| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
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Counterpoints in Science Humagen The history of life on Earth has been punctuated by several great extinctions in which a large percentage of existing animals and plants were wiped out. The most drastic extermination came at the end of the Permian era about 250 million years ago, when about half the species on the planet were erased, possibly due to a change in climate. The most recent and most publicized holocaust occurred 65 million years ago, at the end of the Cretaceous period, when a large asteroid slammed into the Yucatan, raising lethal clouds of dirt, dust, glass pellets, and water vapor into the atmosphere and either causing or accelerating the demise of the dinosaurs. When can we expect another cosmic disaster? Recently there was some excitement about an asteroid that might, or might not, come close to Earth in 2028. Actually, the next great extinction may already be underway. The exponential growth of the human population is displacing thousands of other life forms. We're cutting down the forests and polluting the water supplies. The Ganges, the holy river of India in which the faithful bathe, is becoming more and more like the human intestinal tract, with counts of coliform bacilli several thousand times greater than a normally acceptable level. It seems the Earth itself is becoming more and more like an extension of the human physiology, with the receding rainforests resembling a balding skull and smog simulating the clouded vision that afflicts aging lenses with cataracts. Despite these images of advancing age, humankind is unquestionably thriving these days, multiplying like the sands of the seashore, just as the Old Testament enjoins us to do. How long can this irrational exuberance continue before the market crashes? Ecologists are prophesying a devastating population collapse, as Homo sapiens outbreeds its food supply. Some predict that an AIDS-like virus spread by droplet infection, like the common cold, will sweep the globe clean of its human infestation. I wonder, though, whether we as a species may not be participating in a drastic change in Earth's ecology comparable to the one created by oxygen-producing microbes about two billion years ago. This was the most dramatic change in the history of life on Earth. When the oxygen level rose, it effectively fumigated the global habitat, killing off most of the bacteria and their sister group, the archaea, that had adapted so well to oxygenless conditions of early Earth. The anaerobic forms that had flourished for so long were mostly done for, though some still survive today, having found low-oxygen refuges by infecting other organisms or living in environments like hot springs or pools with high salt concentration. The oxygen-makers had created their own milieu, in which aerobes could exercise their dominion over the erstwhile dominant anaerobes. King Louis XIV is reported to have said, "L'etat, c'est moi." I am the state. Increasingly, pullulating humanity is consuming the old ecology and creating a new one. Due to the sheer spread and weight of our numbers we are becoming the ecology. Le milieu, c'est nous. Now let's suppose, just suppose, that our present spin up the logarithmic population slope manages to be sustained for another hundred or another thousand years. That's not a happy thought for me or for most readers of California Wild. But it's not easy to see what's going to stop us, despite the gloomy prognostications of the futurists. So far they have managed to be wrong on just about every prediction of imminent doom. Beginning in the 1970s, the Club of Rome, an organization concerned with global issues, repeatedly foresaw widespread famines, which never materialized. Population Bomb author Paul Ehrlich lost a well-publicized bet that prices of essential commodities would rise within ten years. In fact they fell. Our oil reserves were supposed to have run out a couple of decades ago, but the estimate of reserves keeps rising while prices at the pump fall. Obviously, we will run out of these resources some day, but that day of reckoning keeps getting postponed. Various pandemics afflict many millions, especially in the developing world, but still the babies pop out faster than the viruses can kill. We humans have adapted well to terrestrial niches from the tropics to the arctic, from the shores to the Himalayas. We are invading the oceans, the air, and the solar system. Other species must adapt to us and our ways, or die. The mammoth, the giant ground sloth, the elephant bird, the moa, the dodo, the quagga, the Tasmanian wolf are all history, or paleontology. Forest-living insect species are dying like flies. Just as microscopic one-celled critters that got us going so long ago had to learn to live with oxygen, today's and tomorrow's survivors may have to adjust to the pervasive human presence. We can call it humagen. Humagen is definitely toxic to many living things. But numerous species have already made the transition from an ahumagenic atmosphere to one that is now about 20 percent humagen and rising. Domestication of plants and animals is a relatively recent event in world history, occurring only in the past 10,000 years. As human populations expand, the domestic species proliferate in sync. Cattle, sheep, pigs, chickens, dogs, and cats are doing just fine, thank you. Rice, wheat, corn, potatoes, cabbage, and its kin cover ever larger expanses of the planet's thin skin. Animals that have learned to live conjointly with humans are doing all right, too. Rats, mice, raccoons, skunks, pigeons, sparrows, cockroaches, dust mites, and of course the interminable ants will be with us from here to eternity, or to extinction, whichever comes first. There's a whole ecology of the human skin. Thousands of tiny insects and arachnids live on our scalps, in our eye folds, in our axillary and pubic hair. Colonies of bacteria, a whole circus of them, are adapted to the oils and secretions of our face, others to our chest, back, and orifices. Certain fungi specialize in feeding on the calluses and nails of our feet. Multiply a square meter of skin by several billion and a pretty expansive playing field emerges of the myriad species we support. Then we have that long, warm, moist digestive tube that winds from lips to anus. Bacteria such as staph, strep, and spirochetes colonize our mouth and pharynx. Heliobacter cavorts happily in the hydrochloric acid of our stomachs. Trillions of coliforms and assorted parasites feed and multiply in the congenial caverns of our lower bowels. And are happy to go for a swim in the Ganges. A few years ago an Australian researcher tried to count how many different kinds of organisms lived in his backyard in Sydney. Adding up all the animals, birds, plants, insects, worms, spiders, and microorganisms he could identify, he totted up 4,620 different species and guessed that there were thousands more that he had missed. Some experts estimate that our clearcutting may be extinguishing several hundred species of insects, birds, and other forest-dwellers every hour. But we're big enough, at least our collective skin, intestines, and backyards are, to promote the evolution and divergence of many others. Finally, we have pathology, all the disease agents that are exquisitely attuned to our chemistry: the varieties of malarial parasites that infect billions and kill about one million each year. We should be able to produce a few new species of these clever protozoa over the next thousand years or so. They've been able to elude all our weapons of chemical mass destruction and develop resistant strains. It's not such a huge leap to spawn a new species, especially with the encouragement our antimalarials offer to speed up the evolutionary process. Each new drug works for a few years, until a resistant strain of malaria emerges. For a long time, malaria has been champion executioner of humankind, but according to the New England Journal of Medicine (March 5, 1998), our old foe Mycobacterium tuberculosis has now assumed the crown. The World Health Organization estimated that in 1996 there were 8 million new cases of tuberculosis and 3 million deaths from the disease. So variable is susceptibility to TB, however, that 90 percent of those infected never become ill. Like many people my age, who grew up before the era of antitubercular drugs, I have a positive skin test indicating prior infection, but I never had tubercular symptoms or an abnormal chest X-ray. Untold quadrillions of tubercle bacilli live cozily with their human hosts, without biting the lungs that feed them. Mycobacterium tuberculosis consists of many different strains which are evolving rapidly. Some varieties are more infectious than others. The more lethal types have an affinity for people with AIDS, whose immune system is compromised by the HIV virus. During the past year, a new strain was discovered in AIDS patients that is resistant to all three standard antitubercular drugs. As diseases evolve in the complex struggle for survival, so do people. Segments of the human population have developed resistance to tuberculosis over the thousands of generations of exposure, and so the evolutionary chess game will go on for the next century or millennium, as human hosts and their pathogens attack and counterattack. So far we seem to be winning the evolutionary game, but at the cost of the Earth's biodiversity, which is clearly losing. I have extrapolated present trends to visualize a planet increasingly covered and controlled by one species, Homo sapiens, and the animals, plants, fungi, and microorganisms well adapted to coexist with that dominant species. To most this seems a fairly dismal prospect, but there are those among us who truly believe that the more people there are, the better. Unless a new super AIDS or Ebola virus leaps out of the wounded rainforest, or a new anaerobe proliferates that uses up all the oxygen, or a wayward asteroid smashes into the Planet of the Naked Apes, our descendants can look forward to a world bald of forests, where rivers teem with E. coli like the human colon, and biodiversity means backyards and aquaculture. The elephants, giraffes, and lions will be gone, but there will be plenty of skunks. Jerold M. Lowenstein is professor of medicine at the University of California in San Francisco and chairman of the Department of Nuclear Medicine at California Pacific Medical Center in San Francisco. |
Summer 1998
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