arrived after the resurgence of forests following the calamity at the end of the Cretaceous. In birdsong, we hear the evolutionary legacy of renewal after great loss.
Land vertebrates other than birds—frogs, other reptiles, early mammals—followed a path of sound making only partly shaped by the rise of flowering plants. All modern vertebrate animals possess a larynx, a fleshy valve at the top of the windpipe enclosed in cartilage. The larynx first evolved in lungfish, where it stopped water from choking the air-filled lungs. The larynx retains this function in land vertebrates today, where it directs food and water to the esophagus, not the airways. Muscular tissues at the top of the windpipe can also make sound, and in many vertebrate land animals today the larynx now serves as an anti-choking valve and a sound maker. Curtain-like extensions from the sides of the larynx, the vocal folds, vibrate as air flows out. These fleshy tremulations give voice to animals from frogs to humans.
Vocal folds do not fossilize and so we cannot exactly reconstruct the timing of sonic evolution in these animals. But comparisons among modern species, combined with family trees built using DNA and dated fossils, give our ears a conduit to the past.
The common ancestor of all living singing frogs (a few modern species are voiceless, descendants of ancient pre-vocal lineages) dates to about two hundred million years ago. From then on, the wetlands of Earth rang with frog chirps and trills. It was likely at about this time that reptiles also became more vocal. Until about two hundred million years ago, ancestral reptiles lacked eardrums and could hear only low-frequency sounds, mostly transmitted through their jaw and leg bones to the inner ear. But once higher-frequency hearing evolved, the possibilities for acoustic communication opened up. Modern turtles call with tonal or wheezy pulses during breeding, crocodile youngsters chirp at their mothers and mating adults bellow, geckos chatter with calls richly layered with harmonics, and many other reptiles hiss when threatened. Early reptiles likely used some or all of this palette of vocalizations, supplemented by nonvocal sounds like scale rubbing, jaw snapping, and whip-cracking long tails.
