A World of the Babies, by the Babies, for the Babies

From Nursery Earth: The Wondrous Lives of Baby Animals and the Extraordinary Ways They Shape Our World by Danna Staaf

We’re delighted to be able to share an excerpt from the new book by recent Skaana guest, Danna Staaf, from her book: Nursery Earth. Before we share the chapter, here are the links to our conversation.

There was a child went forth every day

And the first object he look’d upon, that object he became,

And that object became part of him for the day or a certain

part of the day,

Or for many years or stretching cycles of years.

—Walt Whitman, Leaves of Grass

Baby animals are undeniably cute: puppies tumbling over each other, joeys peeking out of kangaroo pouches, ducklings paddling in a wobbly line. Baby animals are also incredibly bizarre: Moth larvae mimic both snakes and feces, featherless finch chicks beg with beaks like Mondrian paintings—and let’s not forget baby humans, with our squishy skulls and taste buds on our tonsils.

Baby animals are very sensitive to the environment: Bird embryos die when their eggshells are thinned by DDT or smothered by spilled oil. Shellfish larvae can’t grow into edible adults until they find the perfect home in a sea full of increasingly imperfect habitat. But baby animals are also powerful enough to change the environment, for both good and ill from the human perspective. Agricultural pests like rootworms and borers are actually infant insects, and they devastate crops on every continent. At the same time, numerous beetle larvae can actually digest plastic, offering hope for pollution cleanup.

Even human babies embody this combination of vulnerability and voracity. When we’re born, we don’t know a single language, leaving us at the mercy of our adult caretakers. Yet we are so ravenous to learn that we can pick up as many languages as we are exposed to, an ability the adults around us have lost. Similarly, as babies, our incomplete immune systems put us at risk from infections that rarely affect adults. But these same baby immune systems are primed to build relationships with beneficial bacteria that will serve us the rest of our lives. If we’re exposed to too many dangers or deprived of necessary resources, our sensitivity becomes a weakness, but in the right environment, it bloom into a superpower.

Scientists who study early life stages, by dripping chemicals on tadpoles or feeding chicks experimental diets or injecting genes into fly eggs, are known as developmental biologists. Developmental biology is the study of how animals build their bodies—an examination of every process between fertilization and maturity. As a field of research, it has progressed through its own fascinating and sometimes turbulent developmental stages. In the nineteenth century, it was called embryology, and its practitioners peered through microscopes to watch an egg cell cleave into two, four, eight, and many more cells, then eventually grow a gut and a brain. The work of embryologists expanded to produce and bud off the entire field of genetics in the twentieth century, and developmental biology is now expanding again to link the microscopic stages of animal growth with global environment and ecology.

Unexpected connections are the forte of developmental biology. “Adult-onset” diseases like cancer and diabetes are increasingly understood to result from influences in early life, even as early as the womb. Not only humans but many other animals face challenges to health, longevity, and survival that trace their roots to chemical exposure or resource limitation in babyhood. And although these challenges highlight the vulnerability of youth, they also illuminate its adaptability. When a developing baby doesn’t get enough nutrition, it can preferentially devote its limited energy to growing critical organs like the heart and brain, leaving more redundant organs like kidneys to suffer the brunt of starvation. This postpones problematic symptoms until later in life, giving the animal a chance to reproduce first. Thus, surviving long enough to grow up and manifest kidney disease is a triumph of the flexibility of early development.

At no other stage in our lives are animals more capable of perceiving and responding to changes in the environment. In intimate conversation with our inanimate surroundings as well as with our fellow cohabitors of Earth, we mold our bodies to match our world. Within this capacity for change lies the future of life as we know it.

The diversity of developmental forms

When we think of the world’s diverse animal life, we usually think of adult animals: frogs and butterflies, jellyfish and echidnas. Not tadpoles and caterpillars, ephyra and puggles. (Believe it or not, those baby names match the parents that precede them.) Even when we turn our minds to baby animals, we tend to forget that they don’t always look like their parents. This leads to the occasional amusing contradiction in children’s books, like a “daddy caterpillar” or a “baby bee.” In reality, daddy caterpillars are moths or butterflies, and baby bees are wingless white larvae.

What is a larva? (Besides the singular form of larvae.) The word describes a baby that goes through a distinct metamorphosis to become an adult. Most animals have one or more larval stages. Because larvae can look so different from adults, this creates a constant puzzle for biologists, who must piece disparate forms into a single life cycle. Some species’ larvae have never been seen. Other larvae are not yet associated with an adult.

Larvae don’t have to be tiny, nor are all tiny babies necessarily larval forms. Bluefin tuna and kangaroos, both of which can grow to well over 3 feet (1 m) as adults, produce babies about an inch (2.5 cm) long. The tuna hatchling is a larva, the kangaroo joey is not. On the other end of the scale are the chick of a kiwi (not a larva) and the maggot of a tsetse fly (a larva), both born nearly the same size as their parents.

This size variation arises because animals can’t invest infinite energy in producing offspring. There’s a trade-off between size and number. Kiwis lay one gigantic egg at a time, while tuna spawn millions of mini eggs. In both cases, the babies that hatch from the eggs are independent, striking out on their own. Kiwi parents allocate their reproductive effort to building mass, producing a baby large enough to have a good chance of solo survival. Tuna parents allocate theirs to quantity, producing enough babies that it doesn’t matter if only a few survive.

Kangaroos follow a third route, pouring their resources into parental care. Although each baby is minuscule, it receives the warm protection of a pouch and a constant infusion of milk for up to a year after birth. Thus, although a newborn kangaroo is as tiny as a newborn tuna, an independent kangaroo is far closer in size to an adult.

Whether a baby turns to its parents or to the wild world to supply its needs, it is exquisitely well-equipped to do so. Kangaroo joeys have tough arms for climbing from the birth canal to the pouch. Larval tuna have (relatively) enormous jaws for swallowing prey nearly as big as they are. Larval parasites may be some of the most specialized babies of all, built to create links between disparate forms of life. As the tiny seeds of mighty trees are adapted to find a new home by hooking onto an animal carrier or hitching a ride on the wind, so baby tapeworms help themselves travel to a new host. A tiny “worm seed” can infect a human through the consumption of undercooked pork, as cleverly described by the

embryologist-poet Walter Garstang:

He’s very small, a mere pin’s head, beset with six small hooklets,

Is whirled about by wind and rain through puddles, fields and

brooklets;

But if a pig should swallow him, as many porkers do,

He’s made a start with no mistake: he’s on the road to you!

A tapeworm baby needs to be eaten in order to complete its life cycle, a typical parasite feature. As it passes from host to host, a single parasite can infect a wide range of animals in a wide range of habitats, connecting snails to birds and wetlands to forests. Similar connections are made even by animal babies that die when they’re eaten. Young life-forms are easier prey than their parents, accessible to a greater range of predators. Many hungry animals depend for their meals on the profusion of progeny produced by their fellows.

The hidden abundance of youth

For many species, babies comprise the majority of their life cycle.

Most animals on Earth are, in fact, babies.

It might be easiest to understand this as an ephemeral springtime truth, since we’re used to seeing one duck parent trailed by a dozen fuzzy offspring. The children’s classic Make Way for Ducklings contains four times as many babies as adults. In March, a square meter of water (about 11 sq ft) in a North Carolina pond can hold fifteen thousand tadpoles, the product of breeding by only a few hundred adults. In both cases, these babies grow to adulthood in a matter of weeks, and for the rest of the year, no ducklings or tadpoles can be found. So we might conclude that only during certain limited times are adults outnumbered by babies. However, as climate change brings spring weather earlier and earlier, the breeding season of many species is extended, and so is the period of time during which babies rule the roost.

What’s more, other animals often linger much longer in childhood and youth. Salmon are born in fresh water and live in streams for up to two years as fry before they mature and move out to the ocean. Many types of salmon spend no more time as seafaring adults than they did as river-dwelling babies. Given the natural population attrition over time, as salmon are eaten by predators or succumb to parasites, the total number of fry is typically greater than that of adults at any time of year.

Although salmon babies stick to rivers, the ocean serves as a giant nursery for uncounted other species. Surface waters froth with billions of baby fish, squid, crabs, and more, and it seems like each new expedition to the deep sea uncovers another astonishing cradle of life. In 2021, in Antarctica’s Weddell Sea, cameras towed at hundreds of meters’ depth revealed an icefish breeding colony of 93 square miles (240 sq km) filled with an estimated 60 million nests. The average number of eggs per nest was 1,735, making the total number of babies in this nursery well over a hundred billion. This previously unknown jackpot of baby fish revised our whole understanding of the Antarctic ecosystem.

As for humans? Currently, about 22 percent of the world’s Homo sapiens population is under the age of eighteen—likely the lowest this percentage has ever been. Less than a century ago, it was 31 percent. The relative proportion of children and teens variesgeographically, with only 17 percent of the Japanese population under the age of twenty, while 60 percent of the Nigerien populationfalls into this category. On average, adult humans outnumber children, but in many places—from the country of Niger to anyschoolyard—the reverse is true.

Including humans with other animals can be a touchy subject. Merriam-Webster offers multiple definitions for “animal”: first, any of a kingdom (Animalia), and second, one of the lower animals as distinguished from human beings. Both definitions have their uses. The species Homo sapiens belongs indisputably to the order Primates, phylum Chordata, kingdom Animalia, a fact encompassed in the first definition. At the same time, Homo sapiens is the only species that engineers global-environment-altering technology, and engages in moral debates about said technology (among many other topics). The second definition allows us to refer to all the animals that don’t do this with a single word.

However, our developmental biology illuminates our kinship with the rest of the kingdom. As a human embryo, I looked pretty fishy for a while. I also at times resembled a reptile and a chick. The similarities are eye-catching enough that some early biologists encoded them in law, contending that each animal displays the evolutionary history of its species over the course of its development. We now know, as we’ll explore further in chapter 8, that this superficially compelling “law” fails to capture the true intersection of development and evolution, but shared embryonic features still inform our understanding of relationships between animals—including humans. While this book is not about human development (many other excellent texts are available on that subject), our species will come up from time to time.

After all, I am a human, and you most likely are one, too. I, like you, began life as a baby. Also probably like you, I don’t remember it. I know that I depended on my parents and other caregivers, and I remain grateful to them for keeping me alive. I know that I was complicit in the process, crying for the attention I needed, producing attractive facial expressions and postures to garner care. I absorbed environmental input, both actively when I put dirt in my mouth and passively as I experienced the hot summers, poor air quality, and minimal rainfall of Los Angeles in the late twentieth century. I was fortunate to be given consistent affection and nutrition and to be brought periodically to a beach where I could enrich my sampling of dirt and dry grass with salt water and sand.

How much of who I am today is shaped by the genes in my mother’s egg cell and my father’s sperm cell, and how much by my experiences from the womb onward? This age-old question of nature versus nurture sits at the very heart of developmental biology.

From field to lab and back again—the development of developmental biology

We are as much a product of our environments as of our genes. Your immune system, your digestive system, even your brain and your bones all develop with environmental input, whether that input is bacteria, exercise, or diet. Within the rest of the animal kingdom, bacteria make it possible for insect embryos to grow, amphibian eggs to hatch, and squid to mature. Temperature determines the sex of certain turtles; location, the sex of certain worms. Diet dictates a bee’s caste; predators make water fleas grow spines. Birds and mammals grow bones and muscles in response to gravity and stress. Larvae of all kinds metamorphose in response to temperature, light, texture, or smell.

Many of these intimate connections between development and environment were already understood more than a hundred years ago. But in the middle of the twentieth century, when the discovery of DNA allowed scientists to begin reading and manipulating genetic code, the study of development moved out of the natural environment into a carefully controlled laboratory setting. Developmental biologists began to not only disregard but actively avoid environmental influences, so they could focus on the results of genetic modifications.

Biology underwent a schism. On one side were ecologists, studying organisms and networks of organisms in their natural habitat. On the other were geneticists, focused on the molecules that constitute and create life. Neither was wrong about the importance of their work, but they had such difficulty talking to each other that many university biology departments divorced, engendering two separate departments and lots of ruffled feathers.

This is what I discovered as a seventeen-year-old college freshman, newly fledged from my nest in Los Angeles and settling in to study marine biology at the University of California, Santa Barbara. My interests and classes aligned mostly with the Department of Ecology, Evolution, and Marine Biology. But if I wanted to learn about embryos, I had to take a class in the opposing Department of Molecular, Cellular, and Developmental Biology. The separation of marine and developmental biology is especially ironic, as the embryos of marine animals like sea urchins catalyzed numerous foundational breakthroughs in developmental biology.

However, none of these marine species became true model organisms—they were too tricky to raise in the lab. Instead, an enormous body of research was built around the laboratory study of genetic inheritance and mutations in six nonmarine model organisms: the fruit fly, the roundworm, the mouse, the chicken, the frog, and the zebra fish. Scientists picked these species because they’re easy to raise and require minimal environmental input, and studies on them have yielded huge discoveries, from the “toolkit genes” that organize development across the entire animal kingdom to developmental genes that are implicated in human cancer.

However, most animals, including humans, are not model organisms. Most of us could not develop successfully to adulthood in a sterile laboratory environment. Even the model organisms themselves can have trouble—scientists raised “germ-free” mice in the complete absence of bacteria, and discovered a suite of metabolic, neurological, and behavioral abnormalities. As for factors like temperature and diet, those would never be as constant in the real world as they are in the lab. The same precisely optimized nutrition is used to raise chicks in universities around the globe. What would happen if these animals were reared in more realistic conditions?

“Will the egg be computable?” asked Lewis Wolpert, a widely respected developmental biologist, in 1994. Which was to say, given complete knowledge of every molecule in a fertilized egg, could its development to adulthood be predicted? By 2006, Wolpert asserted that it would happen “in the next fifty years.” However, nearly everything that we have learned about developmental biology since then points in the opposite direction.

We used to think of development as a computer program. It has revealed itself instead as a collaborative, continuous performance. Development is not merely how we build ourselves, it’s how the world builds us. Rather than a rigid set of instructions defined by genes, it’s a set of possibilities influenced by and adapted to the environment.

Slowly, environmental concerns have forced their way back into our understanding of development. In 1982, a group of scientists realized that attempts to help endangered sea turtles by incubating eggs at controlled temperatures were actually harming the population by producing babies of all one sex. At first, such cases seemed like exceptions. But as the evidence mounts, we begin to understand that the development of all organisms is defined by their environment.

Studying a greater variety of organisms in a greater variety of conditions becomes increasingly important. In 2003, researchers discovered that tadpoles are more vulnerable to damage from pesticides in an environment that also contains predators—which is, let’s be honest, nearly every environment that tadpoles face. As the lead author wrote, “it is the lethality of pesticides under natural conditions that is of utmost interest.” Similarly, tadpoles exposed to different pesticides that may not seem to harm them directly suffer a reduced immune response, leaving them more vulnerable to parasites than they would otherwise be. Laboratory? No parasites. Real world? Crawling with them.

Not only dangers but also opportunities that would never arise in the lab abound in the real world. Toad eggs in northern India have been found developing in small “ponds” that are actually rain-filled elephant footprints. Before witnessing such a specific connection, we might have expected no relationship between toads and elephants—neither preys on the other, and they don’t compete for resources. But when we take each organism’s entire life cycle into account, the links between species rapidly multiply. Do declines in elephant numbers hurt toad populations, by limiting their nursery sites? No one has yet gathered data on this, or myriad other potential developmental dependencies.

The connections between development and environment illuminate the enduring unity of biology, separation of departments notwithstanding. Microscopic molecules record and store information about the environment as animals encounter it, and the animals use this information to build bodies and behavior best suited to their world. Cells aggregate to produce the bodies of larvae and adults, which aggregate to fill ecosystems. Both individual organisms and entire food webs change continuously with the cycling seasons. We think of seasons as wet or dry, hot or cold, but animals also create their own breeding seasons, spawning seasons, growing and dying seasons. The molecules inside caterpillars determine the timing of butterfly emergence season, which becomes a season of abundant food for birds, which is recorded in the molecules inside their developing chicks. Development shows us that the links are nowhere broken, as we shift in scale from molecule to ecosystem. Research on animal babies is facilitating the collaboration across scales that has become crucial to understanding and caring for our world.

An elusive profusion of squid eggs

Like most children, I adored baby animals from an early age. I bonded deeply with a pet kitten; I campaigned (unsuccessfully but perennially) for a puppy. In the margins of my school notebooks, I doodled fluffy little bodies with huge heads and eyes. Even in college, when my biology classes began introducing me to a profusion of larval forms, if I’d picked up a book about “animal babies,” I would have expected it to focus on cute critters like ducklings and baby bunnies.

In this book, we will encounter ducklings (some are parasitic!) and bunnies (eating their mom’s poop!), but we will also find many far stranger babies. Caterpillars, grubs, larvae of all kinds—these babies may be less adorable, but they are no less important. What they lack in immediate visual appeal they make up for in fantastical anatomy, behavior, and transformations. These forms link the ecosystems of today and build the biology of tomorrow.

It was after college that a fascination with the true diversity of animal babies fully gripped me. Beginning my pursuit of a graduate degree in a laboratory focused on the biology of Humboldt squid, Dosidicus gigas, I learned that no one had ever seen the eggs of this species in the wild. The same could be said for plenty of animals, but the Humboldt squid is not the kind of small, rare, or endangered species for which you might expect this to be the case. No, Humboldt squid grow up to 6 feet (2 m) long, swim through both northern and southern hemispheres of the eastern Pacific Ocean, and support the largest squid fishery in the world. People catch and eat nearly a billion tons of Humboldt squid every year.  Humboldt squid, in turn, catch and eat countless fish, crabs, and fellow squid. An animal like this has got to be making plenty of babies. So where were they?

I set out to study the early life stages of Humboldt squid, and because I actually wanted to finish my degree someday, I had to do more than hope that I could find babies in the open ocean where no one had before. When scientists want to study babies that aren’t readily collected in the wild, we try to make them ourselves. In 2006, on a research ship in the Gulf of California, a generous colleague taught me how to collect eggs and sperm from adult squid and conduct in vitro fertilization.

The process began after dark, when Humboldt squid rise from the depths to the surface and are more easily caught. Often, it was past midnight by the time I had both eggs and sperm isolated in glass dishes. Then I would stay up for hours more, carefully mixing, changing water, watching for signs of successful fertilization and attempting to match the eggs’ early development to reference drawings of other squid species (no one had ever published on Humboldt squid development before). The results were encouraging, but hardly thrilling—many eggs failed to develop properly, while others fell prey to fungal or bacterial infection.

During the day, I napped.

One afternoon, near the end of our two-week research cruise, my bunkmate burst into the room and woke me excitedly. “Guess what, Danna!”

I guessed, blearily and correctly: “You found an egg mass!”

Several researchers had been diving regularly throughout the trip. Since we were operating over deep water, far from shore, they had to be specially trained for “blue-water diving”—safely exploring the open ocean in the absence of rocks, kelp, sand, or any structure at all other than the boat itself. On these dives, they had found jellies of many kinds, most small enough to be scooped up in a collecting jar. On this day, however, they had encountered an enormous gelatinous mass.

They had taken video as well as samples. All of us scientists crowded around a TV in the tiny ship laboratory to stare at a blob so large and diffuse a diver could swim through it. It was transparent, studded with tiny embryos like stars. On the screen, a diver reached out to fill an open jar with embryos and their surrounding jelly. In the lab, I was ecstatic to receive one of these jars for my own. I had been struggling to produce babies artificially, and here were the first naturally produced babies ever to be found in the wild!

They began hatching that very day as perfectly formed little squid, smaller than rice grains (see insert, photo 2). Their eyes were huge relative to their body size, just like a human baby’s, and so was their funnel—the tube a squid uses to breathe and swim. These proportions triggered cuteness-recognition algorithms in my human brain, and I felt compelled to care for the squid babies assiduously.

The next day, we disembarked from the ship in Sonora, Mexico, the home of several of our science crew. Most of the babies stayed with them, while I puzzled over how to bring a small sample of hatchlings on the flight back to my lab in Monterey, California.

This was 2006, when you were still allowed to carry a water bottle onto an airplane without emptying it first. I filled my bottle with seawater and eight squid babies, then walked through security in the Guaymas airport, feeling the illicit thrill of smuggling with none of the actual danger. All the hatchlings survived the trip, and I spent the next week studying their swimming behavior and coaxing them unsuccessfully to eat.

We also extracted DNA from a frozen sample and confirmed that the egg mass indeed belonged to a Humboldt squid. (There had been little doubt, but since we hadn’t observed an adult laying the eggs, we couldn’t be positive without genetic identification.) Estimating the size of the mass from the video and the density of eggs in the mass from the jars, I calculated that the whole thing contained between half a million and two million eggs. Such a vast number sounds incredible, but other scientists had counted tens of millions of eggs in Humboldt squid ovaries. We reasoned that one mother could lay a dozen similarly sized masses in her lifetime.

Such profligacy! How was it that no one had ever spotted one before? Eventually, our in vitro work helped us understand why. Humboldt squid develop from fertilized eggs to swimming hatchlings in a week or less, so the egg masses are incredibly transient. Furthermore, as the divers found, these masses hover at a depth that’s invisible from the surface. Unless you happen to dive right next to one, you would never see it.

Yet without these nearly invisible, transitory masses, the largest squid fishery in the world would cease to exist, and a dominant predator of the eastern Pacific would be gone. Throughout the world, animal babies are hidden threads tying together all the planet’s ecosystems more tightly than we realize. They are now growing up in the most interconnected, rapidly changing environment that any generation has ever experienced. It’s time to pay attention to them.

Metamorphosis beyond metaphor

It’s tempting to think of development as a process with an end product. But what is the end product? Is it the baby, caterpillar, or larva that emerges from an egg or a womb? In many cases, there is a distinct transition from subsisting off yolk or placenta to independent movement and feeding. In other cases, there is not, and the organism that emerges carries yolk with it for days, continuing to live off that parental investment, and not moving actively any more than it did in the egg. Where then to draw the line between process and product?

Perhaps the adult is the end product. For metamorphosing species, this is also a clear demarcation. The newly emerged moth is no caterpillar, and it will live as a moth for the remainder of its life. But what about the rest of us? When does a human become an adult? Is it the onset of menstruation, the change in voice, the first credit card? We use different rubrics for different purposes, and we seem to think there is some nebulous point at which adulthood has “arrived.” However, development never stops; we are a work in progress for all of our lives. Our brains and bodies keep changing. Is menopause a developmental process? How about balding?

One butterfly scientist I spoke with emphasized how much we love the metaphor of metamorphosis because we all want to be able to change yet remain ourselves. I reflected that it doesn’t have to be merely a metaphor. We can change; in fact, we can’t not change. When I gave birth to my first child, the experience felt like a rebirth of my own self. I had been an adult human before. Now I was an adult parent. I had changed, physically and mentally and irrevocably, both over the months of pregnancy and at the time of delivery. It felt like an almost uncanny parallel to pupation (what happens inside a cocoon or chrysalis) and eclosion (the moment of emergence).

Metamorphosis, a phenomenon we’ll explore in more depth in chapter 9, is a feature that allows a single organism to build itself multiple bodies, each adapted to the demands of a specific environment. Aquatic insects like mayflies lose their childhood gills and grow wings to carry them on mating flights. Frogs resorb the tails of their watery youth and sprout legs to hop and climb.

It may seem precarious for a species’ survival to depend on two separate habitats. There’s also a distinct advantage: Babies that live in different places and eat different food from their parents experience no competition with the older generation. Adults can consume as much as they need without taking resources away from their children. This is especially useful when certain ravenous children grow to the same size as adults—or even larger. Adult Goliath beetles are some of Earth’s biggest insects, and they are vegetarians, feeding on fruit and tree sap. Their larvae, however, grow up to twice the weight of an adult on an as-yet-unknown diet that likely includes significant protein, suggesting a predatory habit. (In captivity, they readily consume cat kibble.) Paradox frogs, also known as shrinking frogs, engage in the opposite dietary shift. They grow to an enormous size as algae-eating tadpoles, only to metamorphose into much smaller adults that prey on insects.

All animals move between environments as they mature, whether it’s a matter of scale (from the miniature environment of a tiny fish larva to the oceanic environment of a full-grown tuna) or location (from the saltwater habitat of young eels to the freshwater homes of adults) or, very often, both. Some larvae can even be swept like dandelion seeds across continents and seas, connecting far-flung locales and sprouting new populations.

Humans fall into all of these categories, too. Some of us stay in our hometowns and scale up from a playground to an office building, while others emigrate thousands of miles. Wherever we are and however far we travel, we share every environment on the planet with an enormous diversity of animal life.

The young members of all species are active participants in our planet’s drama. They are consumers and producers, competitors and cooperators. The world’s most destructive crop pests are baby moths and beetles, while some of the most effective methods of pest control are baby wasps. Many endangered species are at risk, not because of threats to the adults but because their babies are running out of habitat or suffering from pollution. And speaking of pollution, larval invertebrates play a crucial role in the study of chemical toxicity, their sensitivity helping us make decisions for our own safety and that of our environment—which are, after all, inextricably linked.

Babies as links across space and time

In the pages to come, we’ll explore animal development from egg to metamorphosis, witnessing the intimate interdependence between each form and its environment. We’ll see how important these early life stages are to the ecosystems they inhabit, and how vulnerable to perturbations like pollution and climate change. The field known as ecological developmental biology has expanded rapidly in recent years, with incredible findings since 2020 alone.

Astonishing research has illuminated how babies connect different parts of the world, from fish eggs tough enough to survive traveling between lakes in duck intestines to sea star larvae that clone themselves as they surf ocean currents in a reverse of Vasco de Gama’s famous voyage. Bold and sturdy adventurers, babies also face huge risks, like the warming water that makes baby sharks more visible to predators and the inbreeding that kills endangered bird chicks long before they can hatch. Perhaps most important of all, we are learning more about how microbes guide animal growth. The word microbe is a shortening of microbios, or “tiny life.” The “micro” part implies that microbes are living things that can only be seen with a microscope—but does that make any microscopic embryo a microbe? Nobody would argue for that, since the embryo’s small size is just a phase. And yet, bread mold that can grow large enough for us to see with the naked eye is considered a microbe. Go figure. When I talk about microbes in this book, I’ll be referring mostly to single-celled bacteria, fungi, and viruses. Insect larvae cooperate with microbes like these in their gut to break down plastics, and a mother’s vaginal microbes can stabilize the health of human babies born by C-section.

In addition to creating links across space, animal babies also illuminate connections over time. Development offers a window into the history of life on Earth. The gills of insect larvae evolved into the wings of adults. Neoteny, or retaining childlike features, is a part of how we humans evolved from other apes. Life began with single-celled organisms, and embryos daily demonstrate the wonder of building one cell into multicellular life.

In fact, when you consider that the single-celled ancestors of animals simply replicated themselves, it’s incredibly weird that we make babies at all. Plenty of multicellular animals can still pull off asexual replication: Anemones bud clone after clone to fill tide pools. Flatworms break into pieces, each of which becomes a whole worm. So why don’t we all reproduce that way? As it turns out, starting the next generation with a single, fragile cell provides surprising advantages, from clearing out disease to promoting cooperation within the new body.

Most people don’t talk about miracles much, but one place the word seems to crop up again and again is in reference to a new baby. “It’s a miracle,” people say, in awe at the arrival of a new human, or if they’re kindly inclined toward the rest of our kingdom, a new calf or chick or puggle. Researchers who study development experience no less awe—perhaps even more, as they uncover layers of marvelous detail. Many of the scientists I spoke with for this book are parents; all were once babies themselves. Again and again I heard them express wonder at the developmental process. Is this what drives so many biologists to poetry, or are the poetically minded drawn to the science of development? In the chapters to come, we’ll encounter verses by eminent researchers as well as words by full-time poets that speak to the topics at hand.

We say “it takes a village to raise a child” and we consider ourselves responsible, as a society, for the care and safety of all children. We build public schools and playgrounds and institute child protection systems. What if we broadened this view to include all the world’s young? Human or hyena, squid or scorpion—we were all young once. This book will show you why that matters.

Credit line: From Nursery Earth: The Wondrous Lives of Baby Animals and the Extraordinary Ways They Shape Our World ⒸDanna Staaf, 2023. Reprinted by permission of the publisher, The Experiment. Available everywhere books are sold. theexperimentpublishing.com