No Reptile Can Suck.

The sounds of fish in deep time are hard to discern. We can, though, conclude that the many fish voices that now animate waters worldwide come almost entirely from groups of recent origin. It seems that for hundreds of millions of years, the fish, crustaceans, and other animals of the oceans made few, if any, communicative sounds. Then, starting around two hundred million years ago and accelerating around one hundred million years ago, most of the voices of the ocean arrived. Starting 180 million years ago, the supercontinent Pangaea fragmented, a process that continued for another 120 million years. These fractures created the major continents and oceans as we know them today. New shorelines and coastal habitats opened worldwide, increasing the extent and diversity of ocean habitat and opening new opportunities for colonization and adaptation. A long period of greenhouse climate also increased sonic diversity. Temperatures were so hot during most of the Cretaceous that ocean waters were tropical almost from pole to pole. There were no permanent ice sheets, and the sea level was up to two hundred meters higher than it is today, further broadening marine habitat as Pangaea broke up. North America was bisected by a large sea. Most of northern Europe and North Africa were underwater.

Easy From Now On

Photosynthetic plankton, the base of the ocean food web, was abundant and evolved a burst of new forms. Fish, crustaceans, snails, and echinoderms also multiplied. Sound making was a luxury enjoyed only by those at the top of a rich food chain. Prey animals at this time stayed silent and evolved thicker shells, and many took to living buried in mud and sand. Many sea creatures, unlike any land animal, shed sperm and eggs into the water without ever coming near another member of their own species. Clams, many snails, corals, and others breed without intimate contact. These species are also generally silent. With no nearby mates, why sing? During the breakup of Pangaea, species that bred this way showed no increase in diversity. But animals that breed by coming into close physical contact, rubbing bodies together or grasping one another, tripled in diversity during this time. Crabs and lobsters both woo partners and spar with rivals by stridulating their exoskeletons. The diverse arrays of thumps, squeaks, growls, and pulsed tones among fish are mostly breeding signals. Why should intimate mating behaviors increase species diversity? Animals that breed by copulating do so only with mates that live nearby.

Don't Stop Until You Get Enough

This keeps gene exchange local, allowing species to break into regional variants and, eventually, new species. But species that broadcast eggs and sperm in water currents have widespread and homogenous gene pools. They are like large, monolithic human corporations. These giants may be good at what they do, but they cannot break into specialized, innovative subgroups. This likely yielded many new species during the time when Pangaea’s breakup created new habitats. In a deliciously convoluted evolutionary path, the structure that stopped water flowing into the lungs of the lungfish and first land vertebrates, the larynx, returned to the water and sang. By blocking their blowholes or nostrils, these marine mammals use the vibrations of vocal folds in their larynx to send sounds through their body tissues and out into the water. When a solid object reflects the beam, the whale uses the echoes to home in on prey, avoid obstacles, or see its companions. Because sound penetrates tissues, this echolocating vision also reveals the inner form of other creatures. Seals and their kin are carnivores and arrived in the water later, twenty million years ago. The teeth and limbs of the transitional ancestors suggest that both groups were drawn to the water in search of the abundant food in nearshore habitats, just as polar bears and sea otters today spend much of their time foraging in the water or at its edge. To the creative forces of climate, biogeography, and mating that brought forth the sounds of fish and crustaceans, we can add the later opportunistic colonization of the seas by hungry mammals.

The Curtains Close

We hear the result in whale cries loud enough to traverse entire ocean basins or in the squealing of seals where fish abound in nearshore habitats. Today ocean waters are a tumult of engine noise, sonar, and seismic blasts. Sediments from human activities on land cloud the water. Industrial chemicals befuddle the sense of smell of aquatic animals. Sound is one of animal life’s ancient creative processes. The title of Jacques Cousteau’s film Le Monde du Silence was a manifestation of our ignorance about aquatic sounds. It was also an unintended warning about the consequences of our actions for other species. As we get louder and more voracious, we silence other living voices, cutting back both the diversity and the evolutionary creativity of the oceans. Specifically, the milk that ancient protomammalian mothers fed to their young. Before the evolution of lactation, protomammalian youngsters nourished themselves on whatever the environment supplied, sometimes brought to them by parents but often foraged for themselves. This diet of seeds, plant material, and small animal prey demanded guts able to digest complex and sometimes hard foods. Energy and nutrients were often in short supply, limiting the growth rate of the young. The invention of nutritive skin secretions broke these constraints and supercharged infancy. Nursing offspring connected directly to the strength and generosity of their mothers’ bodies. In addition to changes in the physiology and behavior of mothers, this new method of feeding demanded a reworking of the throats of infants. Much later, these innovations would allow humans to speak. Our languages are bequests from these ancient mothers. No reptile can suck. Their mouths, tongues, and throats are weak and lack skeletal support for complex muscles. This changed early in the evolution of mammals. Muscles attached to these fingers, strengthening and stabilizing the tongue, mouth, larynx, and esophagus. Judging by fossil evidence, by 165 million years ago the mammalian hyoid and its muscles had turned the slack, open maw of reptiles into a powerful and coordinated sucking device. The diversification of the mammalian clan was built on the unique nutritive bond between mothers and offspring, a connection made possible by both mammary glands and throat anatomy.