with a long presence in Western thought.
In Aristotle’s History of Animals, written during the fourth century BCE, the tree of life is not yet a tree. It’s more like a ladder of nature or—as later Latinized from his Greek—a scala naturae. According to Aristotle, the diversity of the natural world “proceeds” from lifeless things such as earth and fire to living creatures such as animals “little by little,” in a progression so incremental that it’s impossible to draw absolute lines between one form and another. This idea remained useful throughout the Middle Ages and beyond, turning up in woodcuts during the sixteenth century as a Great Chain of Being or a Ladder of Ascent and Descent of the Intellect, which typically rose step-by-step from inanimate substances such as stone or water, to plants and then beasts, then humans, then angels, and finally to God. By that point it was a “Stairway to Heaven,” almost five centuries before Led Zeppelin.
The Swiss naturalist Charles Bonnet reverted to this linear, stair-step model as late as 1745, even while other Enlightenment thinkers and artists were allowing images of nature’s diversity to burgeon sideways with limbs and branches. Bonnet’s treatise on insects, published that year, included a foldout diagram of his “Idea of a Scale of Natural Beings,” arranged in vertical ascent from fire, air, and water, through earth and various minerals, upward to mushrooms, lichens, plants, and then sea anemones, followed by tapeworms and snails and slugs, upward further to fish and then flying fish in particular, and then birds, above which came bats and flying squirrels, then four-legged mammals, monkeys, apes, and lastly man. See the logic? Flying fish are superior to other fish because they fly; bats and squirrels exist on a higher level than birds because bats and squirrels are mammals; orangutans and humans are the best of mammals, and humans are more best than anybody. Bonnet made his living as a lawyer but much preferred studying insects and plants. He was a lifelong citizen of the Republic of Geneva, his French ancestors having been chased out of France by religious persecution, and so maybe it’s no accident that his ladder diagram culminated in people, not God.
The other notable absence from Bonnet’s scale of natural beings, besides God, are microbes. He paid no attention to microorganisms, although the pioneering Dutch microscopist Antoni van Leeuwenhoek had discovered the existence of bacteria, protozoans, and other tiny “animalcules” about seventy years earlier. We all know Leeuwenhoek’s name from our reading in high school of Paul de Kruif’s Microbe Hunters (a terrible book full of concocted dialogue and bogus detail, but an influential doorway to the subject) or other storybook histories of science, though we might not remember that Leeuwenhoek was a draper in Delft who started making his own magnifying lenses in order to better inspect the thread-count of textiles. Then he turned the lenses onto other materials, out of sheer curiosity, and made astonishing discoveries: he found menageries of tiny creatures living in lake water, in rain water, in water from drain pipes, even in scrapings of crud from his own teeth.
Leeuwenhoek’s revelatory observations of microbial life were reported in the journal of the Royal Society of London and became famous in scientific circles throughout Europe, but Charles Bonnet wasn’t interested enough in those “very wee animals” to fit them into his rising scale—not even where they might dismissively have been slotted, somewhere between asbestos and truffles. That omission presages a lasting discomfort with placing microbes on the ladder of life or, harder still, arranging their diverse forms on the tree—and it’s a discomfort to which I’ll return, because it became acute in 1977.
The linear approach to depicting life’s diversity was on the way out, notwithstanding Charles Bonnet’s scale of nature, and being replaced by its more complicated and dimensional successor, the tree. By the late eighteenth century and the start of the nineteenth, natural philosophers (we’d call them scientists, but that word didn’t yet exist) tried to classify and arrange living creatures into distinct groups and subgroups, reflecting their similarities and differences and some sort of organizing schema. The linear alignment, in order of what passed for increasing sublimity, the ladder raised toward God, was no longer satisfactory. There had been a knowledge explosion in Europe since the great age of sailing explorations began—knowledge of diverse animals, plants, and other creatures from all over the world—and scholars wanted to set that explosive abundance of new facts within hierarchical categories so that it could be easily accessed and used.
This wasn’t evolutionary thinking; it was just data management. The knowledge would fill volumes (one man alone, the German naturalist Alexander von Humboldt, published a thirty-volume account of his travels in South America), making all the more necessary an overview, an organizing principle, that could be apprehended at a glance: an illustration. But the illustrators now needed two dimensions, not one, and so the ladder turned into a trunk, and the trunk sprouted limbs, and the limbs diverged into branches. This offered more scope, sideways as well as up and down, for arranging the varied abundance of known creatures.
The tree of life was an old symbol by then, an old phrase, dating back at least to those mentions in Genesis and Revelation. The tree had also served as a model for family histories—the genealogical tree or pedigree of a German duke, for instance. Now the secularized tree became useful for organizing biology. Among the first to embrace this convention was another Frenchman, Augustin Augier, who wrote in 1801 that “a figure like a genealogical tree appears to be the most proper to grasp the order and gradation” of what concerned Augier: the diversity of plants.
Augier was an obscure citizen of the French Republic, living in Lyon, working on botany part-time; his real profession was unknown, his biographical details lost, even to a historian of Lyonnais botanists writing a hundred years later. Augier disappeared. But he left behind a book, a little octavo volume, in which he proposed a new classification of plants, “according to the order that Nature appears to have followed.” That is, a “natural order,” as opposed to an artificial classification system based merely on human whim or convenience. And in the book was a figure representing that system: his arbre botanique (botanical tree). Its trunk and limbs look almost as orderly and stiff as a menorah, but its sideways branching and copious leafing suggest a rife multiplicity of plant forms.
Again, this didn’t imply any heretical ideas about origins. Augier was no evolutionist before his time. His natural order wasn’t meant to suggest that all plants had descended from common ancestors by some sort of material process of transformation. God was their maker, shaping the varied forms individually: “It appears, and one can hardly doubt it, that the Creator, when making flowers, followed certain proportions and progressions in the number of their different parts.” Augier’s contribution, as he saw it, was discovering those proportions and progressions—design principles that had satisfied the Deity’s neat sense of pattern—and using them after the fact to organize botanical knowledge into a tidy system.
Augier wasn’t the first naturalist to hanker for a natural order of nature’s diversity. Aristotle had classified animals as “bloodless” and “blooded.” In the first century of our era a Greek physician named Dioscorides, attached to the Roman army, gathered lore on more than five hundred kinds of plants, arranging them in a compendium mainly on the basis of their medicinal, edible, and perfumatory uses. That book, in various reprints and translations, served as a trusted botany text for the next fifteen hundred years. Toward the end of its run, around the time of the Renaissance, as people traveled more widely and paid closer attention to the empirical details of nature, old Dioscorides gave way to newer illustrated herbals. These were essentially field guides to botany, graced with better illustrations based on improvements in drawing and woodcut techniques, but still organized for convenience of use, not natural order.
