The Gene

by Siddhartha Mukherjee

  Two facts stood out that year. First, as Owen and Lyell pored through the fossils, they found an underlying pattern in the specimens. They were typically skeletons of colossal, extinct versions of animals that were still in existence at the very same locations where the fossils had been discovered. Giant-plated armadillos once roamed in the very valley where small armadillos were now moving through the brush. Gargantuan sloths had foraged where smaller sloths now resided. The huge femoral bones that Darwin had extracted from the soil belonged to a vast, elephant-size llama; its smaller current version was unique to South America.

  The second bizarre fact came from Gould. In the early spring of 1837, Gould told Darwin that the assorted varieties of wrens, warblers, blackbirds, and “Gross-beaks” that Darwin had sent him were not assorted or various at all. Darwin had misclassified them: they were all finches—an astonishing thirteen species. Their beaks, claws, and plumage were so distinct that only a trained eye could have discerned the unity lurking beneath. The thin-throated, wrenlike warbler and the ham-necked, pincer-beaked blackbirds were anatomical cousins—variants of the same species. The warbler likely fed on fruit and insects (hence that flutelike beak). The spanner-beaked finch was a seed-cracking ground forager (hence its nutcracker-like bill). And the mockingbirds that were endemic to each island were also three distinct species. Finches and finches everywhere. It was as if each site had produced its own variant—a bar-coded bird for each island.

  How could Darwin reconcile these two facts? Already, the bare outline of an idea was coalescing in his mind—a notion so simple, and yet so deeply radical, that no biologist had dared to explore it fully: What if all the finches had arisen from a common ancestral finch? What if the small armadillos of today had arisen from a giant ancestral armadillo? Lyell had argued that the current landscape of the earth was the consequence of natural forces that had accumulated over millions of years. In 1796, the French physicist Pierre-Simon Laplace had proposed that even the current solar system had arisen from the gradual cooling and condensation of matter over millions of years (when Napoléon had asked Laplace why God was so conspicuously missing from his theory, Laplace had replied with epic cheekiness: “Sire, I had no need for that hypothesis”). What if the current forms of animals were also the consequence of natural forces that had accumulated over millennia?

  In July 1837, in the stifling heat of his study on Marlborough Street, Darwin began scribbling in a new notebook (the so-called B notebook), firing off ideas about how animals could change over time. The notes were cryptic, spontaneous, and raw. On one page, he drew a diagram that would return to haunt his thoughts: rather than all species radiating out from the central hub of divine creation, perhaps they arose like branches of a “tree,” or like rivulets from a river, with an ancestral stem that divided and subdivided into smaller and smaller branches toward dozens of modern descendants. Like languages, like landscapes, like the slowly cooling cosmos, perhaps the animals and plants had descended from earlier forms through a process of gradual, continuous change.

  It was, Darwin knew, an explicitly profane diagram. The Christian concept of speciation placed God firmly at the epicenter; all animals created by Him sprayed outward from the moment of creation. In Darwin’s drawing, there was no center. The thirteen finches were not created by some divine whim, but by “natural descent”—cascading downward and outward from an original ancestral finch. The modern llama arose similarly, by descending from a giant ancestral beast. As an afterthought, he added, “I think,” above the page, as if to signal his last point of departure from the mainlands of biological and theological thought.

  But—with God shoved aside—what was the driving force behind the origin of species? What impetus drove the descent of, say, thirteen variants of finches down the fierce rivulets of speciation? In the spring of 1838, as Darwin tore into a new journal—the maroon C notebook—he had more thoughts on the nature of this driving force.

  The first part of the answer had been sitting under his nose since his childhood in the farmlands of Shrewsbury and Hereford; Darwin had merely traveled eight thousand miles around the globe to rediscover it. The phenomenon was called variation—animals occasionally produced offspring with features different from the parental type. Farmers had been using this phenomenon for millennia—breeding and interbreeding animals to produce natural variants, and selecting these variants over multiple generations. In England, farm breeders had refined the creation of novel breeds and variants to a highly sophisticated science. The shorthorn bulls of Hereford bore little resemblance to the longhorns of Craven. A curious naturalist traveling from the Galápagos to England—a Darwin in reverse—might have been astonished to find that each region had its own species of cow. But as Darwin, or any bull breeder, could tell you, the breeds had not arisen by accident. They had been deliberately created by humans—by the selective breeding of variants from the same ancestral cow.

  The deft combination of variation and artificial selection, Darwin knew, could produce extraordinary results. Pigeons could be made to look like roosters and peacocks, and dogs made short-haired, long-haired, pied, piebald, bowlegged, hairless, crop-tailed, vicious, mild-mannered, diffident, guarded, belligerent. But the force that had molded the selection of cows, dogs, and pigeons was the human hand. What hand, Darwin asked, had guided the creation of such different varieties of finches on those distant volcanic islands or made small armadillos out of giant precursors on the plains of South America?

  Darwin knew that he was now gliding along the dangerous edge of the known world, tacking south of heresy. He could easily have ascribed the invisible hand to God. But the answer that came to him in October 1838, in a book by another cleric, the Reverend Thomas Malthus, had nothing to do with divinity.

  Thomas Malthus had been a curate at the Okewood Chapel in Surrey by daytime, but he was a closet economist by night. His true passion was the study of populations and growth. In 1798, writing under a pseudonym, Malthus had published an incendiary paper—An Essay on the Principle of Population—in which he had argued that the human population was in constant struggle with its limited resource pool. As the population expanded, Malthus reasoned, its resource pool would be depleted, and competition between individuals would grow severe. A population’s inherent inclination to expand would be severely counterbalanced by the limitations of resources; its natural wont met by natural want. And then potent apocalyptic forces—“sickly seasons, epidemics, pestilence and plague [would] advance in terrific array, and sweep off their thousands and tens of thousands”—leveling the “population with the food of the world.” Those that survived this “natural selection” would restart the grim cycle again—Sisyphus moving from one famine to the next.

  In Malthus’s paper, Darwin immediately saw a solution to his quandary. This struggle for survival was the shaping hand. Death was nature’s culler, its grim shaper. “It at once struck me,” he wrote, “that under these circumstances [of natural selection], favourable variations would tend to be preserved and unfavourable ones to be destroyed. The results of this would be the formation of a new species.”I

  Darwin now had the skeletal sketch of his master theory. When animals reproduce, they produce variants that differ from the parents.II Individuals within a species are constantly competing for scarce resources. When these resources form a critical bottleneck—during a famine, for instance—a variant better adapted for an environment is “naturally selected.” The best adapted—the “fittest”—survive (the phrase survival of the fittest was borrowed from the Malthusian economist Herbert Spencer). These survivors then reproduce to make more of their kind, thereby driving evolutionary change within a species.

  Darwin could almost see the process unfolding on the salty bays of Punta Alta or on the islands of the Galápagos, as if an eons-long film were running on fast-forward, a millennium compressed to a minute. Flocks of finches fed on fruit until their population exploded. A bleak season came upon the island—a rotting monsoon or a parched
summer—and fruit supplies dwindled drastically. Somewhere in the vast flock, a variant was born with a grotesque beak capable of cracking seeds. As famine raged through the finch world, this gross-beaked variant survived by feeding on hard seeds. It reproduced, and a new species of finch began to appear. The freak became the norm. As new Malthusian limits were imposed—diseases, famines, parasites—new breeds gained a stronghold, and the population shifted again. Freaks became norms, and norms became extinct. Monster by monster, evolution advanced.

  By the winter of 1839, Darwin had assembled the essential outlines of his theory. Over the next few years, he tinkered and fussed obsessively with his ideas—arranging and rearranging “ugly facts” like his fossil specimens, but he never got around to publishing the theory. In 1844, he distilled the crucial parts of his thesis into a 255-page essay and mailed it to his friends to read privately. But he did not bother committing the essay to print. He concentrated, instead, on studying barnacles, writing papers on geology, dissecting sea animals, and tending to his family. His daughter Annie—the eldest, and his favorite—contracted an infection and died, leaving Darwin numb with grief. A brutal, internecine war broke out in the Crimean Peninsula. Men were hauled off to the battlefront and Europe plunged into a depression. It was as if Malthus and the struggle for survival had come alive in the real world.

  In the summer of 1855, more than a decade and a half after Darwin had first read Malthus’s essay and crystallized his ideas about speciation, a young naturalist, Alfred Russel Wallace, published a paper in the Annals and Magazine of Natural History that skirted dangerously close to Darwin’s yet-unpublished theory. Wallace and Darwin had emerged from vastly different social and ideological backgrounds. Unlike Darwin—landed cleric, gentleman biologist, and soon to be England’s most lauded natural historian—Wallace had been born to a middle-class family in Monmouthshire. He too had read Malthus’s paper on populations—not in an armchair in his study, but on the hard-back benches of the free library at Leicester (Malthus’s book was widely circulated in intellectual circles in Great Britain). Like Darwin, Wallace had also embarked on a seafaring journey—to Brazil—to collect specimens and fossils and had emerged transformed.

  In 1854, having lost the little money that he possessed, and all the specimens that he had collected, in a shipping disaster, an even more deeply impoverished Wallace moved from the Amazon basin to another series of scattered volcanic islands—the Malay Archipelago—on the edge of southeastern Asia. There, like Darwin, he observed astonishing differences between closely related species that had been separated by channels of water. By the winter of 1857, Wallace had begun to formulate a general theory about the mechanism driving variation in these islands. That spring, lying in bed with a hallucinatory fever, he stumbled upon the last missing piece of his theory. He recalled Malthus’s paper. “The answer was clearly . . . [that] the best fitted [variants] live. . . . In this way every part of an animal’s organization could be modified exactly as required.” Even the language of his thoughts—variation, mutation, survival, and selection—bore striking similarities to Darwin’s. Separated by oceans and continents, buffeted by very different intellectual winds, the two men had sailed to the same port.

  In June 1858, Wallace sent Darwin a tentative draft of his paper outlining his general theory of evolution by natural selection. Stunned by the similarities between Wallace’s theory and his own, a panicked Darwin dashed his own manuscript off to his old friend Lyell. Cannily, Lyell advised Darwin to have both papers presented simultaneously at the meeting of the Linnean Society that summer so that both Darwin and Wallace could simultaneously be credited for their discoveries. On July 1, 1858, Darwin’s and Wallace’s papers were read back to back and discussed publicly in London. The audience was not particularly enthusiastic about either study. The next May, the president of the society remarked parenthetically that the past year had not yielded any particularly noteworthy discoveries.

  Darwin now rushed to finish the monumental opus that he had originally intended to publish with all his findings. In 1859, he contacted the publisher John Murray hesitantly: “I heartily hope that my Book may be sufficiently successful that you may not repent of having undertaken it.” On November 24, 1859, on a wintry Thursday morning, Charles Darwin’s book On the Origin of Species by Means of Natural Selection appeared in bookstores in England, priced at fifteen shillings a copy. Twelve hundred and fifty copies had been printed. As Darwin noted, stunned, “All copies were sold [on the] first day.”

  A torrent of ecstatic reviews appeared almost immediately. Even the earliest readers of Origin were aware of the book’s far-reaching implications. “The conclusions announced by Mr. Darwin are such as, if established, would cause a complete revolution in the fundamental doctrines of natural history,” one reviewer wrote. “We imply that his work [is] one of the most important that for a long time past have been given to public.”

  Darwin had also fueled his critics. Perhaps wisely, he had been deliberately cagey about the implications of his theory for human evolution: the only line in Origin regarding human descent—“light will be thrown on the origin of man and his history”—might well have been the scientific understatement of the century. But Richard Owen, the fossil taxonomist—Darwin’s frenemy—was quick to discern the philosophical implications of Darwin’s theory. If the descent of species occurred as Darwin suggested, he reasoned, then the implication for human evolution was obvious. “Man might be a transmuted ape”—an idea so deeply repulsive that Owen could not even bear to contemplate it. Darwin had advanced the boldest new theory in biology, Owen wrote, without adequate experimental proof to support it; rather than fruit, he had provided “intellectual husks.” Owen complained (quoting Darwin himself): “One’s imagination must fill up very wide blanks.”

  * * *

  I. Darwin missed a crucial step here. Variation and natural selection offer cogent explanations of the mechanism by which evolution might occur within a species, but they do not explain the formation of species per se. For a new species to arise, organisms must no longer be able to reproduce viably with each other. This typically occurs when animals are isolated from each other by a physical barrier or another permanent form of isolation, ultimately leading to reproductive incompatibility. We will return to this idea in subsequent pages.

  II. Darwin was unsure how these variants were generated, another fact to which we will return in subsequent pages.

  The “Very Wide Blank”

  Now, I wonder if Mr. Darwin ever took the trouble to think how long it would take to exhaust any given original stock of . . . gemmules . . . It seems to me if he had given it a casual thought, he surely would never have dreamt of “pangenesis.”

  —Alexander Wilford Hall, 1880

  It is a testament to Darwin’s scientific audacity that he was not particularly bothered by the prospect of human descent from apelike ancestors. It is also a testament to his scientific integrity that what did bother him, with far fiercer urgency, was the integrity of the internal logic of his own theory. One particularly “wide blank” had to be filled: heredity.

  A theory of heredity, Darwin realized, was not peripheral to a theory of evolution; it was of pivotal importance. For a variant of gross-beaked finch to appear on a Galápagos island by natural selection, two seemingly contradictory facts had to be simultaneously true. First, a short-beaked “normal” finch must be able to occasionally produce a gross-beaked variant—a monster or freak (Darwin called these sports—an evocative word, suggesting the infinite caprice of the natural world. The crucial driver of evolution, Darwin understood, was not nature’s sense of purpose, but her sense of humor). And second, once born, that gross-beaked finch must be able to transmit the same trait to its offspring, thereby fixing the variation for generations to come. If either factor failed—if reproduction failed to produce variants or if heredity failed to transmit the variations—then nature would be mired in a ditch, the cogwheels of evolution jammed. For Darwin’s
theory to work, heredity had to possess constancy and inconstancy, stability and mutation.

  Darwin wondered incessantly about a mechanism of heredity that could achieve these counterbalanced properties. In Darwin’s time, the most commonly accepted mechanism of heredity was a theory advanced by the eighteenth-century French biologist Jean-Baptiste Lamarck. In Lamarck’s view, hereditary traits were passed from parents to offspring in the same manner that a message, or story, might be passed—i.e., by instruction. Lamarck believed that animals adapted to their environments by strengthening or weakening certain traits—“with a power proportional to the length of time it has been so used.” A finch forced to feed on hard seeds adapted by “strengthening” its beak. Over time, the finch’s beak would harden and become pincer shaped. This adapted feature would then be transmitted to the finch’s offspring by instruction, and their beaks would harden as well, having been pre-adapted to the harder seeds by their parents. By similar logic, antelopes that foraged on tall trees found that they had to extend their necks to reach the high foliage. By “use and disuse,” as Lamarck put it, their necks would stretch and lengthen, and these antelopes would produce long-necked offspring—thereby giving rise to giraffes (note the similarities between Lamarck’s theory—of the body giving “instructions” to sperm—and Pythagoras’s conception of human heredity, with sperm collecting messages from all organs).

  The immediate appeal of Lamarck’s idea was that it offered a reassuring story of progress: all animals were progressively adapting to their environments, and thus progressively slouching along an evolutionary ladder toward perfection. Evolution and adaptation were bundled together into one continuous mechanism: adaptation was evolution. The scheme was not just intuitive, it was also conveniently divine—or close enough for a biologist’s work. Although initially created by God, animals still had a chance to perfect their forms in the changing natural world. The Divine Chain of Being still stood. If anything, it stood even more upright: at the end of the long chain of adaptive evolution was the well-adjusted, best-erected, most perfected mammal of them all: humans.