The Wizard and the Prophet2

by Charles C. Mann

  Instead, what is needed, above and beyond all else, is a change in our relationship with Nature. If people understood the value of the ecosystems in which they are embedded, society would be profoundly different. In the past, Mexico could exist with this incorrect understanding of the world, but soon there will be no more margin for error. The city is rushing in to cover the land. Matters must change in the next few decades. “It is doubtful that in the entire Western Hemisphere there exists a problem that is more important or more pressing,” Vogt says.

  His letter to the foundation begins a long argument that continues to the present day.

  The World Is a Petri Dish

  Nonsense! I hear Lynn Margulis say. Poppycock! Or, rather, I hear her say something more pungent.

  A researcher who specialized in cells and microorganisms, Margulis was one of the most important biologists in the last half century—she literally helped to reorder the tree of life, convincing her colleagues that it did not consist of two kingdoms (plants and animals), but five or even six (plants, animals, fungi, protists, and two types of bacteria).*2 Until her death in 2011, she lived in my town, and I would bump into her on the street from time to time. She knew I was interested in environmental issues, and she liked to needle me. Hey, Charles, she would call out, are you still all worked up about protecting endangered species?

  Margulis was no apologist for unthinking destruction. Still, she couldn’t help regarding conservationists’ fixation on birds, mammals, and plants as evidence of their ignorance about the greatest source of evolutionary creativity: the microworld of bacteria, fungi, and protists. More than 90 percent of the living matter on Earth consists of microorganisms, she liked to remind people. Heck, there are as many bacterial cells in our body as there are human cells!

  Lynn Margulis, 1990 Credit 3

  Bacteria and protists can do things undreamed of by clumsy mammals like us: form giant super-colonies, reproduce either asexually or by swapping genes with others, take in genes from entirely unrelated species, merge into symbiotic beings—the list is as endless as it is amazing. Microorganisms have changed the face of the earth, crumbling stone and even giving rise to the oxygen we breathe. Compared to this power and diversity, Margulis liked to tell me, pandas and polar bears were epiphenomena—interesting and fun, perhaps, but not actually significant.

  I never told her of my image of the two men in Mexico, but I am quite sure what she would have said about it. Homo sapiens, she once told me, is an unusually successful species. And it is the fate of every successful species to wipe itself out—that is the way things work in biology. By “wipe itself out” Margulis didn’t necessarily mean extinction—just that something comprehensively bad would happen, wrecking the human enterprise. Borlaug and Vogt might have wanted to stop us from destroying ourselves, she would have said, but they were kidding themselves. Neither conservation nor technology has anything to do with biological reality.

  Margulis explained these ideas to me while talking about one of her scientific heroes, the Russian microbiologist Georgii Gause. Born in 1910, Gause was a prodigy: he published his first scientific article at the age of nineteen (it appeared in Ecology, the premier journal in the field). Like Vogt, Gause looked with envy at Rockefeller’s funds, so much greater than anything available to him in the Soviet Union. Hoping to impress the foundation, he decided to perform some experiments and include the results in a grant application.

  Gause knew just what to do. In 1920, two Johns Hopkins biologists, Raymond Pearl and Lowell Reed, had published a mathematical formula that described the rate at which the population of the United States grew over time. Their argument was almost completely theoretical. They imagined what the rate of growth should look like, given their knowledge of biology, and sought to match their hypothetical curve to the actual population of the United States as recorded in census data. The two matched well enough that Pearl and Reed believed they were on to something. Pearl was especially excited; he had been conducting parallel research with fruit flies, locking a male and female in a bottle full of food and observing how many flies would be produced in the next few generations. The results looked so similar to his U.S. Census data that he was convinced that he had found a universal law, applicable to fruit flies in bottles and humans in North America alike. “The growth of populations of the most diverse organisms,” he said, “follows a regular and characteristic course.”

  An expert in self-publicity, Pearl proclaimed the new law in a dozen articles and three books. But the onslaught failed to prevent critics from attacking his ideas. Pearl had begun, the critics said, by assuming his hypothesis might be true, then looking for a match in his data; when he found one, he claimed that the match proved him correct. Pearl’s detractors argued that this procedure missed an essential step: demonstrating that no other hypotheses also fit the data. Worse, the law didn’t work very well—Pearl had to wave his hands at the numbers to make them come out right.

  To win Pearl’s support for a Rockefeller grant, Gause decided to try to nail down the case with a series of experiments on fruit flies. He soon discovered that the flies moved around so much that they were hard to count. To obtain better results, Gause decided to work with microorganisms. These could be spread across a microscope slide and counted.

  By today’s standards, his methodology was simplicity itself. Gause placed half a gram—that is, just a pinch—of oatmeal in one hundred milliliters (about three ounces) 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. He stored the tubes for a week and observed the results. The conclusions appeared in a 163-page book, The Struggle for Existence, published in 1934.

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

  What Gause saw in his test tubes—and what Pearl had theorized before him—is often depicted in a graph, time on the horizontal axis, the number of protozoa on the vertical. By squinting a bit, it is possible to imagine that the curve forms a kind of flattened S, which is why scientists often refer to Gause’s curve as an “S-shaped curve.” At the beginning (that is, the left side of the S-shaped curve), 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 growth. The mad rise continues until the organism begins to run out of food, at which time 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.

  One of Gause’s diagrams of his S-shaped curve, with labels modified by the author Credit 4

  Years ago I watched Margulis demonstrate Gause’s conclusions to one of her classes with a time-lapse video of Proteus vulgaris, a bacterium that resides in the intestinal tract. To humans, she said, P. vulgaris is mainly notable as an occasional cause of hospital infections. Left alone, it divides about every fifteen minutes, producing two individuals where before had been one. Margulis switched on the projector. Onscreen was a tiny dot—P. vulgaris—in a shallow, circular glass container: a petri dish, its bottom covered with a layer of reddish nutrient goo. The students gasped. In the time-lapse video, the colony seemed to pulse, doubling in size every few seconds, rippling outward 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 hou
rs after that, the ball of living cells would be the size of Earth.

  Such a calamity cannot happen, Margulis said, because rival organisms and lack of resources prevent the vast majority of P. vulgaris from reproducing. This 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. And all living creatures have a maximum reproductive rate: the greatest number of offspring they can generate in a lifetime. (For people, she told the class, the maximum reproductive rate is about twenty children per couple per generation. The potential maximum for dachshunds is around 330: eleven pups per litter, three litters a year, for roughly ten years.) Natural selection ensures that only a few members of each generation manage to reach this rate. Many individuals do not reproduce at all; blocked, they fall by the wayside. “Differential survival is really all there is to natural selection,” Margulis said. 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 microbes. Thus constrained, its population remains roughly steady.

  Things are different in the petri dish. From P. vulgaris’s position, the dish initially seems limitless, a boundless ocean of breakfast, no storm on the horizon, no competition for sustenance. The bacterium eats and divides, eats and divides. Racing across the nutrient goo, it passes the first inflection point and hurtles up the left side of the curve. But then its colonies slam into the second inflection point: the edge of the petri dish. When the food supply is exhausted, P. vulgaris experiences a vest-pocket apocalypse.

  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 at a terrific rate; they take over large areas, engulfing their environment as if no force opposed them. Then they hit a barrier. They drown in their own wastes. They starve from lack of food. Something figures out how to eat them.

  When I lived in New York City, zebra mussels invaded the lower Hudson River, the western boundary of Manhattan island. An inch or two long, their shells patterned with wriggly bands of brown and white, zebra mussels are capable of spitting out a million eggs a year apiece. The species originated in the Azov, Black, and Caspian seas on Europe’s Russian- and Turkic-speaking periphery. Globalization has been good to it. Escaping their native waters, zebra mussels hitchhiked around the world in ship bilges and ballast water. They have been recorded in Europe since the eighteenth century. The Hudson first saw them in 1991. Within a year zebra mussels constituted half the mass of living creatures in the river. In some places tens of thousands carpeted every square foot. They covered boat bottoms, blocked intake tubes, literally smothered other species of shellfish with a blanket of striped shell. Zebra mussels were shooting up the S-shaped curve.

  Bust followed boom; the population collapsed. In 2011, two decades after the mussel was first sighted in the Hudson, its survival rates were “1% or less of those in the early years of the invasion” (the quote is from one long-range study). Unlike Gause’s bacteria, the mussels had not run into a physical wall—the physical world is always more complex than a test tube. They did exhaust their food supply, but they also were attacked by a local predator, the blue crab, which had learned to eat the newcomers. Their S-shaped curve wiggled more than those in Gause’s book, but the result was the same. Fifteen years ago, when I went to a park at the edge of the Hudson, I couldn’t step into the river—the sharp edges of open mussel shells were too thick underfoot. Nowadays at the park the creatures are mostly gone. Children splash happily in the shallows. Crumbled shells lie in the sediment, testament to the mussel’s collapse.

  Humans are no different, Margulis believed. The implication of evolutionary theory is that Homo sapiens is just one creature among many, no different at base than P. vulgaris. We and they are controlled by the same forces, produced by the same processes, subject to the same fate. When Borlaug and Vogt stood on the tract of bad land, looking at the city, they were on the edge of the petri dish. Wizard or Prophet, it didn’t matter. Homo sapiens, in Margulis’s eyes, was just another briefly successful species.

  Of Lice and Men

  Why and how did humankind become “successful”? And what, to an evolutionary biologist, does “success” mean, if self-destruction is part of the definition? Does that self-destruction include the rest of the biosphere? What are human beings, anyway? With more than 7 billion of us crowding the planet, it’s hard to imagine more vital questions.

  One way to begin answering them came to Mark Stoneking in 1999, when he received a notice from his son’s school warning of a lice outbreak in the classroom. Stoneking was a researcher at the Max Planck Institute for Evolutionary Biology, in Leipzig, Germany. He didn’t know much about lice. As a biologist, it was natural for him to noodle around to find information about them. The most common louse to afflict humans, he learned, is an arthropod, Pediculus humanus, which lives on human bodies, as its name suggests. P. humanus has two distinct subspecies: P. humanus capitis, head lice, which feed and live on the scalp; and P. humanus corporis, body lice, which feed on skin but live in clothing. In fact, body lice are so dependent on the protection of clothing that they cannot survive more than a few hours away from it.

  It occurred to Stoneking that the difference between the two subspecies could be used as an evolutionary probe. P. humanus capitis, the head louse, could be an ancient annoyance, because human beings have always had hair for it to infest. But P. humanus corporis, the body louse, must not be especially old, because its dependence on clothing meant that it could not have existed when humans went naked. Humanity’s great cover-up had created a new ecological niche, and some head lice had rushed to fill it. Natural selection thereupon did its magic; a new subspecies arose. While Stoneking couldn’t be sure that this scenario had taken place, it seemed likely. If his idea was correct, then discovering when the body louse diverged from the head louse would provide a rough date for when people first wore clothing.

  With two colleagues, Stoneking measured the difference between snippets of genes in the two louse subspecies. Because genetic material picks up small, random mutations at a roughly constant rate, scientists use the number of differences between two populations to tell how long ago they diverged from a common ancestor—the more differences, the longer the separation. In this case, the body louse seemed to have separated from the head louse about 107,000 years ago. This meant, Stoneking hypothesized, that clothing also dated from about 107,000 years ago.

  The subject was anything but frivolous: donning a garment is a complicated act. Clothing has practical uses—warming the body in cold places, shielding it from the sun in hot places—but it also transforms the appearance of the wearer, something of inescapable interest to a visually oriented species like Homo sapiens. Clothing is ornament and symbol; it separates human beings from their earlier, unself-conscious state. (Animals run, swim, and fly without clothing, but only people can be naked.) The arrival of clothing was a sign that a mental shift had occurred. The human world was becoming a realm of complex, symbolic artifacts.

  It was not only clothing. As scientists have painstakingly established, a host of innovations were occurring around that time. Human beings were engraving pieces of ochre and ostrich shells in southern Africa. They were carving elegant harpoons from bone in central Africa. They were making ornamental beads in northwestern Africa. They were burying the dead with care in the Levant, just across from northeast Africa. They were, in sum, becoming human.

  In these discussions “human” has many meanings. One is scientific: relating to or characteristic of our species, Homo sapiens, a bipedal primate. A second, somewhat different meaning is also scientific: relating to or characteristic of our genus, Homo. (A genus is a group of closely related species.) Today there is little distinction between the two meanings, because the genus Ho
mo contains only one species, H. sapiens. But 300,000 or so years ago, when Homo sapiens emerged, the meanings were different. Several species of Homo—the exact number is as uncertain as the next archaeological find, as the next anthropological quarrel over taxonomy—were scattered around the world. Homo sapiens (us), Homo neanderthalensis (Neanderthals), Homo denisova (Denisovans), Homo naledi, Homo heidelbergensis, Homo floriensis (nicknamed “hobbits,” because of their small stature). All were human. Nobody knows how all these humans behaved when they met, whether they were amicable, antagonistic, or aloof. At least some of these ancient types of humans bred with each other—Homo sapiens with Homo neanderthalensis, for example—leaving scattered traces of their coupling in our genes. But whatever the sequence of interactions, we do know the outcome. For better or worse, only one species of human now walks the planet.