“Nothing from nothing ever yet was born.”
— Lucretius, On the Nature of Things
IT IS SPRING IN HOUSTON, which means that each day the temperature rises and so does the humidity. The bricks of my house sweat. In my yard the damp air condenses on the leaves of the crepe myrtle tree; a shower falls from the branches with the slightest breeze. The dampness has darkened the flower bed, and from the black mulch has emerged what looks like a pile of snotty scrambled eggs in a shade of shocking, bilious yellow. As if someone sneezed on their way to the front door, but what came out was mustard and marshmallow.
I recognize this curious specimen as the aethalial state of Fuligo septica, more commonly known as “dog vomit slime mold.” Despite its name, it’s not actually a mold—not any type of fungus at all—but rather a myxomycete (pronounced MIX-oh-my-seat), a small, understudied class of creatures that occasionally appear in yards and gardens as strange, Technicolor blobs. Like fungi, myxomycetes begin their lives as spores, but when a myxomycete spore germinates and cracks open, a microscopic amoeba slithers out. The amoeba bends and extends one edge of its cell to pull itself along, occasionally consuming bacteria and yeast and algae, occasionally dividing to clone and multiply itself. If saturated with water, the amoeba can grow a kind of tail that whips around to propel itself; on dry land the tail retracts and disappears. When the amoeba encounters another amoeba with whom it is genetically compatible, the two fuse, joining chromosomes and nuclei, and the newly fused nucleus begins dividing and redividing as the creature oozes along the forest floor, or on the underside of decaying logs, or between damp leaves, hunting its microscopic prey, drawing each morsel inside its gooey plasmodium, growing ever larger, until at the end of its life, it transforms into an aethalia, a “fruiting body” that might be spongelike in some species, or like a hardened calcium deposit in others, or, as with Stemonitis axifera, grows into hundreds of delicate rust-colored stalks. As it transitions into this irreversible state, the normally unicellular myxomycete divides itself into countless spores, which it releases to be carried elsewhere by the wind, and if conditions are favorable, some of them will germinate and the cycle will begin again.
From a taxonomical perspective, the Fuligo septica currently “fruiting” in my front yard belongs to the Physaraceae family, among the order of Physarales, in class Myxogastria, a taxonomic group that contains fewer than a thousand individual species. These creatures exist on every continent and almost everywhere people have looked for them: from Antarctica, where Calomyxa metallica forms iridescent beads, to the Sonoran Desert, where Didymium eremophilum clings to the skeletons of decaying saguaro cacti; from high in the Spanish Pyrenees, where Collaria chionophila fruit in the receding edge of melting snowbanks, to the forests of Singapore, where the aethalia of Arcyria denudata gather on the bark of decaying wood, like tufts of fresh cotton candy.
Although many species are intensely colored—orange, coral pink, or red—others are white or clear. Some take on the color of what they eat: ingesting algae will cause a few slime molds to turn a nauseous green. Physarum polycephalum, which recently made its debut at the Paris Zoo, is a bright, egg yolk yellow, has 720 sexual configurations and a vaguely fruity smell, and appears to be motivated by, among other things, a passionate love of oatmeal.
Throughout their lives, myxomycetes only ever exist as a single cell, inside which the cytoplasm always flows—out to its extremities, back to the center. When it encounters something it likes, such as oatmeal, the cytoplasm pulsates more quickly. If it finds something it dislikes, like salt, quinine, bright light, cold, or caffeine, it pulsates more slowly and moves its cytoplasm away (though it can choose to overcome these preferences if it means survival). In one remarkable study published in Science, Japanese researchers created a model of the Tokyo metropolitan area using oat flakes to represent population centers, and found that Physarum polycephalum configured itself into a near replica of the famously intuitive Tokyo rail system. In another experiment, scientists blasted a specimen with cold air at regular intervals, and found that it learned to expect the blast, and would retract in anticipation. It can solve mazes in pursuit of a single oat flake, and later, can recall the path it took to reach it. More remarkable still, a slime mold can grow indefinitely in its plasmodial stage. As long as it has an adequate food supply and is comfortable in its environment, it doesn’t age and it doesn’t die.
Here in this little patch of mulch in my yard is a creature that begins life as a microscopic amoeba and ends it as a vibrant splotch that produces spores, and for all the time in between, it is a single cell that can grow as large as a bath mat, has no brain, no sense of sight or smell, but can solve mazes, learn patterns, keep time, and pass down the wisdom of generations.
How do you classify a creature such as this? In the ninth century, Chinese scholar Twang Ching-Shih referred to a pale yellow substance that grows in damp, shady conditions as kwei hi, literally “demon droppings.” In European folklore, slime mold is depicted as the work of witches, trolls, and demons—a curse sent from a neighbor to spoil the butter and milk. In Carl Linnaeus’s Species Plantarum—a book that aspires to list every species of plant known at the time (nearly seven thousand by the 1753 edition)— he names only seven species of slime molds. Among those seven we recognize Fuligo in the species he calls Mucor septicus (“rotting mucus”), which he classifies, incorrectly, as a type of fungus.
At the time, life hadn’t been studied in detail at the microscopic level, and Linnaeus’s taxonomic classifications, few of which have withstood the scrutiny of modern science, were based almost entirely on observable phenotype—essentially, how they looked to the naked eye. He placed Mucor septicus in the same genus as Mucor mucedo, because, well, they both looked like mucus. The fruiting bodies of both of these species looked like a type of fungus, and fungus looked like a type of plant.
We now call Linnaeus the “father” of taxonomy. Though he wasn’t the first to try to impose order on nature—naturalists, philosophers, and artists had constructed their own schema as far back as Aristotle—he was the first to classify our own species within his system, naming us Homo sapiens and placing us, scandalously, within the animal kingdom. That idea, that humans were “natural” beings, “Anthropomorpha” in the same order as chimpanzees and gorillas and sloths, drew the ire of Linnaeus’s fellow naturalists, whose intellectual lineage could be traced back at least to Aristotle, who had ordered the physical world along a continuum from inanimate objects through plants and then to animals. These “ladders” or “scales of ascent,” in turn, inspired the “Great Chain of Being”—the Christian worldview, central to European thought from the end of the Roman Empire through the Middle Ages, that ordered all of creation from lowest to highest, beginning with the inanimate world, through plants and animals, placing humans just below angels, and angels just below God. If anything like slime mold appeared there, it would no doubt be near the very bottom, just above dirt.
Over time, Linnaeus revised his classifications of Homo sapiens, naming “varieties” that at first corresponded to what he saw as the four geographic corners of the planet, but which became hierarchical, assigned different intellectual and moral value based on phenotypes and physical attributes. The idea that humans could and should be ordered—that some were superior to others, that this superiority had a physical as well as social component—was deeply embedded in many previous schema. But Linnaeus’s taxonomy, unlike the systems that came before, gave these prejudices the appearance of objectivity, of being backed by scientific proof. When Darwin’s On the Origin of Species was published in 1859, it was on the foundation of this “science,” which had taught white Europeans to reject the idea of evolution unless it crowned them in glory.
But the history of taxonomic classification has always been about establishing hierarchy, beginning with Linnaeus, who offered the world his binomial naming system as well as its first three taxonomic kingdoms: plants (Regnum Vegetabile), animals (Regnum Animale), and minerals (Regnum Lapideum, which Linnaeus himself later abandoned). Ernst Haeckel—biologist, artist, philosopher, and fervent disciple of Darwin—expanded Linnaeus’s model in 1866. To the plant and animal kingdoms, Haeckel added a third: Protista, for the various microscopic organisms known but not understood at that time. These included sponges and radiolaria and myxomycetes, the term Heinrich Friedrich Link had proposed for slime molds in 1833. Developments in microscope technology in the nineteenth century had given Haeckel and his fellow biologists a glimpse into the world of organisms too small to see with the naked eye, and with it, a keen interest in accounting for the evolutionary relationships of all species on Earth in ever more minute detail. Haeckel called this new science “phylogeny,” and he filled pages and pages of his works with intricately illustrated phylogenetic trees—beautiful in their execution but diabolical in their implications. In perhaps his best-known illustration, “The Pedigree of Man,” he places “man” at the highest point of a great oak, while apes, ungulates, “skull-less animals,” worms, and amoeba are lower down because he saw them as less evolved and therefore closer to the root of creation. Elsewhere he similarly categorized humanity into as many as twelve different species with different evolutionary histories— white Europeans, in his view, being the most evolved, important, and civilized.
Taxonomy has evolved in the centuries since Haeckel and Linnaeus, but much of their thinking still remains. Even if science no longer views humans as divided into different and unequal species, we continue to refer to “race” as if it were a natural, biological category rather than a social one created in service of white supremacy. The myth that humans are superior to all other species—that we are complex and intelligent in a way that matters, while the intelligence and complexity of other species does not—also exists in service to white supremacy, conferring on far too many people an imagined right of total dominion over one another and the natural world.
In high school I learned that humans reigned over five kingdoms: animals, plants, fungi, protists, and bacteria. We came only from ourselves; we owed one another nothing. I learned this in my parents’ church too, that the world was made for men, that every life (my own included) was under their dominion. I did not learn until college about a taxonomic category that superseded kingdom, proposed in the 1970s by biologists Carl Woese and George Fox and based on genetic sequencing, that divided life into three domains: Bacteria, Eukarya, and Archaea, a recently discovered single-celled organism that has survived in geysers and swamps and hydrothermal vents at the bottom of the ocean for billions of years.
Perhaps a limit of our so-called intelligence is that we cannot fathom ourselves in the context of time at this scale, and that so many of us fail, so consistently, to marvel at any lives but our own. I remember a recent visit to the Morian Hall of Paleontology at the Houston Museum of Natural Science. I moved with the exhibit through geologic time, beginning with trilobite fossils from more than 500 million years ago, toward creatures that become larger and more terrifying before each of five extinction events, in all of which climate change has been a factor. Each time, millions of species have disappeared from the planet, but thanks to small, simple organisms, life has somehow carried on.
Slime mold might not have evolved much in the past two billion years, but it has learned a few things.
The hall’s high ceilings and gentle lighting make it feel more like a contemporary art exhibit than a scientific display, and though scientists might object to this approach, for a layperson like me, it fostered wonder, and wonder has often been my antidote to despair. At the very end of the winding geologic maze, I encountered mammals and megafauna before arriving in the smallest exhibit in the entire hall, where a wall case contained the fossilized skulls of the various human lineages, mapping the web of their links and connections. So much damage has been done by the lie that this world belongs only to a few, that some lives matter more than others. The consequences of that lie have changed Earth more in a few decades than in the previous several million years. Outside, the next extinction looms.
But it is also possible to move through the exhibit in the opposite way, beginning with the urgency of the present and journeying back through time—to pass through doorways in this history that show us unexpected connections, to see the web of life spread out before us in all its astonishing diversity. Any system that claims to impose a hierarchy of value on this web is, like petri dishes and toasters and even the very idea of nature, a human invention. Superiority is not an inherent reality of the natural world.
Humans have been lumbering around the planet for only a half million years, the only species young and arrogant enough to name ourselves sapiens in genus Homo. We share a common ancestor with gorillas and whales and sea squirts, marine invertebrates that swim freely in their larval phase before attaching to rocks or shells and later eating their own brain. The kingdom Animalia, in which we reside, is an offshoot of the domain Eukarya, which includes every life-form on Earth with a nucleus—humans and sea squirts, fungi, plants, and slime molds that are ancient by comparison with us—and all these relations occupy the slenderest tendril of a vast and astonishing web that pulsates all around us and beyond our comprehension.
The most recent taxonomies—those based on genetic evidence that evolution is not a single lineage, but multiple lineages, not a branch that culminates in a species at its distant tip, but a network of convergences—have moved away from their histories as trees and chains and ladders. Instead, they now look more like sprawling, networked webs that trace the many points of relation back to ever more ancient origins, beyond our knowledge or capacity for knowing, in pursuit of the “universal ancestors,” life-forms that came before metabolism, before self-replication—the several-billion-year-old plasmodial blobs from which all life on Earth evolved. We haven’t found evidence for them yet, but we know what we’re looking for: they would be simple, small, and strange.
A few years ago, near a rural village in Myanmar, miners came across a piece of amber containing a fossilized Stemonitis slime mold dating from the mid-Cretaceous period. Scientists were thrilled by the discovery, because few slime mold fossils exist, and noted that the 100-million-year-old Stemonitis looks indistinguishable from the one oozing around forests today. Perhaps slime mold hasn’t evolved much in that time, they speculated. Recent genetic analyses have suggested that slime molds are perhaps as old as one or two billion years—which would make them hundreds of millions of years older than plants, and would mean they pulled themselves out of the ocean on their cellbows at a time when the only land species were giant mats of bacteria. One special ability of slime molds that supports this possibility is their capacity for cryptobiosis: the process of exchanging all the water in one’s body for sugars, allowing a creature to enter a kind of stasis for weeks, months, years, centuries, perhaps even for millennia. Slime molds can enter stasis at any stage in their life cycle—as an amoeba, as a plasmodium, as a spore— whenever their environment or the climate does not suit their preferences or needs. The only other species who have this ability are the so-called “living fossils” such as tardigrades and Notostraca (commonly known as water bears and tadpole shrimp, respectively). The ability to become dormant until conditions are more favorable for life might be one of the reasons slime mold has survived as long as it has, through dozens of geologic periods, countless ice ages, and the extinction events that have repeatedly wiped out nearly all life on Earth.
Slime mold might not have evolved much in the past two billion years, but it has learned a few things during that time. In laboratory environments, researchers have cut Physarum polycephalum into pieces and found that it can fuse back together within two minutes. Or, each piece can go off and live separate lives, learn new things, and return later to fuse together, and in the fusing, each individual can teach the other what it knows, and can learn from it in return.
Though, in truth, “individual” is not the right word to use here, because “individuality”—a concept so central to so many humans’ identities—doesn’t apply to the slime mold worldview. A single cell might look to us like a coherent whole, but that cell can divide itself into countless spores, creating countless possible cycles of amoeba to plasmodium to aethalia, which in turn will divide and repeat the cycle again. It can choose to “fruit” or not, to reproduce sexually or asexually or not at all, challenging every traditional concept of “species,” the most basic and fundamental unit of our flawed and imprecise understanding of the biological world. As a consequence, we have no way of knowing whether slime molds, as a broad class of beings, are stable or whether climate change threatens their survival, as it does our own. Without a way to count their population as a species, we can’t measure whether they are endangered or thriving. Should individuals that produce similar fruiting bodies be considered a species? What if two separate slime molds do not mate but share genetic material? The very idea of separateness seems antithetical to slime mold existence. It has so much to teach us.
In 1973, in a suburb of Dallas, a sudden, particularly spectacular appearance of Fuligo septica across lawns sparked a panic. Firemen blasted the plasmodia with water, breaking the creatures to pieces, but those pieces continued to slime around and grow larger. The townspeople speculated that an indestructible alien species had invaded Earth, perhaps recalling the plot of the 1958 movie The Blob starring a young Steve McQueen. Scientists arrived in the panicked neighborhood to take samples, reassuring the community that what they had experienced was just a stage in the life cycle of a poorly understood organism: “a common worldwide occurrence,” they said. “Texas scientists think backyard blob is dead,” read a headline in the New York Times.
The slime mold in my yard is also dead, I think. The aethalia is pale, hardened, and calcified, with the texture and color of a summer cast protecting a child’s broken arm, browned by a season without washing. A breath of wind arrives and black dust lifts from the slime mold’s surface, blown toward the edge of my yard, and the next one over. And the next.
Spring in Houston is the season for working in the garden. We replant our tall ornamental grasses, killed in the recent unseasonable freeze; ours are a hybrid Pennisetum species, from the family Poaceae, a large taxonomic group that also contains Zea (a genus that includes corn), Oryza (rice), Saccharum (sugar cane), and Triticum (wheat). Fungi live on and among these plants, bringing them water and nourishment through the threadlike mycelium to keep them alive and aiding their decomposition when they die. As the plants decompose, they provide the food that bacteria eat, and myxomycete amoeba prey on these bacteria when they hatch from their spores. We plunge our shovels and hands in the dirt, the living substrate—alive in ways I have only just begun to fathom. We plant the grasses, fill the holes, lay down fresh mulch. We collect our tools and retreat indoors to the comforts of our home—our refrigerated food, our instant oatmeal, our beloved air conditioning.
Days later, I am leaving my house to walk the dogs, the air hanging dankly all around me, and out of the corner of my eye I see dozens of bright coral pink beads scattered across the surface of the fresh mulch, a new species I learn is Lycogala epidendrum, “wolf’s milk slime mold.” I know very little about it, but receive this marvelous arrival in the only way I know how: we are made by, and for, one another. O
Curious for more? Get up close with Fuligo septica and other slime molds here.
Lacy M. Johnson is the author of several books, including The Reckonings and The Other Side, both National Book Critics Circle Award finalists. She teaches at Rice University and is the founding director of the Houston Flood Museum.