The Tangled Tree

A Radical New History of Life

David Quammen

• Publisher: William Collins
• Year: 2018
• ISBN:
  9780008310691
• Topic: Biological Sciences
• Subtopic: Evolution
• Index: Biological Sciences

PART

I

Darwin’s Little Sketch

1

Beginning in July 1837, Charles Darwin kept a small notebook, which he labeled “B,” devoted to the wildest idea he ever had. It wasn’t just a private thing but a secret thing, a record of his most outrageous thoughts. The notebook was bound in brown leather, with a tab and a clasp; 280 pages of cream-colored paper, compact enough to fit in his jacket pocket. Portable, but no toss-away pad. Its quality of materials and construction reflected the fact that Darwin was an affluent young man, living in London as a naturalist of independent means. He had arrived back in England just nine months earlier from the voyage of HMS Beagle.

That journey, consuming almost five years of Darwin’s life, on sea and land, mostly along the South American coastline and inland to the plains and mountains, though with notable other stops on the roundabout way home, would be the only major travel experience of his sheltered, privileged life. But it was enough. A mind-awakening and transformative opportunity, it had given him some large ideas that he wanted to pursue. It had opened his eyes to an astonishing phenomenon that demanded explanation. In a letter to his biology professor and friend John Stevens Henslow, back at Cambridge University, written from Sydney, Australia, Darwin mentioned his puzzling observations of the mockingbirds (not the finches) of the Galápagos Archipelago, a set of volcanic nubs in mid-Pacific. These gray, long-beaked birds differed from island to island but so subtly that they seemed to have diverged from one stock. Diverged? Three kinds of mockingbird? Varying slightly, this island to that? Yes: they appeared distinct but similar, in a way that suggested relatedness. If that impression were true, Darwin confided to Henslow, confessing an intellectual heresy, “such facts would undermine the stability of species.”

The stability of species represented the bedrock of natural history. It was taken for granted, and important, not just among clergy and pious lay people but scientists too. That all the varied forms of creatures on Earth had been fashioned by God, in special acts of creation, and are therefore immutable, was an article of faith to the Anglican scientific establishment of Darwin’s era. This tenet is known as the special-creation hypothesis, though at the time, it seemed less hypothesis than dogma. It had been embraced and supported by prominent naturalists and philosophers of the scientific culture within which Darwin had been educated at Cambridge. He was now home from his wildcat voyage, a youthful adventure with a bunch of rough English sailors, about which his stern father had been skeptical at the start. The experience had altered him—though not in the ways his father may have feared. He hadn’t become a drunk or a libertine. He didn’t curse like a bosun. Darwin’s wanderlust, satisfied physically, was now intellectual. He intended to investigate, very discreetly, a radical alternative to scientific orthodoxy: that the forms of living creatures weren’t eternally stable, as God had created them, but instead had changed over time, one into another—by some mechanism that Darwin didn’t yet understand.

It was a risky proposition. But he was twenty-seven years old and deeply changed by what he had seen and, in a quiet way, very gutsy.

So he had set himself up in the big city, with lodgings on Great Marlborough Street, a convenient location for his visits to the British Museum. This was just a few doors down from the house where his elder brother, Erasmus, had already settled. Darwin joined scientific clubs, the Geological Society, the Zoological Society, but had no job. Didn’t need one. The same formidable father who had first disapproved of the Beagle voyage—Dr. Robert Darwin, a wealthy physician up in the town of Shrewsbury—was now rather proud of his second son, the young naturalist well regarded within British scientific circles. Grumpy on the outside, generous within, Dr. Darwin had made supportive arrangements for both brothers. And Charles was single. He sauntered around London, he handled follow-up tasks on his specimens from the voyage, he worked on rewriting his Beagle diary into a travel book, and—very privately—he ruminated about that radical alternative to special creation. He read widely, scribbling facts and phrases into various notebooks. The “A” notebook was devoted to geology. The B notebook was first of a series on what, to himself only, he called “transmutation.” You can guess what that meant. Darwin had begun thinking his way toward a theory of evolution.

He opened the B notebook, in July 1837, with a few phrases alluding to a book titled Zoonomia; or the Laws of Organic Life, published decades earlier by his own grandfather, another Erasmus Darwin. Zoonomia was a medical treatise (Erasmus was a physician), but it contained some provocative musings that sounded vaguely evolutionary. All warm-blooded animals “have arisen from one living filament,” according to Zoonomia, and they possess “the faculty of continuing to improve” in ways that could be passed down across the generations, “world without end!” Improvement across generations? Heritable change throughout the history of the world? That was contrary to the special-creation hypothesis, but not too surprising from a gouty, libidinous freethinker and sometime poet such as old Erasmus. Darwin had read Zoonomia during his student days and shown little sign of giving his grandfather’s daring ideas much credit. But now, on revisiting, he took them as a point of departure. Page one, entry one, in the B notebook: his grandfather’s title, Zoonomia, followed by reading notes.

Then again, those wild suggestions didn’t lead anywhere. Erasmus Darwin had offered no material mechanism for “the faculty of continuing to improve,” and a material mechanism was what young Charles wanted, though he may not have fully realized that yet. As reflected in the B notebook, he now went from his grandfather’s work to other readings, other speculations and questions, jotting down clipped phrases, often in bad grammar and punctuation. He wasn’t writing to publish. These were messages to himself.

“Why is life short,” he asked, omitting the question mark in his haste. Why is reproduction so important? Why do animals of a given kind tend to be constant in form across an entire country but to differ at least slightly on separate islands? He remembered the giant tortoises on the Galápagos, where his stopover had lasted only thirty-five days but catalyzed an upheaval in his thinking. He remembered the mockingbirds too. And why had he seen two distinct kinds of “ostriches” (his label for big, flightless birds now known as rheas) on the Argentine Pampas, one living north of the Rio Negro, one south of it? Did creatures somehow become different when isolated? Put a pair of cats on an island, let them breed and inbreed there for generations, with a little pressure from enemies, and “who will dare say what result,” Darwin wrote. He dared. The descendants might come to look different from other cats, might they not? He wanted to understand why.

Another important question: “Each species changes. does it progress.” Do the cats become better cats, or at least better cats for catting on that particular island? If so, how long would it take? How far would it go? What are the logical limits, if “every successive animal is branching upwards” and with “different types of organization improving,” new forms arising, old forms dying out? That one word, branching, was freighted with interesting implications: of directional growth, of divergence, of an arboreal form. And these questions Darwin asked himself, they applied not just to cats and ostriches but also to armadillos and sloths in Argentina, to marsupials in Australia, to those huge Galápagos tortoises, and to the wolflike Falkland Islands fox, all peculiar in certain ways, all unique to their isolated places, but recognizably similar to their correlatives—other cats and tortoises and foxes, etcetera—elsewhere. Darwin had seen a lot. He was an acutely observant and reflective young man. He sensed that he had seen patterns, not just particulars. It almost seemed, he wrote, that there was a “law of adaptation” at work.

All this and more, facts and speculations, crammed into the first twenty-one pages of notebook B. The pages are mostly undated, so we can’t know how many days or weeks passed in the opening burst of effort. Anyway, he didn’t yet have his theory. Big ideas were coming at him like diving owls. He needed some order as much as he needed the jumble of tantalizing clues. Maybe he needed a metaphor. Then, on the bottom of page 21, Darwin wrote: “organized beings represent a tree.”

2

We don’t know whether Darwin sat back after writing that statement and breathed deep with a new sense of clarity, but he might have. And he was entitled.

Then he scribbled on. The tree is “irregularly branched,” he told the B notebook, “some branches far more branched.” Each branch diverges into smaller branches, he wrote, and then twigs, “Hence Genera,” the next higher category above species, which would be the twiglets or terminal buds. Some buds die away without yielding further growth—species extinction, end of a line—while new buds appear, somehow. Although the very idea of extinction had once been problematic among naturalists and philosophers, doubted as a possibility or rejected outright on grounds that God’s acts of special creation couldn’t be undone, Darwin recognized that there’s “nothing stranger in death of species” than in death of an individual. In fact, extinction was not just natural but necessary, making space for new species as old ones die away. He wrote: “The tree of life should perhaps be called the coral of life, base of branches dead,” ancestral forms gone. Darwin knew something about coral, having seen reefs at Keeling Atoll in the eastern Indian Ocean and elsewhere during the Beagle voyage. They fascinated him; he concocted a theory of how reefs are formed; and in 1842, five years after this notebook entry, he would publish a book about coral reefs. Coral seemed apt—branching coral, not brain coral or table coral, was what he had in mind—because the lower limbs and base are lifeless calcitic skeleton, left behind like extinct forms of ancient lineages as the soft polyps advance upward like living species. But even he seems to have sensed that “the coral of life” didn’t have the same memorable ring. He drew a feeble pen sketch, on page 26 in the B notebook, of a three-branched coral of life, with dotted lines depicting the inanimate lower sections. And then he let the coral idea slide, abandoning that metaphor.

The tree of life was better. It was already a venerable notion in 1837, and Darwin could adapt it to his purposes as an evolutionary theorist—easier than inventing a new trope from scratch. Of course, to make that adaptation was to alter its meaning radically. Never mind, he took the step. Ten notebook pages along, he sketched a much livelier and more complex figure in bold strokes, with a trunk rising into four major limbs and several minor ones, each major limb diverging into clusters of branches, one branch within each cluster labeled A, B, C, D. The branches B and C were near neighbors in the treetop, within adjacent clusters, indicating close relationships among the creatures on those branches. The letter A was far away, on the opposite side of the tree’s crown, signaling a more distant relationship—but still a relationship. The letters were placeholders, meant to represent living species, or maybe genera. Felis, Canis, Vulpes, Gorilla. We don’t know exactly what he had in mind, and maybe it was nothing so specific. Anyway, this was a thunderous assertion, abstract but eloquent. You can look at the little sketch today, with its four labeled branches amid the limbs and the crown, and imagine the evolutionary divergence of all life from a common ancestor.

Darwin’s 1837 sketch, redrawn by Patricia J. Wynne.

Just above the sketch, as though gesturing toward it bashfully, Darwin wrote: “I think.”

3

Darwin didn’t invent that phrase, “the tree of life,” nor originate its iconic use, though he put it to new purpose in his theory. Like so many other metaphors embedded deep in our thinking, it came down murkily, modified and reechoed, from early versions in Aristotle and the Bible. (Why do these things always go back to Aristotle? Well, that’s why he’s Aristotle.) In the Bible, it’s a grand bookend motif, invoked in Genesis 3 just as Adam and Eve are booted out of the Garden, and reappearing at the end of Revelation, on the very last page of the King James version—excellent placement for a launch into Western culture. There in Revelation 22, verses 1–2, the authorial prophet describes his ecstatic vision of the “water of life,” flowing out like a pure river from the throne of God, and beside which grows “the tree of life,” bearing fruit every month, plus leaves “for the healing of the nations.” This tree possibly represents Christ, supplying his leafy and fruity blessings to the world; or maybe it’s grace, or the Church. The passage is opaque, and differences in translations (one tree or many?) have confused things further. The point here is simply that the “tree of life” is an ancient poetic image, a resonant phrase, variously construable, 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 ...

Table of contents

• Title Page
• Copyright
• Dedication
• Contents
• Three Surprises: An Introduction
• PART I: Darwin’s Little Sketch
• PART II: A Separate Form of Life
• PART III: Mergers and Acquisitions
• PART IV: Big Tree
• PART V: Infective Heredity
• PART VI: Topiary
• PART VII: E Pluribus Human
• Notes
• Bibliography
• Illustration Credits
• Index
• Acknowledgments
• About the Book
• About the Author
• Also by David Quammen
• About the Publisher