The Violinist's Thumb: And Other Lost Tales of Love, War, and Genius, as Written by Our Genetic Code (15 page)

After mitochondria proved so vital to science, Margulis used her momentum and sudden prestige to advance other offbeat ideas. She began arguing that microbes also donated various locomotor devices to animals, like the tails on sperm, even though those structures lack DNA. And beyond cells merely picking up spare parts, she outlined a grander theory in which endosymbiosis drives all evolution, relegating mutations and natural selection to minor roles. According to this theory, mutations alter creatures only modestly. Real change occurs when genes leap from species to species, or when whole genomes merge, fusing wildly different creatures together. Only after these “horizontal” DNA transfers does natural selection begin,
merely to edit out any hopeless monsters that emerge. Meanwhile the hopeful monsters, the beneficiaries of mergers, flourish.

Though Margulis called it revolutionary, in some ways her merger theory just extends a classic debate between biologists who favor (for whatever reasons you want to psychoanalyze) bold leaps and instant species, and biologists who favor conservative adjustments and gradual speciation. The arch-gradualist Darwin saw modest change and common descent as nature’s law, and favored a slow-growing tree of life with no overlapping branches. Margulis fell into the radical camp. She argued that mergers can create honest-to-goodness chimeras—blends of creatures technically no different than mermaids, sphinxes, or centaurs. From this point of view, Darwin’s quaint tree of life must give way to a quick-spun web of life, with interlocking lines and radials.

However far she strayed into radical ideas, Margulis earned her right to dissent. It’s even a little two-faced to praise someone for holding fast to unconventional scientific ideas sometimes and chastise her for not conforming elsewhere; you can’t necessarily just switch off the iconoclastic part of the mind when convenient. As renowned biologist John Maynard Smith once admitted, “I think [Margulis is] often wrong, but most of the people I know think it’s important to have her around, because she’s wrong in such fruitful ways.” And lest we forget, Margulis was right in her first big idea, stunningly so. Above all, her work reminds us that pretty plants and things with backbones haven’t dominated life’s history. Microbes have, and they’re the evolutionary fodder from which we multicellular creatures rose.

If Lynn Margulis relished conflict, her older contemporary Barbara McClintock shunned it. McClintock preferred quiet rumination over public confrontation, and her peculiar ideas sprang
not from any contrarian streak but from pure flakiness. It’s fitting, then, that McClintock dedicated her life to navigating the oddball genetics of plants like maize. By embracing the eccentricity of maize, McClintock expanded our notions of what DNA can do, and provided vital clues for understanding a second great mystery of our evolutionary past: how DNA builds multicellular creatures from Margulis’s complex but solo cells.

McClintock’s bio divides into two eras: the fulfilled, pre-1951 scientist, and the bitter, post-1951 hermit. Not that things were all daffodils before 1951. From a very young age McClintock squabbled intensely with her mother, a pianist, mostly because Barbara showed more stubborn interest in science and sports like ice skating than the girlish pastimes her mother always said would improve her dating prospects. Her mother even vetoed Barbara’s dream to (like Hermann Muller and William Friedman before her) study genetics at Cornell, because nice boys didn’t marry brainy gals. Thankfully for science, Barbara’s father, a physician, intervened before the fall 1919 semester and packed his daughter off to upstate New York by train.

At Cornell, McClintock flourished, becoming the freshmen women’s class president and starring in science classes. Still, her science classmates didn’t always appreciate her sharp tongue, especially her lashings over their microscopy work. In that era, preparing microscope samples—slicing cells like deli ham and mounting their gelatinous guts on multiple glass slides without spilling—was intricate and demanding work. Actually using scopes was tricky, too: identifying what specks were what inside a cell could flummox even a good scientist. But McClintock mastered microscopy early, becoming legitimately world-class by graduation. As a graduate student at Cornell, she then honed a technique—“the squash”—that allowed her to flatten whole cells with her thumb and keep them intact on one slide, making them easier to study. Using the squash, she became the first
scientist to identify all ten maize chromosomes. (Which, as anyone who’s ever gone cross-eyed squinting at the spaghetti mess of chromosomes inside real cells knows, ain’t easy.)

Cornell asked McClintock to become a full-time researcher and instructor in 1927, and she began studying how chromosomes interact, with assistance from her best student, Harriet Creighton. Both of these tomboys wore their hair short and often dressed mannishly, in knickers and high socks. People mixed up anecdotes about them, too—like which was it exactly who’d shinnied up a drainpipe one morning after forgetting the keys to her second-story office. Creighton was more outgoing; the reserved McClintock would never have bought a jalopy, as Creighton did, to celebrate the end of World War II and cruise to Mexico. Nonetheless they made a wonderful team, and soon made a seminal discovery. Morgan’s fruit fly boys had demonstrated years before that chromosomes probably crossed arms and swapped tips. But their arguments remained statistical, based on abstract patterns. And while many a microscopist had seen chromosomes entangling, no one could tell if they actually exchanged material. But McClintock and Creighton knew every corn chromosome’s every knob and carbuncle by sight, and they determined that chromosomes did physically exchange segments. They even linked these exchanges to changes in how genes worked, a crucial confirmation. McClintock dawdled in writing up these results, but when Morgan got wind of them, he insisted she publish, posthaste. She did in 1931. Morgan won his Nobel Prize two years later.

Although pleased with the work—it earned her and Creighton bios in, well,
American Men of Science
—McClintock wanted more. She wanted to study not only chromosomes themselves but how chromosomes changed and mutated, and how those changes built complex organisms with different roots and colors and leaves. Unfortunately, as she tried to set up a lab, social
circumstances conspired against her. Like the priesthood, universities at the time offered full professorships only to men (except in home ec), and Cornell had no intention of excepting McClintock. She left reluctantly in 1936 and bounced about, working with Morgan out in California for a spell, then taking research positions in Missouri and Germany. She hated both places.

Truth be told, McClintock had other troubles beyond being the wrong gender. Not exactly the bubbly sort, she’d earned a reputation as sour and uncollegial—she’d once scooped a colleague by tackling his research problem behind his back and publishing her results before he finished. Equally problematic, McClintock worked on corn.

Yes, there was money in corn genetics, since corn was a food crop. (One leading American geneticist, Henry Wallace—future vice president to FDR—made his fortune running a seed company.) Corn also had a scientific pedigree, as Darwin and Mendel had both studied it. Ag scientists even showed an interest in corn mutations: when the United States began exploding nukes at Bikini Atoll in 1946, government scientists placed corn seed beneath the airbursts to study how nuclear fallout affected maize.

McClintock, though, pooh-poohed the traditional ends of corn research, like bigger yields and sweeter kernels. Corn was a means to her, a vehicle to study general inheritance and development. Unfortunately, corn had serious disadvantages for such work. It grew achingly slowly, and its capricious chromosomes often snapped, or grew bulges, or fused, or randomly doubled. McClintock savored the complexity, but most geneticists wanted to avoid these headaches. They trusted McClintock’s work—no one matched her on a microscope—but her devotion to corn stranded her between pragmatic scientists helping Iowans grow more bushels and pure geneticists who refused to fuss with unruly corn DNA.

At last, McClintock secured a job in 1941 at rustic Cold
Spring Harbor Laboratory, thirty miles east of Manhattan. Unlike before, she had no students to distract her, and she employed just one assistant—who got a shotgun, and instructions to keep the damn crows off her corn. And although isolated out there with her maize, she was happily isolated. Her few friends always described her as a scientific mystic, constantly chasing the insight that would dissolve the complexity of genetics into unity. “She believed in the great inner lightbulb,” one friend remarked. At Cold Spring she had time and space to meditate, and settled into the most productive decade of her career, right through 1951.

Her research actually culminated in March 1950, when a colleague received a letter from McClintock. It ran ten single-spaced pages, but whole paragraphs were scribbled out and scribbled over—not to mention other fervid annotations, connected by arrows and climbing like kudzu up and down the margins. It’s the kind of letter you’d think about getting tested for anthrax nowadays, and it described a theory that sounded nutty, too. Morgan had established genes as stationary pearls on a chromosomal necklace. McClintock insisted she’d seen the pearls move—jumping from chromosome to chromosome and burrowing in.

Moreover, these jumping genes somehow affected the color of kernels. McClintock worked with Indian corn, the kind speckled with red and blue and found on harvest floats in parades. She’d seen the jumping genes attack the arms of chromosomes inside these kernels, snapping them and leaving the ends dangling like a compound fracture. Whenever this happened, the kernels stopped producing pigment. Later, though, when the jumping gene got restless and randomly leaped somewhere else, the broken arm healed, and pigment production started up again. Amid her scribbling, McClintock suggested that the break had disrupted the gene for making the pigments.
Indeed, this off/on pattern seemed to explain the randomly colored stripes and swirls of her kernels.

In other words, jumping genes controlled pigment production; McClintock actually called them “controlling elements.” (Today they’re called transposons or, more generally, mobile DNA.) And like Margulis, McClintock parlayed her fascinating find into a more ambitious theory. Perhaps the knottiest biological question of the 1940s was why cells didn’t all look alike: skin and liver and brain cells contain the same DNA, after all, so why didn’t they act the same? Previous biologists argued that something in the cell’s cytoplasm regulated genes, something external to the nucleus. McClintock had won evidence that chromosomes regulated themselves from within the nucleus—and that this control involved turning genes on or off at the right moments.

In fact, (as McClintock suspected) the ability to turn genes off and on was a crucial step in life’s history. After Margulis’s complex cells emerged, life once again stalled for over a billion years. Then, around 550 million years ago, huge numbers of multicellular creatures burst into existence. The first beings probably were multicellular by mistake, sticky cells that couldn’t free themselves. But over time, by precisely controlling which genes functioned at which moments in which stuck-together cells, the cells could begin to specialize—the hallmark of higher life. Now McClintock thought she had insight into how this profound change came about.

McClintock organized her manic letter into a proper talk, which she delivered at Cold Spring in June 1951. Buoyed by hope, she spoke for over two hours that day, reading thirty-five single-spaced pages. She might have forgiven audience members for nodding off, but to her dismay, she found them merely baffled. It wasn’t so much her facts. Scientists knew her reputation, so when she insisted she’d seen genes jump about like fleas, most accepted she had. It was her theory about genetic control that
bothered them. Basically, the insertions and jumps seemed too random. This randomness might well explain blue versus red kernels, they granted, but how could jumping genes control all development in multicellular creatures? You can’t build a baby or a beanstalk with genes flickering on and off haphazardly. McClintock didn’t have good answers, and as the hard questions continued, consensus hardened against her. Her revolutionary idea about controlling elements got downgraded
^*
into another queer property of maize.