But there’s a third sense of “human,” one captured in the phrase of “being human.” Human-ness is the quality—a mix of creativity, drive, and moral awareness—that transforms humans into persons. It is a special spark or spirit, unique among living creatures, a flame possessed in abundance by our heroes, possessed in small amounts by all. It is what makes Homo sapiens want to believe they are special, to believe they are unlike the other members of the genus Homo.
Humans weren’t always human in this third sense, as far as we can tell. In the beginning, Homo sapiens seems not to have created art, played music, invented new tools, worked out the motions of the planets, or worshiped gods in the celestial sphere. These capacities accumulated slowly, over tens of thousands of years. Sometimes a new trait—a new kind of art, a new kind of construction—arose, only to fade out. But over the long run, as the other human species disappeared, these attributes built up in us, until perhaps fifty thousand years ago something resembling modern humankind—“behaviorally modern” humans, in the jargon—was loose in the world. Only as humans were reduced to humankind did humans gain their humanity. And only then did we march out of Africa, a conquering horde, carrying our lice into every corner of the world.
This army, the human army, was an army of similars, its soldiers remarkable in their genetic uniformity. DNA, the material of which genes are made, consists of long, skinny, string-shaped molecules. Each molecule is composed of two chains that are twisted around themselves to create the famous double helix. Individual links in the chains are called “bases” or “nucleotides.” Arrays of links—segments of the DNA chain, so to speak—form individual genes. The totality of the genetic information in an individual or species is that individual or species’ “genome.” The lineup of bases and genes in one person—that person’s genome—is barely distinguishable from the lineup in the next person. This similarity is striking to geneticists but hard to describe exactly. Roughly speaking, two peoples’ genomes differ in only about one out of every thousand bases. This is like having two pages in two different books differ by a single letter. The equivalent figure for two Escherichia coli, the most common bacterium in the human gut, might be one out of fifty. By this measure, the bacteria in people’s intestines are twenty times more diverse than their hosts.
These comparisons are incomplete. In addition to differences in single bases, organisms also vary in terms of duplicated or deleted segments of DNA. These disparities are larger than single bases and usually more important. They are also hard to quantify: if one member of a species has ten copies of a particular gene variant and another has twenty copies of that variant, are they alike because they have the same gene variant or different because they have different numbers of it? Still, the overall point remains: compared to bacteria, humans are genetically similar to the point of tedium. Bacteria may not be the best comparison. They are so diverse genetically that researchers into the microworld often object to classifying them as a “species,” because that implies they share single, identifiable pools of DNA. It may be better to look at mammals, which are closer to us. In general, apes are far on the low end of mammal diversity, and humans are less diverse than almost all apes. The genetic differences between one chimpanzee and its neighbor on a single hillside in central Africa can be greater than those of two humans in central Asia and Central America. When scientists list mammals in order of their genetic diversity, humans are at the bottom, along with endangered species like wolverines and lynxes.
Genetic uniformity is usually a legacy of small population size—the descendants of a small group have only the genes bequeathed to them by their few founders. Reasoning backward from humankind’s sparsely filled genetic larder, some researchers have argued that at some point our numbers must have fallen dramatically, perhaps to a breeding population of as few as ten thousand people—the size of a midsize university. (The actual population would have been bigger; this estimate is of the number of people who successfully produced children.)
When a species shrinks in number, chance can alter its genetic makeup with astonishing rapidity. New mutations can arise and spread; a snippet of scrambled DNA in a single gene in a single member of the small group that populated Ice Age Europe apparently led to the blue eyes that predominate in most of Scandinavia. Rare genetic variants that are already present can suddenly become more common, effectively transforming the species within a few generations as once-unusual traits proliferate. Or common genetic variants can, by chance, fall to the wayside. For these and other reasons, researchers have often speculated that in the short span of some tens of thousands of years—a lightning flash in the history of life—something happened in our DNA, something unprecedented, something special, something that made us human. Changes accelerated. And about seventy thousand years ago, perhaps a bit less, our species took a fateful step.
One way to illustrate the impact of this change is to consider Solenopsis invicta, the red imported fire ant. Geneticists believe that S. invicta originated in southern Brazil, an area with many rivers and frequent floods. The floods wipe out ant nests. Over the eons, these small, furiously active creatures have evolved the ability to respond to rising water by knitting their bodies together into floating swarm-balls—workers on the outside, queen in the center—that can ride on the flood for days. Once the waters recede, colonies swarm back onto previously submerged land so rapidly that S. invicta can use the devastation to increase its range. Like criminal gangs, fire ants thrive on chaos.
In the 1930s Solenopsis invicta was transported to the United States, probably in ship ballast, which often consists of haphazardly loaded soil and gravel. An adolescent bug enthusiast named Edward O. Wilson, later a famous biologist, spotted the first colonies in the port of Mobile, Alabama. From the ant’s vantage, it had been dumped onto an empty, recently flooded expanse. S. invicta took off, never looking back.
More than likely, the initial incursion seen by Wilson was just a few thousand individuals—a number small enough to hint that random, bottleneck-style genetic change played a role in what happened next. (The evidence is not yet conclusive.) In its homeland, fire ant colonies constantly fight each other, reducing their numbers and creating space for other types of ant. In North America, by contrast, the species forms cooperative super-colonies, linked clusters of nests that can spread for hundreds of miles, wiping out competitors along the way. Remade by chance and opportunity, new-model S. invictus needed just a few decades to conquer much of the southern United States.
A primary obstacle to its expansion is another imported South American ant, Linepithema humile, the Argentine ant. After escaping its natal territory more than a century ago, L. humile formed its own super-colonies in the United States, Australia, New Zealand, Japan, and Europe (the European colony stretches from Portugal to Italy). In recent years researchers have come to believe that these huge, geographically separate ant societies in fact may be part of a single intercontinental unit, a globe-spanning entity that exploded across the planet with extraordinary speed and rapacity, and is now the most populous society on Earth.
Homo sapiens did something similar as it became human. Our species first clearly appears in the archaeological record about 300,000 years ago (though we may well have emerged before then). Until about 75,000 years ago—that is, for the majority of our existence on Earth—humankind was restricted to Africa, though we sent out occasional forays into the rest of the world, almost all unsuccessful, all limited in scope. Around 70,000 years ago, everything changed. People raced across the continents like so many imported fire ants. Human footprints appeared in Australia within as few as ten thousand years, perhaps within four or five thousand. 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.
No more than a few hundred people initially left Africa, if geneticists are correct. And for a long time their geographic expans
ion was not matched by an increase in population. As recently as ten thousand years ago we numbered perhaps 5 million, about one human being for every five square miles of Earth’s habitable surface. Homo sapiens was a scarcely noticeable dusting on the surface of a planet dominated by microbes.
At about this time—10,000 years ago, give or take a millennium—our species swung around the first inflection point, with the invention of 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 put it, wild cereals “shatter”—individual grain kernels fall off as they ripen, making it impossible to harvest the plants systematically. Only when an unknown genius discovered naturally mutated grain plants that did not shatter—and purposefully selected, protected, and cultivated them—did true agriculture begin. Planting great expanses of these altered crops, first in southern Turkey, later in almost a dozen other places, early farmers created landscapes that, so to speak, waited for hands to harvest them.
Farming transformed our relationship to nature. 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 the world 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, farms are taut, disciplined communities dedicated to the maintenance of a single species: us. Before agriculture, the Middle West, Ukraine, and the lower Yangzi Valley had been sparsely populated domains of insects and grass; they became breadbaskets, as people scythed away suites of species that used soil and water we wanted to control and replaced them with maize, wheat, and rice. To Margulis’s bacteria, a petri dish is a uniform expanse of nutrients, all ready for the taking. For Homo sapiens, agriculture transformed the planet into something like a petri dish.
As in a time-lapse movie, we divided and multiplied across the newly opened land. It had taken Homo sapiens 2.0, aggressively modern humans, barely fifty thousand years to reach the farthest corners of the globe. Homo sapiens 2.0A—A for agriculture—took a tenth of that time to subdue the planet.
Since the beginning of agriculture, farmers have plowed manure and compost into their soil to promote plant growth. They didn’t know it, but the chief reason manure and compost helped their crops was that they replenished a key plant nutrient, the nitrogen in the soil. But this method of recharging the soil had drawbacks. In most places, the supply of manure and compost was limited; importing them from elsewhere was impossibly expensive.
In the early twentieth century, two German chemists, Fritz Haber and Carl Bosch, discovered the key steps to making synthetic fertilizer. Suddenly farmers could go to a store and buy all the fertilizer they wanted—factory-made, cheap, and plentiful. Haber and Bosch are not nearly as well known as they should be; their discoveries, linked into what is called the Haber-Bosch process, have literally changed the chemical composition of the earth. Farmers have injected so much synthetic fertilizer into their fields that soil and groundwater nitrogen levels have risen worldwide. Today, almost half of all the crops consumed by humankind depend on nitrogen derived from synthetic fertilizer. Another way of putting this is to say that Haber and Bosch enabled our species to extract an additional 3 billion people’s worth of food from the same land.
Synthetic fertilizer is not alone in its impact. The improved wheat, rice, and (to a lesser extent) maize varieties developed by Borlaug and other plant breeders in the 1950s and 1960s drove up yields greatly. Antibiotics, vaccines, disinfectants, and water-treatment plants pushed back humankind’s bacterial, viral, fungal, and protozoan enemies. All allowed humankind ever more unhindered access to the planet.
Rocketing up the growth curve, humankind every year takes ever more of the earth’s richness. An often quoted estimate by a team of Stanford biologists is that humans grab “about 40% of the present net primary production in terrestrial ecosystems”—40 percent of the entire world’s output of land plants and animals. This assessment dates from 1986. Ten years later, a second Stanford team calculated that the figure had risen to “39 to 50%.” (Others have suggested a figure closer to 25 percent, still very high for a single species.) In 2000, the chemist Paul Crutzen and the biologist Eugene Stoermer awarded a name to our time: the Anthropocene, the era in which Homo sapiens became a force operating on a planetary scale.
Lynn Margulis, it seems safe to say, would have rolled her eyes at these statements, which in every case that I am aware of do not take into account the enormous impact of the microworld. But she would not have disputed the central idea: Homo sapiens has become a successful species.
As any biologist would predict, this success led to an increase in human numbers—slow at first, then rapid, tracing Gause’s S-shaped curve. We began rising up the steepest part of the slope in the sixteenth or seventeenth century. If we follow Gause’s pattern, growth will continue at delirious speed until the second inflection point, when we have exhausted the global petri dish. After that, human life will be, briefly, a Hobbesian nightmare, the living overwhelmed by the dead. When the king falls, so do his minions; it is possible that in our desperation we might consume most of the world’s mammals and many of its plants. Sooner or later, in this scenario, Earth will again be a choir of bacteria, fungi, and insects, as it has been through most of its history.
It would be foolish to expect anything else, Margulis thought. More than that, it would be strange. To avoid destroying itself, the human race would have to do something deeply unnatural, something no other species has ever done or could ever do: constrain its own growth (at least in some ways). Brown tree snakes in Guam, water hyacinth in African rivers, rabbits in Australia, Burmese pythons in Florida—all these successful species have overrun their environments, heedlessly wiping out other creatures. Not one has voluntarily turned back. When the zebra mussels in the Hudson River began to run out of food, they did not stop reproducing. When fire ants relentlessly expand their range, no inner voices warn them to consider the future. Why should we expect Homo sapiens to fence itself in?
What a peculiar thing to ask! Economists talk about the “discount rate,” which is their term for the way that humans almost always value the local, concrete, and immediate over the faraway, abstract, and distant in time. We care more about the broken stoplight up the street now than social unrest next year in Chechnya, Cambodia, or the Congo. Rightly so, evolutionists say: Americans are far more likely to be killed at that stoplight today than in the Congo next year. Yet here we are asking governments to focus on potential planetary boundaries that may not be reached for decades or even centuries. Given the discount rate, nothing could be more understandable than a government’s failure to grapple with, say, climate change. From this perspective, is there any reason to imagine that Homo sapiens, unlike mussels, snakes, and moths, can exempt itself from the fate of all successful species? This is what Borlaug and Vogt were asking people to do in their very different ways.
To a biologist like Margulis, who spent her career arguing that humans are simply part of evolution’s handiwork, the answer should be clear. All life is similar at base, she and others say. All species seek to make more of themselves—that is their goal. By multiplying until we reach our maximum possible numbers, we are following the laws of biology, even as we take out much of the planet. Eventually, in accordance with those same laws, the human enterprise will wipe itself out. Shouting from the edge of the petri dish, Borlaug and Vogt might as well be trying to hold back the tide.
From this standpoint, the answer to t
he question “Are we doomed to destroy ourselves?” is “Yes.” That we could be some sort of magical exception—it seems unscientific. Why should we be different? Is there any evidence that we are special?
* * *
*1 I use “maize,” rather than “corn,” because Mexican maize—often multicolored and mainly eaten after drying and grinding—is strikingly unlike the sweet, yellow kernels evoked in the United States by the name “corn.”
*2 The two types of bacteria are bacteria proper and archaea, which resemble bacteria but don’t have a cell nucleus. “Protist” is a catch-all category for everything else—anything living that is not an animal, plant, fungus, or bacterium. Examples include amoebas, slime molds, and single-celled algae. Viruses are not usually included in these lists, because they are so simple that most biologists don’t view them as a form of life.
TWO MEN
[ TWO ]
The Prophet
Heaps of Nitrogen
In the southern Pacific is a great circular moil of trade winds and currents, the South Pacific Gyre, which rotates counterclockwise between New Zealand and the western coast of South America. The piece of the gyre that runs along the South American coast is known as the Humboldt Current. Its near-constant winds, dragging against the shore, push away the warm surface water, drawing colder water up from below. The upwelling water is thick with nutrients: organic matter that has sloughed off the coast and sunk to the ocean floor. The soup feeds huge blooms of plankton, which in turn feed great schools of fish, especially mackerel, sardines, and anchovetas (one of the many species of anchovy). By some measures the Humboldt Current is the most productive marine ecosystem on Earth.
The richness of the ocean is not matched on land. To the east, the Andes Mountains shield the shore from the warm, wet winds that blow in from Brazil; to the west, the Humboldt Current is so cold that the air above it cannot hold much moisture. Sandwiched between these two barriers, the Peruvian coast is dry to the point of desolation. In many places it receives less than an inch of rain per year. Equally barren are Peru’s thirty-nine offshore islands. Hot, small, and almost waterless, they are unsuitable for human habitation. But the abundant anchovetas and sardines in the Humboldt Current make the islands attractive to seabirds, which have roosted there for millennia. Like all living things, the seabirds excrete wastes. The parched islands rarely receive enough rain to dissolve them. Over time the bird feces—guano—has accumulated into deposits as much as 150 feet high.