Uprooting the Tree of Life

Applause. I can’t quite keep the word out of my head as I walk around the campground path, looking up at the trunks of enclosing aspen. On each and every one, in their last stretch of summer green, there are innumerable leaves rustling and shaking in the August breeze. The dendrological clamor isn’t happening for me. I may as well be an insect threading a path between the white and sometimes-graffitied trunks. If anything, the standing ovation is for something much more primal: that we are all here, together. 

For most of my life, I misunderstood aspen. Not that I made any great attempt at exploring the depths of botany. Growing up in a coastal state where maples and dogwoods were everywhere, a tree—so far as my neophyte understanding went—was simply a plant with tough bark that grew tall into a crown of leaves that shaded in summer and either stayed evergreen or burst into color in autumn. A tree, as I understood it, was always a single and discrete entity, defined by the borders of its roots, its branches, its trunk. I had no idea that some trees—like the aspen that grow in the mountains and gullies of the high desert home I relocated to—are not single stalks growing into the air. Some trees, like aspen, are communities.

As I walk along the campground loop high in the La Sal Mountains, I can point to any particular trunk in front of me and say, “That’s an aspen.” Populus tremuloides is hard to miss. Some of the trees grow dozens of feet into the air, each branch dotted with gently serrated oval leaves jutting from the pale bark. But by singling out any particular stalk, I’m missing the heart of the tree. Aspen is clonal. Each stand isn’t a grove, but a single tree: a network of towering, gently waving poles growing from an unseen network of roots below. Some are incomprehensibly enormous. The largest organism in the entire world might be an aspen. Nicknamed “Pando,” the giant grows in Utah’s Fishlake National Forest. It covers over 100 acres and is estimated to weigh nearly 13 million pounds—a tree that sprung up as the great Ice Age glaciers receded 10,000 years ago, a living hillside that witnessed the extinction of the mastodons and watches as the twenty-first century grinds on.

A simple fact can flip the world. Wearing down the soles of my boots on the asphalt path, I’m no longer walking among the trees. I’m walking within them, following a path that runs right through the center of a giant. It’s hard not to think of old fantasy motifs: adventurers wandering lost worlds as they trespass through the exposed rib cages of fallen dragons and monsters that left their bones to the land during some ancient time. If I let my imagination run, the susurration of lilting leaves almost feels like breath, as if I’m looking at the blue and wispy-clouded sky through the branching vessels of an enormous organism’s lungs.

I’m not alone in my wanderings. I’m just one expression of life among many. Jays, chickadees, and sapsuckers flit from branch to branch and tree to tree, often in search of the small insects that shelter on the aspen’s leaves and bark. Some of the stalks in this grove undoubtedly house aspen rot, a species of fungus, deep inside where I cannot see. Signs scrawled with erasable marker near the campground let me know that black bears have recently been wandering through this grove, too, and now and then, I can hear the lowing of range cattle left to graze in the comparative cool above the searing stone of the desert below. Each of these living things leaves their mark on this scene, noticed or not, all part of changes so constant that they are essentially imperceptible. And if we follow that thought a little further, from any trunk or leaf through the tree’s connections to everything around it, we will eventually find ourselves at the root of something much grander: the Tree of Life.

I’ve always found the phrase “Tree of Life” to be comforting, reminiscent of the cool of East Coast evenings in the summer as the fireflies are just beginning to shine. The term conjures up visions of tall trees resplendent with verdant leaves, thick around the trunk and towering overhead. But there’s more to it than that, down at the roots.

We all have our own family tree, a smattering of branches that are themselves part of the greater Tree of Life that includes all living things. But I’ve long felt cut off from my closest branches, the genealogical sort that fill family albums. My personal flowering, becoming a woman in the flesh during the mid-season of my life, felt like its own independent growth—as if my seedling self had been identified as one plant only to later grow into another. Flourishing required moving away from my genetic relations to form my own family, but I still felt a longing to understand how I relate to the rest of the world, where I fit in all this.

That thirst has only led me deeper into the past, to organisms whose names are jotted down in technical journals and who occupy distant times. When I think of family, I think of the tiny primates that scrambled through the trees during the last days of Tyrannosaurus, the four-limbed fish that dragged themselves around mudbanks when living above the surface of the water was a new concept, the squiggly little animals with the precursors of a backbone that somehow managed to survive a sea of predators with snatching mouths and compound eyes. If I were to plaster my home with portraits of my family, the gallery would look like scenes from a natural history museum: a Tree of Life to ground me. 

Naturally, there is no single Tree of Life that will give us the answers we seek about the shape of biodiversity on our planet. The Tree of Life is something we make—and have spent centuries remaking. Prior to the recognition of evolution, Trees of Life grew from the fertile grounds of religious imagery: from the world trees of the Maya and Olmec to the Tree of Life vision that self-styled prophet Joseph Smith wrote about in the Book of Mormon, part of the story that drove his followers to colonize the western groves of Utah I’ve come to know. Spiritually minded naturalists picked up the imagery themselves, life seeming best arranged in a vast tree with “lower” forms of life near the base and the “higher” forms—with our own species being the standard for all comparisons—resting on the upper boughs. Then, of course, came Charles Darwin. The single illustration in 1859’s On the Origin of Species is a crude Tree of Life that represents how life might branch and spread “from so simple a beginning.”

While the poetry of the name has remained, the Tree of Life has only become more formalized—and complicated—with time. When you search for the most scientifically accurate, up-to-date Tree of Life, you’ll find a many-spoked diagram that looks more like a kite that’s been shredded by a summer thunderstorm than anything else. Time is taken out of the visualization altogether, instead replaced with a series of evolutionary sticks that are organized by relatedness, arranged and rearranged as if by a florist with too much time on their hands. It’s more of an evolutionary shrub—spreading out low to the ground, feeling prickly to even look at. But there’s a difference between a technique that’s functional and one that actually allows us to find the grand, clear vista we so desperately seek. In a sense, we are missing the forest for the Tree.

No organism is an island. We often speak of species as discrete biological units, with a defined beginning and end around which we can organize the Tree of Life. Yet biologists fight like hawks over how exactly we define a “species.” “A species represents a population of organisms that interbreed and produce fertile offspring” works fine for elementary school biology, but many species are known to hybridize with what experts might define as a different species based on outward appearances. Or many domesticated animals—such as dogs or, Darwin’s favorite, fancy pigeons—can have dramatic variations in form while still being considered members of the same species. Science often looks for a common denominator or a reductive rule by which the natural world can be understood while forgetting that nature is a wonderfully, unabashedly, fascinatingly messy place. If the very idea of species is messy, then so, too, is our Tree.

Even if we are to define a species in terms of reproductive isolation, we’re still missing something critical. How can we even begin to comprehend life on Earth without considering the places where the lives of particular species, however defined, interact with each other? Return with me to the aspen grove, starting to turn a warm gold as the Earth’s spin brings the sun low. The forest is home to innumerable insects, including a moth so notorious that it’s known by the name of its larva: the western tent caterpillar. In spring, the fuzzy and very hungry larvae munch on the ovoid aspen leaves, part of a relationship that has existed for an untold length of time. These caterpillars can’t break down all the nutrition locked in the cell walls of the aspen leaves on their own. Instead, they rely on symbiotic bacteria that live within them and create an enzyme—cellulase—that allows the caterpillars to feed voraciously as they prepare for their metamorphosis. 

A biologist could draw stark lines between the aspen, the caterpillar, and the microbiome within the insect, each with its own name and place on the Tree of Life. That’s certainly a very clean and neat way to organize nature. But what would a tent caterpillar be without the microorganisms that let it feed—or the aspen without the creatures that live within it and feed upon it? And then there are microbial organisms themselves—which are everywhere but so small that we are blind to their world—that can share genes between themselves, circumventing what we might think of as well-defined lines of descent and instead creating a great, big genetic mess. At this point, the imagery of a single Tree of Life starts to tremble.

We could easily draw a hard line and say that the interactions between organisms or species is the realm of ecology, expressions of life rather than part of its definition. The Tree of Life is dictated by its own branch of science, phylogenetics, and the experts of that discipline make the rules. But viewing each species only in terms of its size, color, genetic signature, and range is like trying to cram biology into a baseball card format from which we get the basic stats but no real depth of understanding. Evolution, after all, is reliant on the constant and often-imperceptible interactions between any given organism and the world around it, not just the abiotic details like temperature and annual precipitation but also the small moments when life connects to life.

There is accuracy, and even poetry, in the idea of a single Tree of Life. Even as some of the branches shift or shake with new discoveries, all this cogitation has provided us with a bare outline—a dendrological skeleton—from which we can perceive broader lessons. In striving to assemble such an encompassing tree, we have found that all life on Earth is descended from a common ancestor. These organisms were not necessarily the first life on our planet (we are entirely in the dark about how many potential life-forms or evolutionary spinoffs there were early in Earth’s history), but they are the primordial root back to which we can follow any life. I can’t help but be in awe of that fact, an idea so large that I feel like it’ll jump out of my hands if I try to hold it too tightly. I came from somewhere, and perhaps that’s why I find myself drawn further and further into the tangle of these questions. Even if I were entirely adrift and on my own, I’d still have family: a menagerie of organisms relevant to my past, my story. I understand who I am through these connections, the unbroken tissues of the Tree of Life that can bring me in touch with times and places I feel like I know but can never visit.

Knowing that the Tree of Life grows according to our ideas and theories allows us to question what shape life really takes. If I am to think of myself as a single leaf, perhaps beginning to show some signs of orange with the coming of my own autumn, then the Tree of Life I envision is more like an immense aspen. You can start with any leaf, any branch, and follow that growth all the way down to the root, belowground, where each trunk is connected to the others. No particular trunk is better than the others, nor is any expression of the tree disconnected from the whole. An aspen grove is a tangle of living things, beautiful because it’s so difficult to comprehend in its entirety. The life that relies on and lives within these groves is a reminder that species are not just self-contained entities but entry points into constant interactions—from the cellular level up to what you can see with the naked eye—that are no less important in knowing an organism’s identity and role in the broad panorama of life. To me, the Tree of Life is an aspen, a buzzing, whispering, swaying riot of life so encompassing that sometimes we forget we’re part of it.