Faint Echoes, Distant Stars_The Science and Politics of Finding Life Beyond Earth

by Ben Bova

  It was the American microbiologist Carl Woese, of the University of Illinois, who first realized that the so-called Tree of Life was more complex than previously believed. In the nineteenth century, biologists recognized plants, animals, and single-celled organisms. By the mid-twentieth century, the Tree had grown to include the prokaryotic bacteria, protozoa (single-celled eukaryotes), fungi, plants, and animals.

  In the late 1970s, Woese was studying a fairly rare form of bacteria called methanogens because they produce methane gas as a waste product of their metabolism. By that time, enough had been learned about DNA and RNA to allow researchers to use these nucleic acids as molecular clues to trace out an organism’s family tree. Woese was surprised to find that the methanogens were not bacteria at all. Single-celled, yes. Prokaryotic, yes: There was no nucleus in their single cells. But their DNA showed that these little creatures were not related to bacteria at all. They are a separate form of life.

  Eventually, Woese found a whole domain of prokaryotes that were not bacteria. Indeed, they seemed to be older in their lineage than bacteria, so he dubbed them archaic bacteria. Biologists were not quick to leap onto Woese’s bandwagon; scientists can be very conservative when it comes to shaking up their fundamental ideas. But as the evidence grew, the community of biologists at last agreed that Woese had found a domain of life that they had not realized existed. (One could say he’d found “a new domain of life,” but the archaic bacteria are hardly new. If anything, they are the oldest form of life in existence.)

  Today, biologists arrange the Tree of Life this way:

  Archaea. The earliest form of life: single-celled prokaryotes that include most of the extremophiles.

  Bacteria. Single-celled prokaryotes that include everything from the E. coli living in your gut to the cyanobacteria that made the world safe for us to live in. (More on that in a moment.)

  Eukarya. All the organisms that have eukaryotic cells, from the single-celled amoeba to the mighty blue whale and the even bigger sequoia tree. The Eukarya include the protists, single-celled protozoa such as the amoeba and the paramecium; slime molds; the fungi, funguses such as mushrooms or athlete’s foot; all the plants and all the animals—including the human race in all its multicellular splendor.

  THE FOOD PROBLEM

  Multicelled creatures are a fairly new innovation in the history of life on Earth, dating back a scant 750 million years. As we have seen, there is evidence that life was already in existence 3.87 billion years ago.

  Not long after life began, geologically speaking, life faced a problem with its food supply. Those earliest organisms could exist only by “eating” the organic molecules around them. Biologists call such life-forms heterotrophs, creatures that must ingest organic compounds to live. You and I are heterotrophs; so is a paramecium, a mosquito, and a Bengal tiger.

  Those earliest heterotrophs had to ingest other organic molecules to stay alive. Thus they faced a problem right from the start, although they could not know it: What happens when all the “food” in the environment is used up?

  Perhaps the problem looks larger in hindsight than it actually was. To these earliest forms of life, the organic soup (or banks of clay) in which they lived was a Garden of Eden. Food was there for the taking, plentiful and easy to come by. Organic materials were being washed out of the ground by wind and rain, providing a constant supply of food for the early heterotrophs. Yet, as these earliest forms of life multiplied swiftly, there was the strong possibility that they would outrun the food supply.

  How quickly those molecules evolved from proteinoid microspheres into true cells is unknown. Biologically, the step from a self-replicating molecule such as DNA to a single-celled creature such as a bacterium is a far greater leap in complexity and organization than the step from bacterium to human being.

  Undoubtedly, these first heterotrophs were multiplying faster than the geologic forces of erosion could stock the environment with fresh organic materials. The earliest life faced the conundrum of “The Tragedy of the Commons”: Under ideal conditions, living organisms outpace their food supply. If those creatures ate all the available food and could find no more, they would then die off. Of course, they might be able to cannibalize one another for a while, but sooner or later there would be only one cannibal left, and quickly enough it would die, too. Life would end, a brief candle that starved to death.

  THE ROLE OF SUNLIGHT

  The energy of sunlight saved life on Earth’s surface from early extinction.

  It must have been a time of wild experimentation among those greedily gobbling early creatures. Unknowing, uncaring, they had a whole world of food and energy to play with, and they must have multiplied and diversified enormously.

  They left scant evidence, those first ancestors of ours, for they were merely microscopic collections of atoms. No hard shells to petrify and be found eventually by paleontologists. Feathers and bones and teeth, the kinds of clues that paleontologists deal with, would not develop until billions of years later. Those tiny creatures lived, died, and dissolved without a trace. Yet they must have been multiplying as fast as they could.

  And changing. Constantly changing. In modern biology laboratories, scientists watch viruses, bacteria, even creatures as complex as fruit flies undergoing mutations and changing from one generation to the next within days or even hours. The thriving organic soup and teeming clay banks of those primeval times must have been a biological laboratory par excellence. Mutations came and went. One generation of hungry creatures followed another in minutes.

  Somehow, somewhere, at least one of these earliest creatures developed a molecule that could use the energy of sunlight to produce food for itself. Sunlight, water, carbon dioxide, and simple inorganic compounds were all it needed to sustain itself. It did not have to gobble other living creatures; it did not even need organic molecules to sustain itself. It could manufacture its own organic materials out of sunlight, water, and carbon dioxide from the air.

  The first autotroph was born, a creature that could manufacture its own food. The earliest autotroph’s solar-energy-using molecule was probably not exactly like the molecule of chlorophyll that is at the heart of every green plant cell on Earth today, but nevertheless that molecule could perform the miracle we call photosynthesis.

  Photosynthesis means “putting together with light.” Creatures that can accomplish photosynthesis are able to split the H2O molecule (which requires considerable energy), take carbon dioxide from the air (or dissolved in the sea), and produce a carbohydrate sugar. The energy to do this comes from sunlight. From water, air, and sunlight, photosynthetic creatures make carbohydrate food for themselves. And become food for others.

  Ever since that supremely critical moment when the first autotrophs began to use sunlight directly as their source of energy, the future of life on Earth was assured. Food chains developed, in which autotrophs create food and heterotrophs eat the autotrophs—or one another. Glowing at the apex of the food chain is always the Sun, beaming its unfailing energy, making life on the surface of Earth possible.

  In the deeper, dark regions of the extremophiles, the heat energy welling up from the Earth’s core provides the driving force for life. Chemosynthetic bacteria convert inorganic materials such as sulfur into living tissue.

  Life survived its earliest crisis. The dead end of cannibalism was averted. Photosynthesis, the ability to live on light, saved life on Earth’s surface from extinction while the extremophiles were driven into the dark, hot, strange (to us) ecological niches of this world.

  When did this happen? When did life “invent” photosynthesis? Those early creatures left no fossils for paleontologists to study, but they did have genes—bits of DNA that produced the proteins that made photosynthesis possible. In 2000, a team of biologists from Indiana University and Kanagawa University in Japan, led by Jin Xiong, traced the genes for photosynthesis in bacteria and came to the conclusion that such photosynthetic creatures existed nearly 3 billion years ago a
nd perhaps even earlier.

  In the 1970s, biochemist Bartholomew Nagy (of earlier meteoritic “organized elements” fame) and his wife, Lois Anne, found traces of round structures in rocks laid down some 3.4 billion years ago in Swaziland, Africa. No more than a few thousandths of a millimeter in diameter, these faint shapes may be evidence of single-celled blue-green cyanobacteria, although Nagy cautioned that such evidence is very uncertain.

  The oldest verified evidence of life comes from Rhodesia and Australia, where rocks dated at approximately 3 billion years bear stromatolites, the fossilized remains of filaments and mats of photosynthetic cyanobacteria—once called “blue-green algae,” we now know that they are truly bacteria.

  Stromatolites are still among us. On Australia’s western shore, at the edge of the Indian Ocean, there are beds of “blue-green algae” living in the shallow, lapping waters just as they did 30 million centuries ago, slowly building up rounded rock formations as they live and multiply. Standing among them on that bleak shore gave me an eerie feeling; it was like stepping into a time machine to witness the beginnings of life on Earth.

  If such cyanobacteria existed in colonies 3 billion years ago, and perhaps as individual free-floating units half a billion years earlier, this is even further evidence that life must have begun on Earth closer to 4 billion years ago, soon after the crustal rocks cooled enough for pools of water to form.

  THE OXYGEN CRISIS

  The success of the cyanobacteria came at a terrible price, however.

  The waste product of photosynthesis is oxygen: corrosive, flamma-ble oxygen. At first, the early photosynthetic organisms pumped their waste oxygen into the water in which they lived. For nearly 2 billion years the oxygen was taken up by the rich deposits of iron in the seabeds to produce thick layered sediments that geologists call banded iron formations (BIFs). About 2 billion years ago, BIF formation stopped, possibly because all the available sea-bottom iron was oxidized. With nowhere else to go, the oxygen being given off by the cyanobacteria began to bubble up into the atmosphere.

  At that time, Earth’s atmosphere was mainly carbon dioxide, carbon monoxide, methane, hydrogen sulfide, and water vapor. The new influx of oxygen changed all that. As the cyanobacteria and later the multicelled photosynthetic plants pumped more and more deadly oxygen into the atmosphere, the makeup of the atmosphere began to change radically. Oxygen is a very active element, and it began to combine with the molecules already in the air, changing the atmosphere eventually to the mix of nitrogen and oxygen that we are familiar with.

  Many earlier forms of life were driven into extinction because deadly oxygen killed them. Some forms of bacteria adapted and survived in mutated forms. Many others, and many of the archaea, however, remained anaerobic, and their habitats became restricted to those out of the way places where free oxygen is not present to poison them: fetid swamps, cracks in rocks deep underground, the digestive tracts of humans and other animals.

  Oxygen is a powerful, energy-rich element, and some living organisms mutated to take advantage of the increasing amount of oxygen that was being pumped into the atmosphere by the chlorophyllic autotrophs. Multicellular life began in the seas at first and then on the land. Insects, amphibians, reptiles, dinosaurs, birds, and mammals filled every continent except frozen Antarctica. Human beings evolved, and in time began to gaze at the stars in the night sky and wonder if any of them might harbor life.

  THE GAIA HYPOTHESIS

  Perhaps the first thinker to consider the interdependence of our planet and the life-forms it supports was the pioneering Russian geochemist, Vladimir I. Vernadsky, who coined the term biosphere in his 1926 book, The Biosphere. He used the term to connote the interrelationship of the Earth’s crust, the energy of sunlight and other radiations from space, and the living organisms that inhabit our planet.

  By the 1970s, the realization that life’s actual beginnings took place in space, even before Earth solidified out of the Sun’s accretion disk, made scientists understand that life must be an integral part of a planet’s development. Life is not some separate phenomenon to be studied by itself. It is as fully a part of the Earth’s history and development as sunlight and water. Indeed, life has been a crucial factor in the development of Earth. This realization began to break down some of the barriers between the disciplines of geology, biology, and astronomy. It became clear that, to search for extraterrestrial life, we must understand how life has affected the development of our own world.

  A large part of the credit for this new outlook belongs to another maverick scientist, the British chemist James Lovelock. After a twenty-year-long career with the National Institute for Medical Research in London, Lovelock decided to be an independent researcher, a “freelancer” who works on whatever interests him out of his home in West Devon, Cornwall. This is extremely unusual in an age when most scientists depend on employment in industry, government, or academia.

  Thinking about Earth’s atmosphere, Lovelock realized, “Almost everything about its composition seems to violate the laws of chemistry.” Chemically speaking, our atmosphere is not in equilibrium.

  Compare the atmosphere of Earth with that of the two planets that are closest to us, both in distance and composition. Venus, our so-called “sister planet,” is almost the same size as Earth and is built out of the same materials. Yet its atmosphere is almost entirely composed of carbon dioxide. Mars’ thin, cold atmosphere is also primarily carbon dioxide.

  In contrast, our air is 21 percent oxygen and 78 percent nitrogen, with only a smattering of carbon dioxide. Oxygen is an extremely active element that vigorously combines with other elements. Oxygen is what makes a peeled apple turn brown in just a few minutes. Corrosive and flammable, oxygen should have been removed from the atmosphere eons ago, consumed by metals and rocks (turning iron into rust, for example) or by wildfires that convert free oxygen in the air into carbon dioxide and water vapor. Yet oxygen has been a major constituent of Earth’s atmosphere for hundreds of millions of years or longer. How come so much oxygen is still in the air?

  The answer is well-known: Living organisms constantly replenish it. Chlorophyllic plants, bacteria, and algae are constantly consuming carbon dioxide to produce carbohydrate food and exhaling oxygen as a waste product of their metabolism. Animals breathe in the oxygen, which they use to power their metabolism, and exhale carbon dioxide, which becomes fresh feedstock for the plants.

  Everyone knew that. But Lovelock drew a far-reaching conclusion from this obvious fact: Living creatures alter their environment for their own benefit. The fact that our atmosphere contains plentiful oxygen “can only be an artifact maintained in a steady state far from chemical equilibrium by biological processes,” Lovelock wrote.

  In 1979, he proposed the Gaia hypothesis, which states that our planet’s vast and intricate web of life—the biosphere—is a self-regulating system that cycles carbon, nitrogen, water, oxygen, and the other elements necessary for life in a series of feedback loops so as to optimize the environment for life.

  Gaia is the name of a Greek goddess of the Earth, and Lovelock proposed that our planet, both its living flora and fauna and its inanimate rocks, oceans, and atmosphere, is a single interactive entity, “maintained and regulated by life on the surface.”

  To give a very simplified example, consider the greenhouse effect that appears to be causing a global warming trend. Human societies are pumping huge amounts of carbon dioxide and other heat-trapping gases into the atmosphere, and most scientists fear that this will lead to severe shifts in the global climate and rises in sea levels that could flood coastal cities. In the long run, though, more carbon dioxide in the atmosphere will lead to the growth of more green plants, which will eventually consume the carbon dioxide and bring the world’s temperature down again. While human civilization may suffer devastating effects from “temporary” climate changes, sooner or later Gaia will regulate everything back to conditions that are optimal for the planet’s biosphere (although not necessa
rily optimal for human existence).

  As biologist Margulis, an early and enthusiastic supporter of the Gaia hypothesis, puts it:

  Gaia is a tough bitch. People think the Earth is going to die and they have to save it, that’s ridiculous . . . There’s no doubt that Gaia can compensate for our output of greenhouse gases, but the environment that’s left will not be happy for any people.

  Most scientists scoffed at Lovelock’s concept of the Earth as a single gigantic, interrelated, and self-regulating organism. But his view of Gaia showed that astrobiologists cannot keep astronomy, geology, and biology in separate mental compartments. They affect one another intimately. Life creates environments that are chemically out of equilibrium. Life-bearing worlds will show anomalies, such as an abundance of free oxygen in their atmospheres. This is an important clue in our search for life. We have only one example of a world where we know life has developed. What does it tell us about the chances for finding life elsewhere?

  Section III

  Life in the Solar System

  12

  A Model Planet Called Mars

  Across the gulf of space, minds that are to our minds as ours are to those of the beasts that perish, intellects vast and cool and unsympathetic, regarded this earth with envious eyes, and slowly and surely drew their plans against us.

  —H. G. Wells

  The War of the Worlds

  WHEN PEOPLE START to think about the possibilities of life on other worlds, they first turn their eyes toward Mars.

  “Martians” are the archetype for extraterrestrial creatures, thanks in large part to Herbert George Wells, whose 1898 novel The War of the Worlds has fixed in the public’s mind the idea of malevolent intelligent aliens. The famous Halloween radio broadcast of Wells’ story by Orson Welles in 1938 added a new shudder of fear; thousands of radio listeners actually believed that New Jersey was being invaded by Martians.