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

by Sam Kean

  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.

  So what does all this DNA archaeology add up to? As the field has come into its own, scientists have pulled together an overarching summary of how modern humans emerged and spread across the globe.

  Perhaps most important, DNA confirmed our African origins. A few archaeological holdouts had always maintained that humankind arose in India or Asia, but a species generally shows the highest genetic diversity near its origin, where it’s had the longest time to develop. That’s exactly what scientists see in Africa. As one example, African peoples have twenty-two versions of a particular stretch of DNA linked to the all-important insulin gene—only three of which, total, appear in the rest of the world. For ages anthropologists lumped all African peoples into one “race,” but the genetic truth is that the larger world’s diversity is more or less a subset of African diversity.

  DNA can also embellish the story of human origins with details about how we behaved long ago and even what we looked like. Around 220,000 years ago, the apoE meat-eating gene emerged and began to spread, introducing the possibility of productive old age. Just twenty thousand years later, another mutation allowed the hair on our heads to (unlike monkey hair or body hair) grow indefinitely long—a “haircut gene.” Then, thirty thousand years later, we began wearing skins as clothes, a fact scientists determined by comparing the DNA clocks of head lice (which live only on scalps) and of related but distinct body lice (which live only in clothes) and figuring out when they diverged. In ways large and small, these changes transformed societies.

  Suitably attired and perfectly coiffed, humans seem to have started making inroads from Africa into the Middle East maybe 130,000 years ago (our first imperial impulse). But something—cold weather, homesickness, predators, a Neanderthal NO TRESPASSING sign—halted their sprawl and drove them back to Africa. The human population bottlenecked over the next few tens of thousands of years, perhaps because of Toba. Regardless, humans scraped through and eventually recovered. But this time, instead of cowering and waiting for the next extinction threat, small clans of humans, as few as a few thousand people total, began establishing settlements beyond Africa, in waves beginning about sixty thousand years ago. These clans probably crossed the Red Sea at low tide, Moses style, through a southern point called Bab el Mandeb, the Gate of Grief. Because bottlenecks had isolated these clans for millennia, they’d developed unique genetic traits. So as they spread into new lands and their population doubled and redoubled, these traits blossomed into the unique features of European and Asian populations today. (In a touch Buckland would have appreciated, this multipronged dispersal from Africa is sometimes called the Weak Garden of Eden theory. But this tale is actually better than the biblical version; we didn’t lose Eden but learned to make other Edens across the world.)

  As we expanded beyond Africa, DNA kept a marvelous travelogue. In Asia, genetic analysis has revealed two distinct waves of human colonization: an earlier wave sixty-five thousand years ago that skirted India and led to the settlement of Australia, making the Aborigines the first real explorers in history; and a later wave that produced modern Asians and led to humanity’s first population boom, forty thousand years ago, when 60 percent of humankind lived on the Indian, Malay, and Thai peninsulas. In North America, a survey of different genetic pools suggests that the first Americans paused for perhaps ten thousand years on the Bering land bridge between Siberia and Alaska, as if trembling to break away from Asia and enter the New World. In South America, scientists have discovered MHC genes from Amerindians in native Easter Islanders, and the thorough blending of those genes in the islanders’ otherwise Asian chromosomes indicates that someone was making Kon-Tiki –like sea voyages back and forth to the Americas in the early 1000s, back when Columbus was mere specks of DNA spread among his great-great-great-(…)-great-grandparents’ gonads. (Genetic analysis of sweet potatoes, bottle gourds, and chicken bones also indicates pre-Columbian contact.) In Oceania, scientists have tied the diffusion and winnowing of people’s DNA to the diffusion and winnowing of their languages. It turns out that people in southern Africa, the cradle of humanity, not only have richer DNA than anyone else but richer languages, with up to one hundred distinct sounds, including the famous tchk-tchk clicks. Languages from intermediately diverse lands have fewer sounds (English has forty-some). Languages at the far end of our ancient migration, like Hawaiian, use around a dozen sounds, and Hawaiian people show correspondingly uniform DNA. It all adds up.

  Looking beyond our species a tad, DNA can also illuminate one of the biggest mysteries of archaeology: what happened to Neanderthals? After Neanderthals had thrived in Europe for ages, something slowly strangled them into smaller and smaller territories, and the last ones expired about thirty-five thousand years ago in southern Europe. The profusion of theories to explain what doomed them—climate change, catching diseases from humans, competition for food, homicide (by Homo sapiens), “mad Neanderthal” disease from eating too many brains—is an unmistakable sign that nobody has the faintest. But with the decoding of the Neanderthal genome, we know at last that Neanderthals didn’t disappear, not entirely. We carried their seed inside us across the globe.

  After emerging from Africa around sixty thousand years ago, clans of humans eventually wandered into Neanderthal lands in the Levantine. Boys eyed girls, tyrannical hormones took over, and pretty soon little humanderthals were running around—a replay of when archaic humanoids bedded protochimpanzees (plus ça change). What happened next is hazy, but the groups divorced and did so asymmetrically. Perhaps some outraged human elders stormed off, taking their despoiled children and humanderthal grandchildren with them. Perhaps only Neanderthal men bagged human women, who then left with their clan. Perhaps the groups parted amicably, but all the half-breeds left in Neanderthal care died when the humans pushed on and colon
ized the planet at large. Anyhow, when these Paleolithic Lewises and Clarks parted from their Neanderthal lovers, they carried some Neanderthal DNA in their gene pool. Enough, in fact, that we all still have a few percent’s worth inside us today—equivalent to the amount you inherited from each great-great-great-grandparent. It’s not yet clear what all this DNA does, but some of it was MHC immunity DNA—which means that Neanderthals may have unwittingly helped destroy themselves by giving humans the DNA to fight new diseases in the new lands we took from Neanderthals. Oddly, however, there seems to have been no reciprocity: no uniquely human DNA, disease-fighting or otherwise, has shown up in any Neanderthals so far. No one knows why.

  And actually only some of us absorbed Neanderthal DNA. All the amour took place on the pivot between Asia and Europe, not in Africa proper. Which means that the people who carried Neanderthal DNA forward weren’t ancient Africans (who, insofar as scientists can tell, never hooked up with Neanderthals) but the early Asians and Europeans, whose descendants populated the rest of the world. The irony is too rich not to point out. When arranging the different human races in tiers, from just below the angels to just above the brutes, smug racialist scientists of the 1800s always equated black skin with “subhuman” beasts like Neanderthals. But facts is facts: pure Nordic Europeans carry far more Neanderthal DNA than any modern African. One more time, DNA debases.

  Just to frustrate archaeologists, however, evidence turned up in 2011 that Africans had their own extra-species liaisons. Certain tribes who stayed at home in central Africa and never saw a Neanderthal in their lives seem to have acquired chunks of noncoding DNA from other, unnamed, and now-extinct archaic humans, and did so well after the early Asians and Europeans had left. As scientists continue to catalog human diversity around the world, DNA memories of other assignations will no doubt surface in other groups, and we’ll have to ascribe more and more “human” DNA to other creatures.

  Really, though, tallying whether this or that ethnic group has less archaic DNA than another misses the point. The emerging and vital truth isn’t who is more Neanderthal than whom. It’s that all peoples, everywhere, enjoyed archaic human lovers whenever they could. These DNA memories are buried deeper inside us than even our ids, and they remind us that the grand saga of how humans spread across the globe will need some personal, private, all-too-human amendments and annotations—rendezvous here, elopements there, and the commingling of genes most everywhere. At least we can say that all humans are united in sharing this shame (if shame it is) and in sharing these scarlet A’s, C’s, G’s, and T’s.

  11

  Size Matters

  How Did Humans Get Such Grotesquely Large Brains?

  The expansion of our ancestors across the globe required more than luck and persistence. To dodge extinction after extinction, we also needed us some brains. There’s clearly a biological basis for human intelligence; it’s too universal not to be inscribed in our DNA, and (unlike most cells) brain cells use almost all the DNA we have. But despite centuries of inquiry, by everyone from phrenologists to NASA engineers, on subjects from Albert Einstein to idiot savants, no one quite knows where our smarts come from.

  Early attempts to find the biological basis of intelligence played off the idea that bigger was better: more brain mass meant more thinking power, just like more muscles meant more lifting power. Although intuitive, this theory has its shortcomings; whales and their twenty-pound brains don’t dominate the globe. So Baron Cuvier, the half Darwin, half Machiavelli from Napoleonic France, suggested that scientists also examine a creature’s brain-body ratio, to measure its relative brain weight as well.

  Nonetheless scientists in Cuvier’s day maintained that bigger brains did mean finer minds, especially within a species. The best evidence here was Cuvier himself, a man renowned (indeed, practically stared at) for the veritable pumpkin atop his shoulders. Still, no one could say anything definitive about Cuvier’s brain until 7 a.m. on Tuesday, May 15, 1832, when the greatest and most shameless doctors in Paris gathered to conduct Cuvier’s autopsy. They sliced open his torso, sluiced through his viscera, and established that he had normal organs. This duty dispatched, they eagerly sawed through his skull and extracted a whale of a specimen, sixty-five ounces, over 10 percent larger than any brain measured before. The smartest scientist these men had ever known had the biggest brain they’d ever seen. Pretty convincing.

  By the 1860s, though, the tidy size-smarts theory had started unraveling. For one, some scientists questioned the accuracy of the Cuvier measurement—it just seemed too outré. No one had bothered to pickle and preserve Cuvier’s brain, unfortunately, so these later scientists grasped at whatever evidence they could find. Someone eventually dug up Cuvier’s hat, which was indeed commodious; it fell over the eyes of most everyone who donned it. But those wise in the ways of milliners pointed out that the hat’s felt might have stretched over the years, leading to overestimates. Tonsorial types suggested instead that Cuvier’s bushy hairdo had made his head merely appear enormous, biasing his doctors to expect (and, because expecting, find) a vast brain. Still others built a case that Cuvier suffered from juvenile hydrocephaly, a feverish swelling of the brain and skull when young. In that case, Cuvier’s big head might be accidental, unrelated to his genius.*

  Baron Cuvier—a half-Darwin, half-Machiavelli biologist who lorded over French science during and after Napoleon—had one of the largest human brains ever recorded. (James Thomson)

  Arguing about Cuvier wasn’t going to solve anything, so to get more data on more people, cranial anatomists developed methods to gauge the volumes of skulls. Basically, they plugged every hole and filled the skulls (depending on their preference) with a known amount of peas, beans, rice, millet, white peppercorn, mustard seed, water, mercury, or lead buckshot. Imagine rows of skulls on a table, funnels protruding from each one, an assistant humping around buckets of quicksilver or burlap bags of grain from the market. Whole monographs were published on these experiments, but they produced still more baffling results. Were Eskimos, who had the largest brainpans, really the smartest people on earth? What’s more, the skulls of the newly discovered Neanderthal species were actually roomier than human skulls by an average of six cubic inches.

  As it turns out, that’s just the start of the confusion. Again, without correlating strictly, a bigger brain does generally make a species smarter. And because monkeys, apes, and humans are all pretty sharp, scientists assumed that intense pressure must have come to bear on primate DNA to boost brain size. It was basically an arms race: big-brain primates win the most food and survive crises better, and the only way to beat them is to get smarter yourself. But nature can be stingy, too. Based on genetic and fossil evidence, scientists can now track how most primate lineages have evolved over many millions of years. It turns out that certain species’ bodies, and not infrequently their brains, shrank over time—they became cranial runts. Brains consume a lot of energy (around 20 percent of human calories), and in times with chronic food shortages, the DNA that won out in primates was the miserly DNA that scrimped on building up brains.

  The best-known runt today is probably the “hobbit” skeleton from the Indonesian island of Flores. When it was discovered in 2003, many scientists declared it a stunted or microcephalic (tiny-headed) human; no way evolution was irresponsible enough to let the brains of a hominid dwindle that much, brains being about all we hominids have going. But nowadays most scientists accept that the brains of hobbits (officially, Homo floresiensis) did shrink. Some of this diminution might relate to so-called island dwarfism: islands, being severely finite, have less food, so if an animal can tune down some of the hundreds of genes that control its height and size, it can get by with fewer calories. Island dwarfism has shrunk mammoths, hippos, and other stranded species to pygmy sizes, and there’s no reason to think this pressure wouldn’t squash a hominid, even if the cost is a punier brain.*

  By some measures, modern humans are runts, too. We’ve probably all gon
e to a museum and snickered over the wee suit of armor that a king of England or some other big swinging dick from history wore—what a shrimp! But our ancestors would giggle at our clothes just the same. Since about 30,000 BC, our DNA has diminished the average human body size by 10 percent (roughly five inches). The vaunted human brain dwindled by at least 10 percent over that span, too, and a few scientists argue it has shrunk even more.

  Scientists filling skulls with buckshot or millet in the early 1900s didn’t know about DNA, of course, but even with their crude tools, they could tell the brain size–intelligence theory didn’t add up. One famous study of geniuses—it got a two-page spread in the New York Times in 1912—did find some truly capacious organs. Russian writer Ivan Turgenev’s brain topped seventy ounces, compared to a human average of fifty. At the same time, the brains of statesman Daniel Webster and mathematician Charles Babbage, who dreamed up the first programmable computer, were merely average. And poor Walt Whitman had to sound his barbaric yawp over the rooftops with a command center of just forty-four ounces. Even worse was Franz Joseph Gall. Though an intelligent scientist—he proposed for the first time that different brain regions have different functions—Gall also founded phrenology, the analysis of head lumps. To his followers’ eternal shame, he weighed in at a measly forty-two ounces.

  To be fair, a technician dropped Whitman’s brain before measuring it. It crumbled into pieces like a dried-out cake, and it’s not clear whether they found all of them, so maybe Walt could have made a better showing. (No such mishap with Gall.) Regardless, by the 1950s the size-smarts theory had received some fatal wounds, and any lingering association between brain heft and braininess died for good a few hours after Albert Einstein himself died in 1955.