The Tangled Tree

by David Quammen

  Haeckel threw everything together in his great Stammbaum der Organismen, a tree of all organisms, portrayed in a complicated figure rising vertically but sliced horizontally with three baselines across the bottom parts of the image, reflecting three alternate hypotheses about the origins of all life. Take each baseline as a starting point, and you get a different hypothesis. The tree has nineteen secondary limbs, all rising vertically, all cut across by the topmost baseline: suggesting that life began independently nineteen times. The tree has three major limbs from which those nineteen diverge: suggesting three independent origins of life—one leading to plants, one to animals, one to creatures of all other kinds, such as Haeckel’s beloved radiolarians. (He called that third group Protista, and it represented an important departure from orthodox thinking, to which I’ll return in a moment.) At the base of the tree is its single big trunk: suggesting all life as one tree, derived from one origin, one primordial ancestor. He labeled the trunk Moneres, by which he seems to have meant the simplest of single-celled organisms, resembling bacteria. (For technical reasons, their name, meaning “single,” was later corrected to Monera.) This hypothesis, among his three, was the boldest application of Darwinian theory that could be made at the time. It asserted that all living creatures, including humans, have descended from some common ancestor resembling a bacterium. But in 1866 and for some years after, Haeckel himself was still undecided about which of his three hypotheses was correct.

  The other innovation presented in Haeckel’s big tree was that group he named Protista—his kingdom of nonplant, nonanimal organisms. From the time of Aristotle, through the work of Linnaeus and until Haeckel’s era, naturalists had viewed all life as divided into two kingdoms: plants and animals. That was simple. It conformed to common sense. If the large living forms we see all around us—the trees and the grasses, the birds and the fishes, the flowers and the elephants—fall neatly into two categories (as they seemed to do, before any microscopic investigation of fungi), then all life must be binary. A creature moved, or it didn’t. It ate, or it greened. Even microbial organisms—the bacteria and the amoebae, the radiolarians and the ciliates, the diatoms and all the others—were considered either animals or plants. That’s why Leeuwenhoek wrote of the animalcules he saw through his lenses, and why the misleading term protozoa (earliest animals) was coined in 1818. But Haeckel said no, wait: life isn’t binary. It’s trinary. And he placed those puzzling little creatures in a kingdom all their own. It was a visionary act of almost Copernican daring, which went largely unnoticed and unfussed-about at the time.

  Generelle Morphologie was a dense book, written stubbornly amid sorrow, and though it influenced scientists, it didn’t reach a broad audience. He rectified that two years later, producing a work for the general public under a title that best translates as Natural History of Creation. This one emphasized the human part of the big story, always a good tactic when you’re writing for a human readership, and it celebrated Darwin, Darwin’s theory, and some of Darwin’s precursors (Lamarck, Lyell, Darwin’s grandfather Erasmus, Wallace, and, because Haeckel was German, Goethe) in language somewhat more accessible and fluid than Darwin’s. It presented the biogenetic law, about ontogeny and phylogeny, with help from an illustration showing similarities among the embryos of a turtle, a chicken, a dog, and a human. It sold well, blazing through twelve German editions and being widely translated. In English, it appeared as The History of Creation (the word natural dropped evidently because it sounded too materialistic), and that version went through a number of printings too. By the early twentieth century, one historian called this book “the chief source of the world’s knowledge of Darwinism.” It also contained trees, mostly depicting relationships among the higher animals. There was a distinctly racist tree of human ethnicities, purporting to show which were primitive and which advanced. By strange coincidence, Germans were on the uppermost branch.

  Haeckel’s tree of organisms, 1866.

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  Busy and prolific, Haeckel continued his popularizing as well as his research, and in 1874 he published a book devoted entirely to human evolution—or, as put in his subtitle, The Developmental History of Man. His tree making came to a crescendo here, in an image known later as Haeckel’s “great oak.” This is probably his most widely recognized work of art. You may have seen it as a poster. You could get it on a T-shirt. It’s his version, in graphic form, of the lineal ancestry of humans, from Monera, through worms and amphibians and reptiles, straight up to us. This tree has no canopy and not much branching. It’s thick at the bottom and tapers skyward, looking less like a great oak than like an enormous rutabaga, slightly hairy, pulled from the ground and turned point up. At its very topmost branch sits the word Menschen, “people,” flanked below (not side by side) with gorillas and orangutans and chimps.

  Haeckel’s great oak, English version, 1879.

  Does this drawing reveal Ernst Haeckel as a throwback anthropocentrist in the guise of a Darwinian, as some scholars have argued? Was his great oak just another ladder of nature, of the sort Aristotle proposed and Charles Bonnet had been designing back in 1745? Does it show that Haeckel took humans as the crown of creation, the end point toward which a teleological evolutionary process was directed? Not necessarily. It might look that way at first glance, yes, but his thinking and his illustrating were more complicated.

  Haeckel is controversial among historians of biology, who disagree about his real or supposed views, confusions, and sins. He lived a long time (dying in 1919, at age eighty-five), published many works, embraced many variations on Darwinian theory and many “laws” supposedly under its rubric, changed his mind on some points, was vague or self-contradictory on others, and thereby provided plenty of fodder for such disagreements. One school of thought dismisses him as more Lamarckian than Darwinian, deluded by the notion that acquired characteristics can be inherited (although Darwin believed that too). His biogenetic law has been discredited. During his lifetime, in a nasty crisis, Haeckel stood accused of presenting bogus evidence—drawings, falsified either intentionally or by mistake—in support of that hypothesis. He has been criticized for making paleontological assertions without much fossil evidence. He has been called “superficial, inconsistent, and just plain muddleheaded.” And then there’s the charge that, with his progressionist view of evolution, placing mankind at the apex of a directional process toward perfection, Haeckel was “Darwinian in name alone.” The historian Peter J. Bowler made that charge among others in The Non-Darwinian Revolution, his 1988 study devoted to all the “pseudo-Darwinians” and “anti-Darwinians” who influenced ideas about evolution in the decades after The Origin first appeared. The cover illustration of Bowler’s book, pressing this point, was Haeckel’s great oak.

  The oak appeared inside Bowler’s book too, offered in evidence of what Bowler called “the essentially linear character of Haeckel’s evolutionism.” That was probably unfair. Other scholars, including the biographer Richards, have noted that Haeckel drew two kinds of trees, with two distinct purposes. The first kind were phylogenetic illustrations, such as his Stammbaum der Organismen, intended to show the breadth of all life as a full canopy of branches and twigs. The second kind were genealogical trees such as the great oak, intended primarily to illuminate one lineage. The oak was labeled Stammbaum der Menschen, after all: a tree of mankind, not a tree of everything. It showed humans at the apex because it was about humans. It was drawn to illustrate Haeckel’s bold (and, yes, Darwinian) assertion that we have descended from a line of other forms, traceable back to the simplest single-celled creatures: his Monera. That’s why its trunk is thick and tapering, with few limbs. It wasn’t meant to show life’s diversity and interrelatedness. It was meant to show lineage—human lineage. Haeckel may have been a Romantic, and a progressionist, and an inconsistent Darwinian, but even he knew that the tree of life is not a rutabaga.

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  Haeckel had started something that didn’t stop. Well into the middle of t
he twentieth century, paleontologists and biologists created phylogenetic trees, graphically executed by their own hands or with help from illustrators, portraying inferred evolutionary relationships and histories of descent. They based these trees mainly on similarities and differences among the shapes of living creatures (comparative morphology), on the fossil record, and on embryology. That was the evidence they had. Molecular phylogenetics didn’t yet exist.

  Some of these images were trees of all life and some were trees of particular groups. Henry Fairfield Osborn, a paleontologist at the American Museum of Natural History in New York, published a tree of proboscidean mammals, in 1936, festooned with mammoths and mastodons and elephants. The tree limbs were just simple arrows, but the pachyderms were nicely sketched. A British entomologist named William Edward China, a specialist in the taxonomy of the Hemiptera (true bugs, which live by sucking fluids from plants or other animals), produced a tree of hemipteran families in 1933. It resembled a geisha fan as much as a tree, but it showed a stinkbug’s degree of kinship with a bedbug, for those who wanted to know. Herbert F. Copeland, a biology teacher tucked away at Sacramento Junior College in the late 1930s, published a memorable paper on the kingdoms of life that, besides reprinting Haeckel’s tree of all organisms, included a graphic figure of Copeland’s own. Copeland’s was not exactly tree shaped, but it expressed the same basic idea: that lineages have risen through geological time and, as they diverged, expanded through ecological space. Instead of branches, the Copeland illustration shows smoothly tapered cones ascending vertically in a cluster, like organ pipes ready to belch out a fugue.

  Alfred S. Romer, a paleontologist at Harvard, was one of the great tree makers of the midcentury, and his reached a broad audience of young scientists-in-training through his influential textbooks, most notably Vertebrate Paleontology. That book was first published in 1933, revised in 1945, revised again, and still a standard source when I bought my copy in 1982. Its tree branches rise and thicken like plumes of black ink on the page, then in some cases grow thinner again, their breadth representing the comparative abundance and diversity of different groups throughout the vicissitudes of time. Romer shaped a tree of all vertebrates, trees of the vertebrate classes, and trees of some of their subdivisions: fishes, amphibians, reptiles, mammals, even-toed ungulates, whales, rodents. Glancing at his figures, you can see that dinosaurs crashed at the end of the Cretaceous period but, for some reason, crocodiles didn’t, and that rats thrived in the Pliocene Epoch. Romer’s trees speak of the shifting fortunes, the pulses and fades, that are so basic to evolution’s story.

  Then in 1969 came an unusual tree from Robert H. Whittaker, a plant ecologist at Cornell University for whom “broad classification,” as he called it—numbering and delineating the kingdoms of life—was a sidelight. Whittaker’s tree had little to recommend it by way of artistic flair but delivered some provocative content. It looked like an annotated balloon animal. Or maybe, to be more polite and botanical: a prickly pear cactus. It consisted of five roughly oval lobes: three, stacked upon one, stacked upon another. The lobes represented what Whittaker proposed as the five kingdoms of all life.

  Five kingdoms? That was a new count at the time, a radical proposal, and Whittaker had come to it in stages. At the start of his career, studying insects and plant communities, he gave no hint that he would ever be interested in, let alone drastically revise, how life-forms are classified at the highest level.

  Robert Whittaker grew up in eastern Kansas during the Dust Bowl and the Depression, collecting butterflies as a boy, wandering through meadows and woods, becoming a man of sternly traditional values, with a personality that some colleagues would later call “stoic” and “intense.” But he was capable of recognizing ambiguity where he saw it. He went to college in Topeka, did a hitch as a weatherman for the Army Air Forces during World War II, then went to grad school in a place that keeps reappearing in this story: Urbana, Illinois. He wanted to study ecology. Rejected by the Botany Department, he joined Zoology instead but became a plant ecologist anyway. He distinguished himself at Illinois by presenting a dissertation in zoology that had nothing to do with animals.

  During that PhD research, on the vegetation of the Great Smoky Mountains, Whittaker began to challenge a canonical principle in his branch of science: the idea that plant communities are stable, highly integrated associations, with consistent species compositions, clear boundaries, and hard reality as units, almost as though they were living organisms. This idea had been enshrined in ecological thinking through the influence of an ecologist named Frederic Clements. Whittaker dismantled it in his dissertation. He showed that plant communities are loose associations, not integrated units, with blurry boundaries and a “low degree of reality.” By way of this experience and his other work, he brought two forceful predispositions to his efforts, years later, on the tree of life: he saw things ecologically, and he appreciated that boundaries are often ambiguous.

  Somewhere along the way, Whittaker got interested in delineating not just plant communities but also the most fundamental categories of life. In 1957 he published his first effort, a short paper titled “The Kingdoms of the Living World.” How many, and what were they? The traditional view recognized just two kingdoms of life: plants and animals. Haeckel had said three: plants, animals, and his Protista, the everything-else category composed mainly of microbes. Several earlier naturalists, including Richard Owen and John Hogg in Britain, had also found a third kingdom of life, Owen calling his Protozoa and Hogg coining the name Protoctista (“first created beings”). But neither Owen nor Hogg was an evolutionist, and their influence on phylogenetics was far less than Haeckel’s. Herbert F. Copeland, who published that 1938 paper with the organ-pipe illustration, came up with four kingdoms. He took Haeckel as his inspiration but, drawing on better microscopy and newer thinking about microbes, separated Monera (bacteria) from Protista (simple creatures with cell nuclei). So for Copeland, using the formal names, it was Monera, Protista, Plantae, and Animalia. He argued his case more fully in a 1956 book, which is full of intricate stipple illustrations of microbial creatures but, oddly, omits any tree-of-life art whatsoever. What it does contain, as its reverential frontispiece, is a photo of Ernst Haeckel in his prime—with the beard, the wavy blond hair, the blazing eyes—suggesting how forcefully Haeckel haunted this field even into the middle twentieth century. Robert Whittaker seems to have been provoked to his own declarations on kingdoms in reaction to Copeland’s book.

  Whittaker’s approach was unique in that he tried to answer the big question with ecology, not with morphology. “Ecologists are familiar with divisions of the living world which correspond neither to Copeland’s nor to the two-kingdom conception,” he wrote. Ecologists see distinctions that microscopists miss. Among the boldest of those distinctions, Whittaker noted, are three categories of organism: producers, consumers, and decomposers. Animals are consumers, swallowing other creatures for their sustenance. Plants are producers, gaining sustenance from sunlight and water, creating their bodily substance from nonliving materials. Bacteria and fungi are decomposers, taking their sustenance by gently dismantling other creatures, dead or alive, and putting the pieces to new use. Each of those three categories, in Whittaker’s 1957 proposal, represents a kingdom. Another way of explaining it, he wrote, was that “kingdoms are, most essentially, major directions of evolution” and that those directions reflect three different means of gaining nutrition: eating, photosynthesis, and absorption.

  “The kingdoms are man’s classification,” he added, and their meaning derives only from the fact that we humans choose to recognize them, purely for our convenience in organizing biological knowledge. This is similar to what he had said about plant communities: that the communities, as distinct from individual plants or populations of this species or that, have a “low degree of reality” in the tangible world. Plant communities are how we think about plant diversity, which tends to get arranged so that plants with similar needs share
similar habitats. Kingdoms of life are how we think about life’s diversity, so that we don’t get nosebleeds and despair of the whole enterprise of biology. It’s worth noting too that Robert Whittaker, like Herbert Copeland, was a professor as well as a scientist. He understood that teaching biological information, along with organizing and retrieving it, required putting its bounteous variousness into handy categories.

  Two years later, after some further thought, Whittaker revised his schema, in a paper titled “On the Broad Classification of Organisms.” Evidently he felt he had simplified too far. Three kingdoms, however broad, didn’t cover the full breadth of nature’s diversity. There were four—but not precisely the same four as Copeland’s. For the Whittaker of 1959, it was Protista, Plantae, Animalia and . . . Fungi. Although fungi absorb their nutrients, he had become uncomfortable at lumping them with all those single-celled absorbers among the Protista. So the plants (producers), the animals (consumers), and the fungi (absorbers) were defined by ecology, he explained, and the protists (unicellular) were defined by morphology. “These themes are inconsistent,” he admitted, but it was the best he could do.

  This paper is also memorable for containing his first prickly-pear-of-life illustration. It showed four lobes: a basal lobe, labeled Protista, with three lobes for Plantae, Fungi and Animalia sticking up from it. The reason he had chosen such an unconventional form—a figure with lobes rather than limbs—wasn’t obvious at the time. But it would become more clear in a later iteration.

  Whittaker returned to the problem in 1969. His paper “New Concepts of Kingdoms of Organisms” appeared in Science that year and, somewhat surprisingly, given his previous waffling, influenced a generation’s worth of biology textbooks. The new concepts it offered were primarily Whittaker’s old concepts from 1957 and 1959—classification should be ecological, except for single-celled organisms—plus one important addition: another kingdom. Meanwhile, as I’ve described, Roger Stanier and C. B. van Niel had offered their own forceful idea on broad categories in 1962—dividing all life into prokaryotes and eukaryotes. Whittaker nodded to that dichotomy, used it for characterizing bacteria, then otherwise essentially ignored it. He split the bacteria away from Protista and gave them their own realm, going back to Haeckel’s label: Monera. His new illustration was a prickly pear cactus with five lobes: Monera at base, Protista rising from that, then Plantae and Fungi and Animalia. Five kingdoms, not four, comprising all life on Earth.