The Violinist's Thumb: And Other Lost Tales of Love, War, and Genius, as Written by Our Genetic Code (29 page)

After all this preparation, the actual DNA sequencing begins. We’ll talk about this process in detail later, but basically scientists determine the A-C-G-T sequence of each individual DNA fragment, then use sophisticated software to piece the many, many fragments together. Paleogeneticists have successfully applied this technique to stuffed quaggas, cave bear skulls, woolly mammoth tufts, bees in amber, mummy skin, even Buckland’s beloved coprolites. But the most spectacular work along these lines comes from Neanderthal DNA. After the discovery of Neanderthals, many scientists classified them as archaic humans—the first (before the metaphor became tired) missing link. Others put Neanderthals on their own terminal branch of evolution, while some European scientists considered Neanderthals the ancestors of some human races but not others. (Again,
sigh,
you can guess which races they singled out, Africans and Aborigines.) Regardless of the exact taxonomy, scientists considered Neanderthals thick-witted and lowbrow, and it didn’t surprise anyone that they’d died out. Eventually some dissenters began arguing that Neanderthals showed more smarts than they got credit for: they used stone tools, mastered fire, buried their dead (sometimes with wildflowers), cared for the weak and lame, and possibly wore jewelry and played bone flutes. But the scientists couldn’t prove that Neanderthals hadn’t watched humans do these things first and aped them, which hardly takes supreme intelligence.

DNA, though, permanently changed our view of Neanderthals. As early as 1987, mitochondrial DNA showed that Neanderthals weren’t direct human ancestors. At the same time, when the complete Neanderthal genome appeared in 2010, it turned out that the butt of so many
Far Side
cartoons was pretty darn human anyway; we share well north of 99 percent of our genome with them. In some cases this overlap was homely: Neanderthals likely had reddish hair and pale skin; they had the most common blood type worldwide, O; and like most humans, they couldn’t digest milk as adults. Other findings were more profound. Neanderthals had similar MHC immunity genes, and also shared a gene,
foxp2,
associated with language skills, which means they may have been articulate.

It’s not clear yet whether Neanderthals had alternative versions of
apoE,
but they got more of their protein from flesh than we did and so probably had some genetic adaptations to metabolize cholesterol and fight infections. Indeed, archaeological evidence suggests that Neanderthals didn’t hesitate to eat even their own dead—perhaps as part of primitive shamanic rituals, perhaps for darker reasons. At a cave in northern Spain, scientists have discovered the fifty-thousand-year-old remains of twelve murdered Neanderthal adults and children, many of them related. After the deed, their probably starving assailants butchered them with stone tools and cracked their bones to suck the marrow, cannibalizing every edible ounce. A gruesome scene, but it was from this surfeit of 1,700 bones that scientists extracted much of their early knowledge of Neanderthal DNA.

Like it or not, similar evidence exists for human cannibalism. Each hundred-pound adult, after all, could provide starving comrades with forty pounds of precious muscle protein, plus edible fat, gristle, liver, and blood. More uncomfortably, archaeological evidence has long suggested that humans tucked into each other even when not famished. But for years questions persisted
about whether most nonstarvation cannibalism was religiously motivated and selective or culinary and routine. DNA suggests routine. Every known ethnic group worldwide has one of two genetic signatures that help our bodies fight off certain diseases that cannibals catch, especially mad-cow-like diseases that come from eating each other’s brains. This defensive DNA almost certainly wouldn’t have become fixed worldwide if it hadn’t once been all too necessary.

As the cannibalism DNA shows, scientists don’t rely entirely on ancient artifacts for information about our past. Modern human DNA holds clues as well. And about the first thing scientists noticed when they began surveying modern human DNA is its lack of variety. Roughly 150,000 chimps and around the same number of gorillas are living today, compared to some seven billion humans. Yet humans have less genetic diversity than these monkeys, significantly less. This suggests that the worldwide population of humans has dipped far below the population of chimps and gorillas recently, perhaps multiple times. Had the Endangered Species Act existed way back when,
Homo sapiens
might have been the Paleolithic equivalent of pandas and condors.

Scientists disagree on why our population decreased so much, but the origins of the debate trace back to two different theories—or really, two different weltanschauungs—first articulated in William Buckland’s day. Virtually every scientist before then upheld a catastrophist view of history—that floods, earthquakes, and other cataclysms had sculpted the planet quickly, throwing up mountains over a long weekend and wiping out species overnight. A younger generation—especially Buckland’s student Charles Lyell, a geologist—pushed gradualism, the idea that winds, tides, erosion, and other gentle forces shaped the earth and its inhabitants achingly slowly. For various reasons (
including some posthumous smear campaigns), gradualism became associated with proper science, catastrophism with lazy reasoning and theatrical biblical miracles, and by the early 1900s catastrophism itself had been (and this puts it mildly) annihilated in science. Eventually the pendulum swung back, and catastrophism became respectable again after 1979, when geologists discovered that a city-sized asteroid or comet helped eradicate the dinosaurs. Since then, scientists have accepted that they can uphold a proper gradualist view for most of history and still allow that some pretty apocalyptic events have taken place. But this acceptance makes it all the more curious that one ancient calamity, the first traces of which were discovered within a year of the dino impact, has received far less attention. Especially considering that some scientists argue that the Toba supervolcano almost eliminated a species far more dear to us than dinosaurs:
Homo sapiens.

Getting a grasp on Toba takes some imagination. Toba is—or was, before the top 650 cubic miles blew off—a mountain in Indonesia that erupted seventy-odd thousand years ago. But because no witnesses survived, we can best appreciate its terror by comparing it (however faintly) to the second-largest known eruption in that archipelago, the Tambora eruption of 1815.

In early April 1815, three pillars of fire straight out of Exodus blasted out of Tambora’s top. Tens of thousands died as psychedelic orange lava surfed down the mountainside, and a tsunami five feet high and traveling at 150 miles per hour battered nearby islands. People fifteen hundred miles away (roughly from New York to mid–South Dakota) heard the initial blast, and the world went black for hundreds of miles around as a smoke plume climbed ten miles into the sky. That smoke carried with it enormous amounts of sulfurous chemicals. At first, these aerosols seemed harmless, even pleasant: in England, they intensified the pinks, oranges, and bloody reds of the sunsets that summer, a celestial drama that probably influenced the land- and sunscapes
of painter J. M. W. Turner. Later effects were less cute. By 1816—popularly known as The Year Without a Summer—the sulfurous ejecta had mixed homogeneously into the upper atmosphere and began reflecting sunlight back into space. This loss of heat caused freak July and August snowstorms in the fledgling United States, and crops failed widely (including Thomas Jefferson’s corn at Monticello). In Europe Lord Byron wrote a dire poem in July 1816 called “Darkness,” which opens, “I had a dream, which was not all a dream. / The bright sun was extinguish’d… / Morn came and went—and came, and brought no day, / And men… / Were chill’d into a selfish prayer for light.” A few writers happened to holiday with Byron that summer near Lake Geneva, but they found the days so dreary that they mostly sulked indoors. Channeling their mood, some took to telling ghost stories for entertainment—one of which, by young Mary Shelley, became
Frankenstein.

Now, with all that in mind about Tambora, consider that Toba spewed for five times longer and ejected a dozen times more material—millions of tons of vaporized rock per second
^*
at its peak. And being so much bigger, Toba’s enormous black basilisk of a plume could do proportionately more damage. Because of prevailing winds, most of the plume drifted westward. And some scientists think that a DNA bottleneck began when the smoke, after sweeping across south Asia, scythed into the very grasslands in Africa where humans lived. According to this theory, the destruction happened in two phases. In the short term, Toba dimmed the sun for six years, disrupted seasonal rains, choked off streams, and scattered whole cubic miles of hot ash (imagine wading through a giant ashtray) across acres and acres of plants, a major food source. It’s not hard to imagine the human population plummeting. Other primates might have suffered less at first because humans camped on the eastern edge of Africa, in Toba’s path, whereas most primates lived inland,
sheltered somewhat behind mountains. But even if Toba spared other animals initially, no one escaped the second phase. Earth was already mired in an Ice Age in 70,000 BC, and the persistent reflection of sunlight into space might well have exacerbated it. We have evidence that the average temperature dropped twenty-plus degrees in some spots, after which the African savannas—our ancient homes—probably contracted like puddles in August heat. Overall, then, the Toba-bottleneck theory argues that the initial eruption led to widespread starvation, but the deepening Ice Age is what really pinned the human population down.

Macaque, orangutan, tiger, gorilla, and chimpanzee DNA also show some signs of bottlenecking right around Toba, but humans really suffered. One study suggested that the human population, worldwide, might have dropped to forty adults. (The world record for fitting people in a phone booth is twenty-five.) That’s an outlandishly pessimistic guess even among disaster scientists, but it’s common to find estimates of a few thousand adults, below what some minor-league baseball teams draw. Given that these humans might not have been united in one place either, but scattered in small, isolated pockets around Africa, things look even shakier for our future. If the Toba-bottleneck theory is true, then the lack of diversity in human DNA has a simple explanation. We damn near went extinct.

Not surprisingly—more infighting—many archaeologists find that explanation for low genetic diversity way too pat, and the theory remains contentious. It’s not the existence of a bottleneck per se that rankles. It’s well established that the protohuman breeding population (roughly equivalent to the number of fertile adults) dropped alarmingly at times in the past million years. (Which, among other things, probably allowed a freakish trait like forty-six chromosomes to spread.) And many scientists see strong evidence in our DNA for at least one major bottleneck after anatomically modern humans arose 200,000 years ago.
What rankles scientists is linking any bottleneck to Toba; suspicion of the old, bad catastrophism looms.

Some geologists contest that Toba wasn’t as powerful as their colleagues claim. Others doubt Toba could have decimated populations thousands of miles away, or that one puny mountain could throw up enough sulfurous spume to intensify a global ice age. Some archaeologists have also found evidence (disputed, inevitably) of stone tools right above and below some six-inch-thick Toba ash layers, which implies not extinction but continuity right where Toba should have done the most damage. We have genetic reasons to question a Toba bottleneck as well. Most important, geneticists simply cannot distinguish, retroactively, between the lack of diversity induced by a short but severe bottleneck and the lack of diversity induced by a longer but milder bottleneck. In other words, there’s ambiguity: if Toba did crush us down to a few dozen adults, we’d see certain patterns in our DNA; but if a population was held down to a few thousand, as long as it was held down consistently, those people’s DNA would show
the same
signatures after maybe a thousand years. And the wider the time frame, the less likely Toba had anything to do with the bottleneck.

William Buckland and others would have recognized this debate instantly: whether small but persistent pressures could have held our clever species down for so long, or whether it took a cataclysm. But it’s a measure of progress that, unlike the rout of catastrophism in Buckland’s day and the century of scorn that followed, modern scientific catastrophists can make their case heard. And who knows? The Toba supervolcano may yet join the dinosaur-killing space rock as one of the world’s premier disasters.