Best American Magazine Writing 2013

by The American Society of Magazine Editors

  And not just clothing. As scientists have established, a host of remarkable things occurred to our species at about that time. It marked a dividing line in our history, one that made us who we are, and pointed us, for better and worse, toward the world we now have created for ourselves.

  Homo sapiens emerged on the planet about 200,000 years ago, researchers believe. From the beginning, our species looked much as it does today. If some of those long-ago people walked by us on the street now, we would think they looked and acted somewhat oddly, but not that they weren’t people. But those anatomically modern humans were not, as anthropologists say, behaviorally modern. Those first people had no language, no clothing, no art, no religion, nothing but the simplest, unspecialized tools. They were little more advanced, technologically speaking, than their predecessors—or, for that matter, modern chimpanzees. (The big exception was fire, but that was first controlled by Homo erectus, one of our ancestors, a million years ago or more.) Our species had so little capacity for innovation that archaeologists have found almost no evidence of cultural or social change during our first 100,000 years of existence. Equally important, for almost all that time these early humans were confined to a single, small area in the hot, dry savanna of East Africa (and possibly a second, still smaller area in southern Africa).

  But now jump forward 50,000 years. East Africa looks much the same. So do the humans in it—but suddenly they are drawing and carving images, weaving ropes and baskets, shaping and wielding specialized tools, burying the dead in formal ceremonies, and perhaps worshipping supernatural beings. They are wearing clothes—lice-filled clothes, to be sure, but clothes nonetheless. Momentously, they are using language. And they are dramatically increasing their range. Homo sapiens is exploding across the planet.

  What caused this remarkable change? By geologists’ standards, 50,000 years is an instant, a finger snap, a rounding error. Nonetheless, most researchers believe that in that flicker of time, favorable mutations swept through our species, transforming anatomically modern humans into behaviorally modern humans. The idea is not absurd: in the last 400 years, dog breeders converted village dogs into creatures that act as differently as foxhounds, border collies, and Labrador retrievers. Fifty millennia, researchers say, is more than enough to make over a species.

  Homo sapiens lacks claws, fangs, or exoskeletal plates. Rather, our unique survival skill is our ability to innovate, which originates with our species’ singular brain—a three-pound universe of hyperconnected neural tissue, constantly aswirl with schemes and notions. Hence every hypothesized cause for the transformation of humankind from anatomically modern to behaviorally modern involves a physical alteration of the wet gray matter within our skulls. One candidate explanation is that in this period people developed hybrid mental abilities by interbreeding with Neanderthals. (Some Neanderthal genes indeed appear to be in our genome, though nobody is yet certain of their function.) Another putative cause is symbolic language—an invention that may have tapped latent creativity and aggressiveness in our species. A third is that a mutation might have enabled our brains to alternate between spacing out on imaginative chains of association and focusing our attention narrowly on the physical world around us. The former, in this view, allows us to come up with creative new strategies to achieve a goal, whereas the latter enables us to execute the concrete tactics required by those strategies.

  Each of these ideas is fervently advocated by some researchers and fervently attacked by others. What is clear is that something made over our species between 100,000 and 50,000 years ago—and right in the middle of that period was Toba.

  Children of Toba

  About 75,000 years ago, a huge volcano exploded on the island of Sumatra. The biggest blast for several million years, the eruption created Lake Toba, the world’s biggest crater lake, and ejected the equivalent of as much as 3,000 cubic kilometers of rock, enough to cover the District of Columbia in a layer of magma and ash that would reach to the stratosphere. A gigantic plume spread west, enveloping southern Asia in tephra (rock, ash, and dust). Drifts in Pakistan and India reached as high as six meters. Smaller tephra beds blanketed the Middle East and East Africa. Great rafts of pumice filled the sea and drifted almost to Antarctica.

  In the long run, the eruption raised Asian soil fertility. In the short term, it was catastrophic. Dust hid the sun for as much as a decade, plunging the earth into a years-long winter accompanied by widespread drought. A vegetation collapse was followed by a collapse in the species that depended on vegetation, followed by a collapse in the species that depended on the species that depended on vegetation. Temperatures may have remained colder than normal for a thousand years. Orangutans, tigers, chimpanzees, cheetahs—all were pushed to the verge of extinction.

  At about this time, many geneticists believe, Homo sapiens’ numbers shrank dramatically, perhaps to a few thousand people—the size of a big urban high school. The clearest evidence of this bottleneck is also its main legacy: humankind’s remarkable genetic uniformity. Countless people have viewed the differences between races as worth killing for, but compared to other primates—even compared to most other mammals—human beings are almost indistinguishable, genetically speaking. DNA is made from exceedingly long chains of “bases.” Typically, about one out of every 2,000 of these “bases” differs between one person and the next. The equivalent figure from two E. coli (human gut bacteria) might be about one out of twenty. The bacteria in our intestines, that is, have a hundredfold more innate variability than their hosts—evidence, researchers say, that our species is descended from a small group of founders.

  Uniformity is hardly the only effect of a bottleneck. When a species shrinks in number, mutations can spread through the entire population with astonishing rapidity. Or genetic variants that may have already been in existence—arrays of genes that confer better planning skills, for example—can suddenly become more common, effectively reshaping the species within a few generations as once-unusual traits become widespread.

  Did Toba, as theorists like Richard Dawkins have argued, cause an evolutionary bottleneck that set off the creation of behaviorally modern people, perhaps by helping previously rare genes—Neanderthal DNA or an opportune mutation—spread through our species? Or did the volcanic blast simply clear away other human species that had previously blocked H. sapiens’ expansion? Or was the volcano irrelevant to the deeper story of human change?

  For now, the answers are the subject of careful back-and-forth in refereed journals and heated argument in faculty lounges. All that is clear is that about the time of Toba, new, behaviorally modern people charged so fast into the tephra that human footprints appeared in Australia within as few as 10,000 years, perhaps within 4,000 or 5,000. Stay-at-home Homo sapiens 1.0, a wallflower that would never have interested Lynn Margulis, had been replaced by aggressively expansive Homo sapiens 2.0. Something happened, for better and worse, and we were born.

  One way to illustrate what this upgrade looked like is to consider Solenopsis invicta, the red imported fire ant. Geneticists believe that S. invicta originated in northern Argentina, an area with many rivers and frequent floods. The floods wipe out ant nests. Over the millennia, these small, furiously active creatures have acquired the ability to respond to rising water by coalescing into huge, floating, pullulating balls—workers on the outside, queen in the center—that drift to the edge of the flood. Once the waters recede, colonies swarm back into previously flooded land so rapidly that S. invicta actually can use the devastation to increase its range.

  In the 1930s, Solenopsis invicta was transported to the United States, probably in ship ballast, which often consists of haphazardly loaded soil and gravel. As a teenaged bug enthusiast, Edward O. Wilson, the famed biologist, spotted the first colonies in the port of Mobile, Alabama. He saw some very happy fire ants. From the ant’s point of view, it had been dumped into an empty, recently flooded expanse. S. invicta took off, never looking back.

  The initial incursio
n watched by Wilson was likely just a few thousand individuals—a number small enough to suggest that random, bottleneck-style genetic change played a role in the species’ subsequent history in this country. In their Argentine birthplace, fire-ant colonies constantly fight each other, reducing their numbers and creating space for other types of ant. In the United States, by contrast, the species forms cooperative supercolonies, linked clusters of nests that can spread for hundreds of miles. Systematically exploiting the landscape, these supercolonies monopolize every useful resource, wiping out other ant species along the way—models of zeal and rapacity. Transformed by chance and opportunity, new-model S. invictus needed just a few decades to conquer most of the southern United States.

  Homo sapiens did something similar in the wake of Toba. For hundreds of thousands of years, our species had been restricted to East Africa (and, possibly, a similar area in the south). Now, abruptly, new-model Homo sapiens were racing across the continents like so many imported fire ants. The difference between humans and fire ants is that fire ants specialize in disturbed habitats. Humans, too, specialize in disturbed habitats—but we do the disturbing.

  The World Is a Petri Dish

  As a student at the University of Moscow in the 1920s, Georgii Gause spent years trying—and failing—to drum up support from the Rockefeller Foundation, then the most prominent funding source for non-American scientists who wished to work in the United States. Hoping to dazzle the foundation, Gause decided to perform some nifty experiments and describe the results in his grant application.

  By today’s standards, his methodology was simplicity itself. Gause placed half a gram of oatmeal in one hundred cubic centimeters of water, boiled the results for ten minutes to create a broth, strained the liquid portion of the broth into a container, diluted the mixture by adding water, and then decanted the contents into small, flat-bottomed test tubes. Into each he dripped five Paramecium caudatum or Stylonychia mytilus, both single-celled protozoans, one species per tube. Each of Gause’s test tubes was a pocket ecosystem, a food web with a single node. He stored the tubes in warm places for a week and observed the results. He set down his conclusions in a 163-page book, The Struggle for Existence, published in 1934.

  Today The Struggle for Existence is recognized as a scientific landmark, one of the first successful marriages of theory and experiment in ecology. But the book was not enough to get Gause a fellowship; the Rockefeller Foundation turned down the twenty-four-year-old Soviet student as insufficiently eminent. Gause could not visit the United States for another twenty years, by which time he had indeed become eminent, but as an antibiotics researcher.

  What Gause saw in his test tubes is often depicted in a graph, time on the horizontal axis, the number of protozoa on the vertical. The line on the graph is a distorted bell curve, with its left side twisted and stretched into a kind of flattened S. At first the number of protozoans grows slowly, and the graph line slowly ascends to the right. But then the line hits an inflection point, and suddenly rockets upward—a frenzy of exponential growth. The mad rise continues until the organism begins to run out of food, at which point there is a second inflection point, and the growth curve levels off again as bacteria begin to die. Eventually the line descends, and the population falls toward zero.

  Years ago I watched Lynn Margulis, one of Gause’s successors, demonstrate these conclusions to a class at the University of Massachusetts with a time-lapse video of Proteus vulgaris, a bacterium that lives in the gastrointestinal tract. To humans, she said, P. vulgaris is mainly notable as a cause of urinary-tract infections. Left alone, it divides about every fifteen minutes. Margulis switched on the projector. Onscreen was a small, wobbly bubble—P. vulgaris—in a shallow, circular glass container: a petri dish. The class gasped. The cells in the time-lapse video seemed to shiver and boil, doubling in number every few seconds, colonies exploding out until the mass of bacteria filled the screen. In just thirty-six hours, she said, this single bacterium could cover the entire planet in a foot-deep layer of single-celled ooze. Twelve hours after that, it would create a living ball of bacteria the size of the earth.

  Such a calamity never happens, because competing organisms and lack of resources prevent the overwhelming majority of P. vulgaris from reproducing. This, Margulis said, is natural selection, Darwin’s great insight. All living creatures have the same purpose: to make more of themselves, ensuring their biological future by the only means available. Natural selection stands in the way of this goal. It prunes back almost all species, restricting their numbers and confining their range. In the human body, P. vulgaris is checked by the size of its habitat (portions of the human gut), the limits to its supply of nourishment (food proteins), and other, competing organisms. Thus constrained, its population remains roughly steady.

  In the petri dish, by contrast, competition is absent; nutrients and habitat seem limitless, at least at first. The bacterium hits the first inflection point and rockets up the left side of the curve, swamping the petri dish in a reproductive frenzy. But then its colonies slam into the second inflection point: the edge of the dish. When the dish’s nutrient supply is exhausted, P. vulgaris experiences a miniapocalypse.

  By luck or superior adaptation, a few species manage to escape their limits, at least for a while. Nature’s success stories, they are like Gause’s protozoans; the world is their petri dish. Their populations grow exponentially; they take over large areas, overwhelming their environment as if no force opposed them. Then they annihilate themselves, drowning in their own wastes or starving from lack of food.

  To someone like Margulis, Homo sapiens looks like one of these briefly fortunate species.

  The Whip Hand

  No more than a few hundred people initially migrated from Africa, if geneticists are correct. But they emerged into landscapes that by today’s standards were as rich as Eden. Cool mountains, tropical wetlands, lush forests—all were teeming with food. Fish in the sea, birds in the air, fruit on the trees: breakfast was everywhere. People moved in.

  Despite our territorial expansion, though, humans were still only in the initial stages of Gause’s oddly shaped curve. Ten thousand years ago, most demographers believe, we numbered barely 5 million, about one human being for every hundred square kilometers of the earth’s land surface. Homo sapiens was a scarcely noticeable dusting on the surface of a planet dominated by microbes. Nevertheless, at about this time—10,000 years ago, give or take a millennium—humankind finally began to approach the first inflection point. Our species was inventing agriculture.

  The wild ancestors of cereal crops like wheat, barley, rice, and sorghum have been part of the human diet for almost as long as there have been humans to eat them. (The earliest evidence comes from Mozambique, where researchers found tiny bits of 105,000-year-old sorghum on ancient scrapers and grinders.) In some cases people may have watched over patches of wild grain, returning to them year after year. Yet despite the effort and care the plants were not domesticated. As botanists say, wild cereals “shatter”—individual grain kernels fall off as they ripen, scattering grain haphazardly, making it impossible to harvest the plants systematically. Only when unknown geniuses discovered naturally mutated grain plants that did not shatter—and purposefully selected, protected, and cultivated them—did true agriculture begin. Planting great expanses of those mutated crops, first in southern Turkey, later in half a dozen other places, early farmers created landscapes that, so to speak, waited for hands to harvest them.

  Farming converted most of the habitable world into a petri dish. Foragers manipulated their environment with fire, burning areas to kill insects and encourage the growth of useful species—plants we liked to eat, plants that attracted the other creatures we liked to eat. Nonetheless, their diets were largely restricted to what nature happened to provide in any given time and season. Agriculture gave humanity the whip hand. Instead of natural ecosystems with their haphazard mix of species (so many useless organisms guzzling up resources!), farms ar
e taut, disciplined communities conceived and dedicated to the maintenance of a single species: us.

  Before agriculture, the Ukraine, American Midwest, and lower Yangzi were barely hospitable food deserts, sparsely inhabited landscapes of insects and grass; they became breadbaskets as people scythed away suites of species that used soil and water we wanted to dominate and replaced them with wheat, rice, and maize (corn). To one of Margulis’s beloved bacteria, a petri dish is a uniform expanse of nutrients, all of which it can seize and consume. For Homo sapiens, agriculture transformed the planet into something similar.

  As in a time-lapse movie, we divided and multiplied across the newly opened land. It had taken Homo sapiens 2.0, behaviorally modern humans, not even 50,000 years to reach the farthest corners of the globe. Homo sapiens 2.0.A—A for agriculture—took a tenth of that time to conquer the planet.

  As any biologist would predict, success led to an increase in human numbers. Homo sapiens rocketed around the elbow of the first inflection point in the seventeenth and eighteenth centuries, when American crops like potatoes, sweet potatoes, and maize were introduced to the rest of the world. Traditional Eurasian and African cereals—wheat, rice, millet, and sorghum, for example—produce their grain atop thin stalks. Basic physics suggests that plants with this design will fatally topple if the grain gets too heavy, which means that farmers can actually be punished if they have an extra-bounteous harvest. By contrast, potatoes and sweet potatoes grow underground, which means that yields are not limited by the plant’s architecture. Wheat farmers in Edinburgh and rice farmers in Edo alike discovered they could harvest four times as much dry food matter from an acre of tubers than they could from an acre of cereals. Maize, too, was a winner. Compared to other cereals, it has an extra-thick stalk and a different, more productive type of photosynthesis. Taken together, these immigrant crops vastly increased the food supply in Europe, Asia, and Africa, which in turn helped increase the supply of Europeans, Asians, and Africans. The population boom had begun.