Here Be Dragons: The Scientific Quest for Extraterrestrial Life / Edition 1

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In Here Be Dragons, astronomer David Koerner and neurobiologist Simon LeVay offer a scientifically compelling and colorful account of the search for life beyond Earth.
The authors survey the work of biologists, cosmologists, computer theorists, NASA engineers, SETI researchers, roboticists, and UFO enthusiasts and debunkers as they attempt to answer the greatest remaining question facing humankind: Are we alone? From their "safe haven of skepticism" the authors venture into the "rough seas of speculation," where theory and evidence run the gamut from hard science to hocus pocus. Arguing that the universe is spectacularly suited for the evolution of living creatures, Koerner and LeVay give us ringside seats at the great debates of Big Science. The contentious arguments about what really happens in evolution, the acrimonious UFO controversy, and the debate over intelligence versus artificial intelligence shed new light on the wildly divergent claims about the universe and life's place in it.

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Editorial Reviews

From Barnes & Noble
The Barnes & Noble Review
David Koerner (planetary science) and Simon LeVay (neuroanatomy) begin their book, oddly enough, at the Museum of Creation and Earth History -- the point being that any study of exobiology ultimately starts with the question of how life arose on our own planet. There isn't yet a consensus on how life did arise from nonlife. Some theories, like the idea that Earth was seeded with the organic molecules that are now known to be quite widespread throughout the galaxy, suggest that life could be common elsewhere, given hospitable conditions. Before the recent discovery of bacteria living in (to us, at least) extreme environments here on Earth, the so-called "habitable zone" for life on other planets was considered to be much smaller. Researchers now spend their time in places like Death Valley, looking for the type of life that might be lurking on Mars or one of the other planets' moons. Although some might be disappointed if life on other planets in our solar system consists solely of bacteria, the fact that another planet harbors any life at all would have huge implications for the possibility that more complex forms might have evolved in distant solar systems. The real hope is that there exists other intelligent life. A chapter on SETI (the Search for Extraterrestrial Intelligence) gives some history and rationale for the present radio search for signals from a technologically advanced civilization, and another chapter, "Dreamland," gives a nod to UFOs. (Laura Wood, Science & Nature Editor)
From the Publisher
"A wide-ranging, well-written, and very wise examination of the fascinating search for life in the universe, filled with the latest information from the frontiers of scientific investigation and philosophical thought."—Ben Bova, author of Return To Mars and Immortality and President Emeritus of the National Space Society
Library Journal
Planetary scientist Koerner and neuroanatomist LeVay have written a clear, concise, and engaging overview of the hypotheses, experiments, explorations, and issues that surround exobiology, the search for life forms, intelligent beings, and advanced civilizations elsewhere in the universe. Their open-minded and level-headed presentation takes readers from the Arecibo radio telescope in Puerto Rico to emerging star systems throughout our galaxy. One becomes acutely aware of both the technological challenges of exobiological research and the philosophical ramifications should life as we know it (or don't know it) be found on other worlds. Topics critically discussed range from the origin of life on Earth to extrasolar planets and the Taurus-Auriga molecular cloud. Koerner and LeVay excel at making complex empirical information both comprehensive and relevant. Fascinating, and inspiring, and rich in ideas, this is highly recommended for all science collections.--H. James Birx, Canisius Coll., Buffalo, NY Copyright 2000 Cahners Business Information.
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Product Details

  • ISBN-13: 9780195146004
  • Publisher: Oxford University Press, USA
  • Publication date: 11/28/2001
  • Edition description: REPRINT
  • Edition number: 1
  • Pages: 280
  • Product dimensions: 9.00 (w) x 5.80 (h) x 0.90 (d)

Meet the Author

David Koerner is an Assistant Professor of Astronomy at the University of Pennsylvania. Simon LeVay is an Independent Consultant and former Associate Professor at the Salk Institute for Biological Studies in La Jolla, California.

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Read an Excerpt

Chapter One


How Life on Earth Began

The Museum of Creation and Earth History stands on a freeway frontage road in Santee, California, a nondescript suburb tucked among the low hills east of San Diego. We pull into the museum's forecourt and park backward in our space, thus concealing the "Darwin fish" that adorns the rear of the car. We feel a slight anxiety.

    The receptionist is a pleasant elderly gentleman, who nevertheless increases our unease by asking, "You're not reporters, are you?" "No, no," we assure him truthfully, but it feels perilously close to a falsehood. A sense of transparent culpability, not experienced since Sunday school decades ago, accompanies us into the exhibition rooms. For this museum raises one of the most profound questions that humanity can ask—Where do we come from?—and offers an unambiguous answer: Scientists like ourselves have got it all wrong, and the Bible has got it exactly right.

    We move through a series of small halls, named for the Six Days of Creation. In the first, light is divided from the darkness; in the second, a firmament appears; in the third, the seas are divided from the dry land. All this is visualized with the help of rather schematic artwork, to the accompaniment of classical music. Things get a little more animated on the Fourth Day with the appearance of the heavenly bodies: NASA-supplied color photographs reveal the beautiful and unexpectedly diverse faces of the planets and their moons. But the hall of the Fifth Day really comes alive: there'san aquarium with real fish and an aviary with real birds, although the birds' trills and warbles are piped in. The next hall has even more living creatures: a poisonous-looking frog, a snake (or its shed skin, at least), and even a herd of giant Madagascan cockroaches. And humans, of course—represented by their skulls. For the Sixth Day was the culmination of Creation, when God created Man in His own image and gave Man dominion over the Earth. In the next hall, the significance of the Seventh Day is explained: God rested from His labors, thus marking the end of the period in which He created the universe and the beginning of the period, still continuing today, in which He actively upholds His Creation.

    The Seven Days of Creation by no means exhaust the museum's exhibits. Other rooms take us inside Noah's Ark, to the eruption of Mount St. Helens, to the Grand Canyon, to the interior of a glacier. We see the tower of Babel and pass through the Ishtar Gate, and we file down a corridor between portraits of creationists on the left, and evolutionists on the right—the saints and sinners of the Great Debate.

    If the museum based its case simply on a divinely inspired faith in what the Bible says, it would be of limited interest to us. But far from it: The museum's whole purpose is to show how we can deduce the truth of the Bible story from objective study of the world around us—from science, in fact. It could properly be called the Museum of Natural Theology, for that is the name of the venerable branch of philosophy that seeks to recognize God through Reason and the study of His Works.

    In making this case, of course, the museum has to face serious obstacles. Because of the detailed genealogies recounted in Genesis, the museum needs to place the beginning of all things no more than about 10,000 years in the past, while most astronomers and cosmologists claim that our universe is a million times older. The museum must compress into a mere six days processes that, in the view of the majority of scientists, took more than ten billion years. And it must make intentional what most scientists consider, in a deep sense, accidental.

    The museum does not shirk this challenge. It expresses open antipathy toward Christians who try to smooth over the gulf by, for example, asserting that the "days" of Creation were metaphors for longer periods of time: that they were in fact "ages" or "eons." No, "days" were days—periods of 24 hours.

    It also rejects the strategy, favored by some Christian groups, of pushing God's creative role backward in time, allowing the latter part of Creation to go forward by purely natural processes. Some believe, for example, that God lit the spark of life on Earth but allowed natural selection to do the job of getting from microbes to humans. This, in fact, was the view publicly espoused by Charles Darwin, though his private beliefs, as we shall see later, were different. With discoveries in physics and astronomy, there has been pressure to push God's role back even further. The British cosmologist Stephen Hawking, in his book A Brief History of Time, tells how he attended a scientific meeting at the Vatican at which the Pope admonished the conferees not to discuss what happened before the Big Bang, because that was God's province. Yet Hawking's lecture at the conference concerned the possible circularity of time, a hypothesis that, if true, would make the phrase "before the Big Bang" meaningless! The Museum of Creation wisely refuses to set foot on the slippery slope of biblical revisionism.

    How then, does the museum propose to explain the apparent discrepancies between the Bible story and the usual teachings of science? There are several basic points. One is that, according to the museum, God created all things, including living creatures, in a fully functioning, mature state. Thus, Adam and Eve were created as normal adults, in possession of navels, for example—just as they are portrayed by Dürer and a hundred other artists. But seeing their navels, we think of umbilical cords and therefore assume that Adam and Eve were once fetuses—which they were not. And seeing the Tree of Knowledge, we assume that it was once a seed, and so on. There is the deceiving appearance of a past.

    The same phenomenon, the museum argues, could explain how stars appeared in the sky on the Fourth Day, even though it would take many years for photons, traveling at the speed of light, to reach us from the newly created stars. God may have created a "functionally mature" state, including both the stars and the entire stream of photons traveling from it to us, in a single act. But seeing the photons, we naturally imagine that they originated from the star many years previously.

    Of course, this line of thought can lead us into dangerous territory. Is it not equally possible that the universe is much younger than the Bible tells us? Perhaps God created the universe just a few hours or minutes ago, rather than 10,000 years ago? That vivid memory we have of reading this morning's newspaper, and every earlier memory—were they perhaps implanted in our brains to make us "functionally mature"? Do our past lives resemble those wildlife dioramas we loved as children: a couple of stuffed gazelles up front, and the rest painted on the backdrop? How to distinguish reality from illusion becomes an insoluble dilemma, once one posits the intentional creation of "mature" systems.

    The museum presents a second line of argument to explain the discrepancies between creationism and conventional science. Most scientists, it argues, assume that natural processes have always occurred at the same rate. If the half-life of a radioactive isotope (the time required for half of the atoms to decay into other atoms) is now a million years, it was always a million years, because the physical laws that control radioactive decay have not changed since atoms first existed. But, the museum reminds us, we can't go back into the distant past and measure the decay rate then; therefore, the assumption of a constant rate is unjustified, and so is any finding based on that assumption, such as the age of a rock or of a fossil embedded in that rock. The Seventh Day of Creation, when God rested, was one particular time when the rates of physical processes might well have changed. Before then, light may have traveled at infinite speed, for example, thus providing an alternative explanation for how stars were seen on the day they were created.

    As a matter of fact, it is not quite right to say that scientists simply assume the constancy of process rates. Many processes on Earth, such as the rate of deposition of sedimentary rocks, have been shown to vary greatly over time. Even the constancy of the great "constants," such as the strength of the gravitational force, is open to scientific debate: there are cosmologists who have developed models in which the force of gravity has changed since the Big Bang. But we can study process rates in the past with the same kinds of certainties and uncertainties with which we study them today. Some kinds of radioactive decay, for example, leave permanent tracks in rocks—rocks whose age can be estimated by other means, such as their degree of weathering or chemical transformation, their position in a sedimentary series, and so forth. One can count these tracks and thus determine whether the process of radioactive decay took place at the same rate in ancient times as it does today. In the end, our knowledge of process rates in the past is built on the mutual consistency of events that happened then, just as our knowledge of process rates today is built on the consistency of events happening now. To believe that the apparent great age of the universe is an illusion caused by decreasing process rates is really to say that time itself ran faster in the past—an assertion that belongs to metaphysics, not science.

    Finally, the museum confronts the findings of conventional science by contesting the findings on science's own terms—by getting into the nitty-gritty of the data and challenging every piece of evidence, and every interpretation, that runs counter to the Bible story. Does radiometric dating of rocks at the bottom of the Grand Canyon prove them to be a billion years old? No, because if one applies the same technique to obviously recent lava flows near the canyon's rim, one gets an even earlier date—or so the museum's experts allege. Therefore the dating technique is patently untrustworthy. Did the dinosaurs go extinct 65,000,000 years ago, as the fossil record suggests? No, because dinosaurs were frequently and unambiguously sighted by humans—they called them "dragons"—as recently as the Middle Ages. Dinosaur fossils, like all other fossils, are merely the remains of the animals that drowned in Noah's Flood. Others survived, either by swimming or by being taken on board the Ark. At the museum, a painting of the Ark's interior shows what seems to be a stegosaurus lounging peaceably in its stall. The accompanying panel goes through the arithmetic to show that the Ark was plenty big enough to hold all 50,000 "kinds" of animals.

    The Museum of Creation is an offshoot of the Institute for Creation Research, whose offices are located in the same building, and the Institute's Senior Vice President, Duane Gish, is a Berkeley-trained biochemist who yields to no one in the discussion of scientific minutiae, whether it be the proper interpretation of an indistinct band in a sedimentary rock or the assessment of transitional forms between various fossil hominids. Woe to the "evolutionist" who agrees to debate Gish on a college campus or at a church meeting: he or she will be buried under an avalanche of particularities that collectively obliterate the conventional scientific worldview. Gish and the institute's founder, Henry Morris, have written a series of books that promote creationism as a science and label the theory of evolution a "religion"—and a false one, to boot. Of course, creationism should be taught in schools.

    Where does the institute stand on extraterrestrial life? Bill Hoesch, the institute's Public Relations Officer, tells us that nonintelligent life—such as microbes—poses no problems. Creationists do not have the same need for them that "naturalists" do, since the Creator might well have chosen to put life on the Earth alone. But there is nothing to say that microbes do not exist elsewhere. With intelligent life it's a different story, especially if that life is in an "unfallen" state. In retribution for mankind's Original Sin, God put His Curse on the entire universe, Saint Paul tells us in Romans 8:22 ("For we know the whole creation groaneth and travaileth in pain together until now"). If innocent extraterrestrial creatures are laboring under this Curse, it would raise the question of whether God had acted unjustly. "That would raise some hoary theological problems for us," Hoesch says. So creationists doubt that such beings exist.

    As we leave the museum and stand blinking in the afternoon sunlight, we have the sense of having torn ourselves free from a dark web of unreason, a web that might have held us in its threads until the brains were sucked out of our skulls. We feel the impulse to flag down one of those trucks hurtling by on Route 67, to breathlessly recount our trip to Eden and the saurian Ark, as if we had just returned from an alien abduction. Surely the driver would comfort us with the assurance that everything we saw and heard was an illusion?

    Perhaps not. Creationism, in one form or another, is the majority worldview. Most people believe that the universe was brought into existence by divine intention, and about 40 percent of the population of the United States, according to a 1991 survey by U.S. News and World Report, believes in the literal truth of the Genesis story. Henry Morris, Duane Gish, and their colleagues at the Institute for Creation Research are unusual only in the fervor with which they explore the ramifications of that belief.

    Of course, the Museum of Creation does represent something of an extreme position within theology. Natural theology, as practiced today, has many different perspectives on the identity of God and His role in the creation of the universe and life. For example, one school of liberal theology speaks of God as a process that is coming into being, rather than as a substance coexistent with but transcendent over matter. Process theologians, and many other liberal theologians, would not dream of contesting the date that dinosaurs went extinct, or any other scientific findings related to our origins. Our purpose in visiting the museum was not to gain an overview of current theological perspectives, but to sample the least naturalistic among them, in order to provide a contrast with what follows: The effort to explain the origins of life by natural processes.

* * *

Lucretius, whose belief in extraterrestrial life we mentioned in the Introduction, had an uncompromisingly naturalistic view of Creation. The gods exist, he said, but they are irrelevant. Our world assembled itself spontaneously, by the aggregation of atoms moving through a boundless extent of space. No Prime Mover was needed. Nor did the origin of life require divine intervention. "As I believe," he wrote, "no golden rope let down living things from on high into the fields ... rather, this same earth that now nourishes them from herself gave them birth."

    To explore this alternative vision, we visit the beachside community of La Jolla, 15 miles and a world away from the Museum of Creation. For La Jolla is home to the science-focused University of California, San Diego (UCSD) and to a host of satellite institutes and research corporations. Here we call on a group of five scientists—Jeff Bada, Stanley Miller, Gustav Arrhenius, Leslie Orgel, and Gerald Joyce—who are the closest thing to disciples of Lucretius that one may hope to find in the world today. Not that they are concerned with the entire panoply of Creation. It's that "golden rope" part that obsesses them. Can one explain the origin of life without it? It turns out to be a Herculean undertaking.

    The group is called the NASA Specialized Center of Research and Training in Exobiology, one of a pair of such centers in the US. Yet the La Jolla scientists actually devote the bulk of their attention to terrestrial life—to "endobiology," if you like. "Certainly, our effort is to figure out how life began on Earth," the center's Director, Jeff Bada, tells us. "But of course that provides a model for everything else. Admittedly, we're biased by what we know about life on Earth. But I think the consensus is, if we can understand the processes that lead up to the origin of life, then given the proper conditions, it will probably be a universal process."

    Jeans clad, with weather-beaten face and graying beard, Bada could be mistaken for an aging sailor. In fact, his research has taken him onto the high seas—he has sampled fluids emitted by deep-sea volcanic vents, for example. His office is no more than a few hundred feet from the ocean, at the Scripps Institution of Oceanography. And the ocean, Bada and his colleagues believe, is where life most probably originated.

    "Water is best," said the first philosopher, Thales of Miletus, about six centuries before Christ. Water gave rise to all things, he claimed, including life—and certainly, water seems like the most natural place for a Life to get started: it's an excellent solvent, and there's plenty of it, on Earth at least. But water alone isn't enough. Terrestrial life is made of carbon-containing molecules—organic compounds—many of which also contain nitrogen, oxygen, and other elements. And assembling these molecules takes energy.

    As mentioned earlier, Charles Darwin publicly expressed a belief that Earth's first creatures were divinely made. Perhaps he felt that he had rocked the boat sufficiently with his theory of evolution—that he would endanger the seaworthiness of his whole enterprise if he went further. But in a private letter, written in 1871, he did put forward the idea that life arose spontaneously, "in some warm little pond, with all sorts of ammonia and phosphoric salts, light, heat, electricity, etc. present."

    If so, what were these chemicals and where did they come from? In 1936 the Russian chemist A.I. Oparin suggested an answer. The Earth's early atmosphere, he proposed, was rich in ammonia (NH3) and methane (CH4), and lacked oxygen. In this "reducing" (hydrogen-donating) atmosphere, a large variety of organic molecules formed and were washed by rain into the ocean, gradually building up a "prebiotic soup." (The "soup" metaphor was actually introduced by the British geneticist J.B.S. Haldane, who had been thinking independently along the same lines.) The very first organisms, Oparin believed, were extremely simple: they didn't need to have complex metabolic pathways because everything was available in the soup—both molecules to make up their structure (such as amino acids) and molecules to break down for energy. It was the ultimate free lunch. Eventually, of course, the goodies ran out, and organisms had to learn how to make an honest living. However long that initial period may have lasted—a hundred thousand years, a million years, ten million years—it could have been no more than a moment in the Earth's history.

    One of the La Jolla scientists, Stanley Miller of the UCSD Chemistry Department, tested Oparin's ideas in the laboratory. In 1952, as a graduate student working in the laboratory of Harold Urey at the University of Chicago, Miller performed an experiment that made him famous and established origin-of-life research as an experimental science. He tested Oparin's hypothesis by (1) filling a flask with a "reducing atmosphere" (he chose a mixture of methane, ammonia, and hydrogen gas—H2) over an "ocean" (a cupful or so of water) and (2) subjecting the milieu to "lightning strikes" (electrical discharges). After a week, he analyzed what was in the water and found glycine and alanine—two of the amino acids that are building blocks of proteins. Subsequent experiments of a similar kind have revealed that a wide variety of amino acids, as well as the nucleosides that are the building blocks of DNA and RNA, are readily formed in experiments of this kind. Thus, Miller's work suggested that the building blocks of life were indeed there, free for the taking, in the Earth's primordial ocean. It was just a matter of putting them together into an organism.

    Asked what it was like to have performed such a famous experiment while a graduate student, Miller tells us: "I'm sure it helped my career. But in terms of famousness—I don't know. A lot of people felt that it wasn't really science. It was attacking a problem that people didn't think about."

    With the passage of the years, however, Miller has evolved from radical wunderkind to conservative defender of a possibly outmoded theory. This is on account of changing views about the composition of the Earth's early atmosphere. To understand this change, we must take a look at how scientists think the Earth and its atmosphere were created. According to current consensus, the Earth formed by the gathering together ("accretion") of smaller objects, or "planetesimals," in the disk of gas and dust orbiting the evolving Sun, 4.6 billion years ago. The main period of accretion lasted about 100 million years. During this period, the heat generated by frequent impacts kept the Earth in a molten state. For several hundred million years after that, sporadic large impacts probably prevented life from establishing itself. One such impact—by an object at least as large as Mars—is thought to have kicked a large amount of material from the Earth's mantle into orbit around the remainder of the planet. This orbiting material eventually accreted to form the Moon.

    It was once generally believed that the Earth's original atmosphere was drawn directly from the disk of gas and dust from which the solar system formed. If so, it would have resembled the present atmosphere of Jupiter and Saturn, being rich in hydrogen and hydrogen-containing molecules, such as ammonia and methane, and lacking molecular oxygen (O2). This would have been a strongly reducing atmosphere and would have been appropriate for the synthesis of organic compounds by the methods that Urey and Miller proposed. But according to the majority of contemporary researchers, the Earth was too small to attract or hold on to such a primordial atmosphere. Instead, the first atmosphere was composed of volatiles that were released from infalling planetesimals as they crashed into the magma ocean and were vaporized, or of volatiles that were outgassed from volcanoes. The main gases produced by these processes would probably have been water vapor, nitrogen, carbon monoxide (CO), carbon dioxide (CO2), and hydrogen. Hydrogen, however, is light enough to escape from the atmosphere into space and, therefore, would not have accumulated in significant concentrations. Compounds such as methane and ammonia, if they were generated at all, would likely have been kept at very low concentrations by the destructive effect of the Sun's ultraviolet radiation.

    Geochemists have had a much harder time figuring out how organic molecules could have been generated in this neutral or mildly reducing atmosphere, compared with the strongly reducing atmosphere favored by Oparin, Urey, and Miller. It's not completely impossible. Miller himself, for example, has shown that electrical discharges in a mildly reducing CO2/N2/H2O atmosphere can give rise to formaldehyde, and hydrogen cyanide can be produced in a similar atmosphere by ultraviolet irradiation. These compounds can go on to build larger organic molecules. Still, the process is not very efficient. "If you're going to make enough organic compounds," says Miller, "it has to be methane or ammonia, or else hydrogen and carbon dioxide and nitrogen."

    So Miller tries to find ways to rescue the original scheme. He suggests to us, for example, that methane might have been released from the deep-sea volcanic vents. The vents don't release methane now, admittedly, but they might have done so, Miller says, when the Earth's atmosphere and oceans lacked oxygen. There would still be the problem of how to protect that methane from the Sun's ultraviolet radiation once it entered the atmosphere. But by happenstance, the Cornell astronomer Carl Sagan (shortly before his death in 1996), along with Chris Chyba, came up with a theory to explain how methane might have been protected. They suggested that a layer of organic haze high in the Earth's atmosphere—smog, in effect—filtered out the ultraviolet radiation before it could reach the deeper layers where methane would be located. Sagan and Chyba came upon this idea because just such a smog layer does surround another body in the solar system—Saturn's largest moon, Titan (see Chapter 3).

    We say "by happenstance" because Chyba and Sagan had not set out to rescue the Miller-Urey hypothesis. They wanted methane in the Earth's early atmosphere for a quite different reason. Early in the Earth's history, the Sun was not as bright as it is at present because its nuclear fires were concentrated in a relatively small sphere near its center. Thus, the early Earth received about 30 percent less sunlight than it does now. By rights, this should have allowed the oceans to freeze solid. And once they had frozen solid, even the present-day Sun would be powerless to melt them because the ice would reflect most of the Sun's rays back into space. Chyba and Sagan were therefore looking for some way by which the early Earth might have been kept warm in spite of the Sun's faintness. A blanket of methane would do the job nicely, since methane is a greenhouse gas: it allows incoming sunlight to pass through to warm the Earth's surface, but it blocks the outgoing infrared radiation.

    Ammonia, another powerful greenhouse gas, may also have played a more significant role than previously thought. According to a group led by Robert Hazen of the Carnegie Institution in Washington, D.C., who simulated the environment of the deep-sea vents in their laboratory, ammonia is generated in copious amounts from nitrogen thanks to the catalytic action of minerals present at the vents. It's possible that, on the early Earth, ammonia was formed at the vents at a high enough rate that it built up to a significant concentration in the atmosphere, particularly if it was protected from the ultraviolet radiation by Sagan and Chyba's "smog." If so ammonia, like methane, could have contributed both to keeping the planet warm and to providing the raw material for the synthesis of organic compounds.

In spite of these various mechanisms by which the Milley-Urey hypothesis might be rescued, enough doubts have been sown to motivate a search for alternatives. Jeff Bada, as well as Chyba and Sagan, have explored the viability of another theory: the idea that the organic compounds in the prebiotic soup were not synthesized on Earth at all but were brought to Earth by infalling meteorites, comets, and dustgrains.

    All three of these kinds of objects can contain significant amounts of organic compounds. Meteorites, especially the class of meteorite known as carbonaceous chondrites, can contain as much as 5 percent organic material. The Murchison meteorite, for example, a carbonaceous chondrite that fell in Australia in 1972, has been found to contain over seventy different amino acids, including eight of the twenty amino acids that are the building blocks of terrestrial proteins. Where these organic compounds originally came from is a topic we explore in Chapter 4.

    A particularly interesting issue concerns the handedness (also called "chirality") of the organic compounds found in meteorites. Many organic compounds come in two mirror-image versions, which differ only in the three-dimensional arrangement of the bonds around one or more of the carbon atoms. The two arrangements are called, by convention, "left-handed" or "right-handed." For some reason, terrestrial life only uses amino acids of the left-handed variety. Most probably, the choice of left-handed amino acids was made because there was a small excess of left- over right-handed amino acids in the prebiotic soup. But it is very difficult to see how the local synthesis of amino acids on Earth (the hypothesis favored by Stanley Miller) could lead to more than the tiniest excess of one handedness over the other.

    In the early 1970s, several groups of organic chemists reported finding an apparent excess of left-handed amino acids in carbonaceous meteorites. This opened the door to a new hypothesis: the prebiotic soup was biased toward left-handed amino acids because meteorites, comets, and dust grains imported more left handed than right-handed molecules. The findings were not terribly convincing because of the possibility that the meteorite samples had become contaminated with left-handed amino acids of terrestrial origin. In 1997, however, John Cronin and Sandra Pizzarello, of Arizona State University in Tempe, reexamined the issue by looking at the chirality of individual amino acids in the Murchison meteorite, as well as in another carbonaceous chondrite. They found that it wasn't just the familiar amino acids used in terrestrial biochemistry that were biased toward the left-handed form, so were some exotic amino acids that are never found on Earth except in meteorites. This finding seems to have disposed of the contamination issue and suggests that there was indeed a left-handed bias in the supply of amino acids that reached Earth from space. This in turn raises the question of why the extraterrestrial supply should be biased toward one chirality, a question that we will discuss in Chapter 4.

    To evaluate the possible contributions of terrestrial-versus-extraterrestrial supplies of prebiotic chemicals, Bada tried to estimate how much organic material is reaching the Earth today. He suspected (as originally suggested by Edward Anders of the University of Chicago) that the bulk of the material would be brought in on very small dust grains—perhaps 50 microns in diameter or so (a micron is one-thousandth of a millimeter, or 0.0004 inches). Grains of this size don't heat up excessively as they enter the atmosphere; instead, they quickly decelerate and drift safely to the surface. To find these grains, Bada used the services of a group of French researchers, who collect and melt ice from the Antarctic—the land surface least contaminated by human activities. Starting with tons of ice, Bada ended up with micrograms (millionths of a gram—1 gram is 0.035 ounces) of material that might be of extraterrestrial origin. He then analyzed this material for the presence of organic compounds and found amino acids—not just the usual amino acids that are found in terrestrial organisms (which might represent contamination) but also amino acids that play no part in terrestrial biochemistry and that therefore are almost certainly of extraterrestrial origin.

    Micrometeorites in the millimeter-size range are generally heated to incandescence as they enter the Earth's atmosphere, producing the familiar "shooting stars," and any organic freight is therefore destroyed. But there is a range of larger objects—say cabbage sized or thereabouts—that bring organic compounds to the surface intact. The surface layers of these meteorites do heat to incandescence during passage through the atmosphere, but their interiors remain cold.

    Much larger objects strike the Earth's surface with such force that the heat of impact destroys any organic compounds they contain. However, large objects not uncommonly break up in the atmosphere. This happened, for example, with the "Tunguska object"—thought to have been a stony meteorite about 160 meters across—that exploded in the air over Siberia in 1908. In such cases, the resulting fragments could descend to the surface more gently.

    There is another potential mechanism by which meteorites can contribute to the Earth's inventory of organic compounds, even if its own organics are destroyed on impact. Shock waves produced by the meteorite as it races through the atmosphere can provide the energy to form organic compounds, rather in the same way as did the lightning strikes in the original Miller-Urey mechanism. But like that mechanism, the shock-synthesis of organic compounds works best in a strongly reducing atmosphere and is therefore not really a means to explain how organics could form in a more plausible early atmosphere.

    Bada, as well as other researchers such as Chyba and Sagan, have gone through the arithmetic to see if enough extraterrestrial organic material could have reached the early Earth to make a reasonably thick "prebiotic soup." One might think that a soup of any desired thickness could be generated simply by waiting long enough: after all, there were no bacteria to eat it up. But in fact there is a process that destroys organic compounds in the ocean: this is the cycling of seawater though the deep-sea volcanic vents. Seawater in the vicinity of the midocean ridges percolates down to the magma beneath the seafloor, where it is heated and returned to the ocean through the vents. In the process, the seawater reaches a temperature of about 500°c (over 900°F)—a temperature that should be hot enough to destroy organic compounds. Bada has verified this by directly sampling the water as it passes through the vents, both in the Pacific Ocean and in the Gulf of Mexico: it is indeed "clean." Rough calculations indicate that the entire ocean circulates through the vents about once every 10 million years. Therefore one doesn't have forever to build up a prebiotic soup, and the rate at which organic compounds accumulate is crucial.

    The bottom line, according to Bada, Chyba, and Sagan, is as follows. If the early Earth had a strongly reducing atmosphere, organic compounds from all sources would have built up to a steady-state concentration of about 0.1 percent—a gram of organic compounds for every kilogram of seawater. Not quite chicken broth, but in the same ballpark. If the atmosphere were neutral or mildly reducing, on the other hand, the prebiotic soup would have been a thousandfold more dilute—only about a milligram per kilogram of seawater.

    Of course, no one knows how thick a soup would be needed to allow life to get rolling. And it's possible that the concentration of organic material could build up to higher levels in certain favorable locations—in drying tidal pools, for example. But still, the thousandfold difference makes the original reducing atmosphere, and the Miller-Urey mechanism, very attractive. What's more, says Bada, the Miller-Urey mechanism produces "better-quality" chemicals: a wide range of amino acids and nucleosides, whereas what comes in from space tends to be much more restricted—glycine (the simplest amino acid) and a lot of pretty useless compounds.

    There is a third possibility, put forward by, of all people, a patent attorney in Munich, Germany, by the name of Günter Wächtershäuser. In 1992, Wächtershäuser, who has had a lifelong passion for organic chemistry, suggested that the reduction needed to produce organic chemicals was carried out, not by gases in the atmosphere, but by inorganic chemicals at the hot deep-sea volcanic vents. Specifically, he pointed out that ferrous (iron) sulfide and hydrogen sulfide, both of which are present at the vents, constitute a powerful reducing system that might be able to convert carbon dioxide (a gas that is released from the vents) into a variety of organic compounds. These compounds, he suggested, might remain adsorbed onto the iron sulfide crystals where they were generated, thus building up high concentrations of organic molecules near the vents. If so, a prebiotic soup in the free ocean might be completely unnecessary. Wächtershäuser's "iron-sulfur world" has garnered considerable attention, particularly in the light of evidence, discussed later, that heat-loving organisms are the ancestors of all present life-forms on Earth.

    According to Miller and Bada, however, Wächtershäuser's scheme doesn't hold up in practice. When they (with colleagues Anthony Keefe and Gene McDonald) put together carbon dioxide, ammonia, ferrous sulfide, and hydrogen sulfide in conditions that Wächtershäuser predicted would produce amino acids, no amino acids were in fact generated. Miller and Bada believe that the proposed reactions, although theoretically feasible, require the reactants to jump over energy barriers that are not easily crossed in the absence of catalysts. "Wächtershäuser made an awful lot of noise," comments Miller, "but it doesn't work. The vents don't make organics, they destroy them."

For Gustaf Arrhenius, whose lab is also at Scripps Oceanographic, origin-of-life research is a family tradition. Gustaf is the grandson of Svante Arrhenius, a brilliant and unorthodox Swedish physicist who won a Nobel Prize in 1903 for a fundamental discovery in electrochemistry—that electricity is carried in solution by charged ions. But Svante also wrestled with profound questions about life and the universe.

    "Yes, my grandfather was very interested in the origin of life," Arrhenius tells us. "Or rather, he was very uninterested. For like everyone in that period, he thought that the universe was infinite in time. Therefore, there was no reason to think that an event like the origin of life would take place just on this little speck of dust in space. It was more natural to believe that it was everywhere, that it cruised around from one place to another. Rather than worry about how life was created here on Earth, it made more sense to think how it might be transported here from elsewhere. He felt that he had hit upon a new way by which spores might be transported across space, through the pressure exerted by light. This phenomenon of light pressure, or radiation pressure, had just recently been discovered." Svante Arrhenius calculated that spores might be transported from Earth to Mars in 20 days and from the solar system to the nearest star in 9000 years.

    Svante Arrhenius's theory of "panspermia" ran into a serious problem in 1910 when the French plant physiologist and radiation pioneer Paul Becquerel showed that small organisms in space would be rapidly killed by the sun's ultraviolet radiation. Later, the discovery of cosmic rays made the survival of tiny spores in space even less plausible. Yet, Svante Arrhenius held on to his theory with almost mystical fervor. He found enormously appealing the notion that all organisms in the universe were related. Perhaps life had always existed, and the question of how it might be created was therefore superfluous.

    Gustaf Arrhenius, now in his seventies, betrays his Scandinavian origin with his accent, his careful choice of words, and his urbane good humor. And something still draws him to the chilly North, for he has recently spent time prospecting for geological specimens on the western coast of Greenland. There, he and his colleagues found what may be the most ancient remaining traces of life on earth.

    Searching for the origins of life in the geological record has, of course, occupied scientists for generations. Fossils are abundant in sedimentary rocks as old as the Cambrian, a period about 500 to 600 million years ago when life diversified into all kinds of exotic and soon-to-be-discarded forms. But before the Cambrian, it gets difficult. Organisms were mostly microscopic; or, if larger, they lacked the hard body parts that readily fossilize. Even worse, the more ancient sedimentary rocks themselves become harder to find, and when they can be located, they mostly turn out to have spent time at high temperature and pressure in the depths of the Earth, an experience that plays havoc with any fossils that the rocks may have contained.

    The oldest clearly recognizable fossilized organisms were found in the late 1980s in Western Australia by William Schopf of UCLA. Radioactive dating of the rocks in which they were embedded gave an age of 3.5 billion years. Schopf's organisms were filamentous microbes, very much resembling certain kinds of modern cyanobacteria—bacteria that get their energy from sunlight and that release oxygen in the process. The morphological similarity between the 3.5-billion-year-old fossils and their modern counterparts suggests that saying no to evolution can pay off handsomely in the long run.

    Arrhenius knew that Schopf's organisms were not likely to have been the Earth's first inhabitants—they were too complicated. So Arrhenius and research fellow Steve Mojzsis went looking in the Isua formation of West Greenland, which contains the oldest known sedimentary rocks—rocks that were laid down about 3.9 billion years ago, a mere 600 million years or so after the Earth's formation. Arrhenius and Mojzsis did not find any recognizable organisms in these rocks, which have been subjected to periods of intense heat and pressure since their deposition. But they did find microscopic grains of apatite, a form of calcium phosphate that is generally produced by living organisms rather than by geochemical processes. And within these grains were patches of carbonaceous material that might be the charred residues of ancient organisms.

    To pin down the matter a little more closely, Arrhenius and Mojzsis wanted to know the isotopic composition of the carbon in these patches. If the patches were indeed derived from living organisms, they should contain relatively more [sup.12]C and less [sup.13]C than is found in nonliving matter. That is because the enzymes that handle carbon work better on the light [sup.12]C isotope than on the heavier [sup.13]C isotope. (The even heavier [sup.14]C isotope does not come into the picture, because it disappears by radioactive decay over much shorter time periods than we are considering here.)

    A high [sup.12]C/[sup.13]C ratio was reported for Isua rocks some years ago by Manfred Schidlowski of the Max Planck Institute for Chemistry in Mainz, Germany. Schidlowski had interpreted this finding to mean that the carbon was of biogenic origin. But Arrhenius and Mojzsis wanted to know specifically about the carbon in the microscopic patches. They therefore called in Mark Harrison of UCLA, an expert in a technique known as ion microprobe analysis. In a machine somewhat resembling an electron microscope, a tiny beam of positively charged cesium ions was aimed at the individual carbonate patches. The carbon atoms were vaporized and fed into a mass spectrometer, which sorted them out by mass and electric charge. It turned out that the carbon isotope ratio was indeed that expected for carbon of biogenic origin, confirming Schidlowski's interpretation.

    Thus, Arrhenius is confident that living organisms existed on Earth at least 3.9 billion years ago. He can say nothing about what kind of organisms they might have been, except that, given their selectivity for [sup.12]C over [sup.13]C, they probably had enzyme-catalyzed metabolic pathways not unlike those of organisms existing today. Earth's first organisms, therefore, should have lived long before that period.

    These discoveries radically alter the scenario for the origin of life on Earth. Before the discoveries were made, it was possible to believe that the Earth lay fallow for a billion years or more, cool enough to sustain life, and rich in every nutrient required for life—yet stubbornly lifeless. What was missing, it seemed, was only the pinch of fairy dust that set everything into jangling motion. The origin of life was a probability barrier—an unlikely event that took long ages to happen, even when all circumstances seemed to favor it. But now it begins to look like the very opposite: that life arose at the very first possible moment—or even earlier!

    But when was the "first possible moment"? That depends on what happened to the Earth during its first billion years, and unfortunately the history of this period is hard to reconstruct. Very large impacts, massive enough to completely vaporize any oceans that may have existed, probably continued until at least 4.3 billion years ago. Thus, any life that may have taken hold earlier should have been wiped out. Even after that period, however, major impacts apparently continued. In fact, to judge from the craters on the Moon, whose ages have been established by the study of samples returned by the Apollo astronauts, intense bombardment continued until at least 3.8 billion years ago. How can this be reconciled with an apparent origin of life well before 3.9 billion years ago?

    One possible answer is that life arose on the deep-ocean floor, in an environment protected from all but the most cataclysmic impacts. This environment would also have been safe from another hazard—ice. For according to some theorists, the oceans did indeed freeze over repeatedly, to a depth of thousands of feet, only to be melted by the next all-incinerating impact. Not for nothing are the Earth's first 600 million years referred to as the Hadean Eon—the "age of Hell." Perhaps the deepest reaches of the ocean, especially the zones around the volcanic vents, did double duty as incubators and bomb shelters for the Earth's first inhabitants.

    Arrhenius has a different, somewhat unconventional answer to the paradox. "I believe that the late bombardment of the Moon has nothing to do with the evolution of the Earth," he says. "It was caused by collision with objects in the Moon's own orbit—the final remains of the ring of debris that accreted to form the Moon." According to this hypothesis, none of these objects struck the Earth, and therefore the Earth may have been habitable much earlier than generally believed.

    A third possibility remains unspoken—that the elder Arrhenius was right, and the Earth was indeed seeded by living organisms from other worlds. As we shall see, that could have happened otherwise than by means of light-borne spores.

Leslie Orgel, whose laboratory is at the Salk Institute, across the road from UCSD, is another distinguished elder statesman of science. Like Arrhenius, he has eschewed retirement in favor of a continued struggle with the vexatious problem of how life originated. As if worn down by the effort, the British-born chemist affects a hangdog world-weariness. When asked his opinion of some new theory, Orgel is likely to remark that "it's no worse a possibility than the others." He is emphatic only about the depths of human ignorance. Hearing that we were working on a book about life in the cosmos, Orgel comments simply: "My opinion is that we have no way of knowing anything about the probability of life in the cosmos. It could be everywhere, or we could be alone."

    Yet Orgel, more than anyone else, is responsible for a profound insight about the evolution of life on Earth. He has concerned himself with the question of how one might get from a prebiotic soup of organic chemicals to an actual living system. He has concluded that the life that we're familiar with today—based on the triumvirate of DNA, RNA, and protein—was almost certainly not the original form of life on Earth. Instead, it was likely preceded by another, radically simpler form of life, in which RNA ruled alone.

    In today's world, the famous double helix of DNA is the almost universal repository of genetic information. RNA plays several key roles in the execution of that information, that is, in carrying out the synthesis of proteins under genetic instructions. Proteins are largely responsible for the structure of living matter and, as enzymes, for catalyzing the innumerable metabolic reactions of `life'—including the reactions that lead to the assembly of DNA and RNA. Thus, there is a classic chicken-and-egg problem: Which came first, nucleic acids (DNA and/or RNA) or proteins? Seemingly, neither could come into being without the other, yet the idea that they arose simultaneously and by chance strains credulity.

    About thirty years ago, Orgel, along with Francis Crick and Carl Woese, suggested that RNA came first. They rejected proteins as the primordial macromolecules because there was no obvious way by which proteins could replicate themselves. Nucleic acids, on the other hand, have the nucleotide base-pairing mechanism: guanine to cytosine and adenine to thymine (or to uracil in RNA). This mechanism would offer at least the theoretical possibility of replication without any assistance from proteins. Of the two nucleic acids, Orgel, Crick, and Woese favored RNA as the first-comer because of the relative ease with which the building blocks of RNA could be synthesized. In addition, several aspects of DNA biochemistry are dependent on RNA and its constituents, suggesting that RNA already existed when DNA biochemistry began.

    There are three challenging requirements for an "RNA world." First, the building blocks of RNA must be available. Second, there must be a mechanism for an RNA molecule to assemble (polymerize) from those building blocks without the help of protein enzymes. And third, there must be a mechanism for RNA molecules to form copies of themselves (replicate), again without the help of proteins.

    Nucleotides—the building blocks of both RNA and DNA—are composed of a base (adenine, cytosine, guanine, and thymine in DNA—uracil replaces thymine in RNA) linked to a sugar (deoxyribose in DNA, ribose in RNA), which in turn is linked to a phosphate group (PO4). (Nucleosides are nucleotides without a phosphate group.) As we've mentioned, the bases may have been available from reactions in the primordial soup, or may have been imported on micrometeorites. But making ribose and adding it on to the bases is more problematic. The difficulty is that, while it is possible to find circumstances in which ribose is made, it generally shows up as a small constituent in a mixture of many different molecules. Some of these molecules are likely to interfere with subsequent processes, just as mixing different-sized ball bearings will bring a machine grinding to a halt. Still, Orgel is reasonably optimistic that a plausible pathway will eventually be found, perhaps involving inorganic catalysts such as mineral surfaces.

    Mineral surfaces might also play a role in the polymerization of RNA. James Ferris, who directs the New York Center for Studies of the Origins of Life, has managed to get ribonucleotides to assemble into short chains on the surface of a kind of clay called montmorillonite. Some slightly altered ribonucleotides will form chains of an RNA-like polymer containing more than fifty bases. Again, it seems as if further research may find circumstances in which long chains of RNA will be formed.

    Getting RNA to replicate is the toughest problem. For this to happen, new ribonucleotides must bind to the existing chain, following the rules of base pairing: uracil binds to adenine, cytosine to guanine. Then, the new ribonucleotides must polymerize into a chain and separate from the original chain. The new chain, with a base sequence complementary to the original one, must now serve as the template for a second round of ribonucleotide binding, which produces a third-generation chain identical to the first. Unfortunately, although Orgel has had some limited success with the first round of copying, he has not been able to get both steps to happen. No RNA molecule has been replicated in the laboratory without the aid of protein enzymes.

    "Something has to give," says Orgel. "Maybe someone will find an easy way of making nucleotides. If someone found a magic mineral which you shook up with formaldehyde or cyanide and phosphate, and out came ribose phosphate—then the whole problem would be different."

    But more likely, Orgel thinks, the RNA world was not the first living system on Earth—it's just too complicated. Some simpler system had to precede it. Among the candidates for such a system is a macromolecule called peptide nucleic acid (PNA). This molecule is not known to exist in nature; it was designed by Peter Nielsen of the University of Copenhagen. PNA uses base-pairing like RNA, but has a much simpler, proteinlike backbone. Another macromolecule, called pyranosyl-RNA (pRNA), resembles RNA except that it contains a different, more abundant, version of ribose. Albert Eschenmoser, of the Swiss Federal Institute of Technology in Zurich, has coaxed pRNA into replicating under certain conditions.

    But for Orgel, PNA and pRNA are still too complicated. "We want something really simple, like a polymer of aspartate and glutamate [two very similar amino acids]. Anything much more complicated than that is implausible. It's so hard to make RNA. If nothing simpler can replicate, that would be a strong argument for the existence of God."

    Orgel believes that living organisms may travel from world to world, and therefore that terrestrial life may have come from another planet, as Svante Arrhenius suggested. The organisms would not travel as free-floating spores, of course. Rather, they would travel in the interior of meteorites. It's now well established that meteorites have traveled from the Moon and from Mars to Earth. Doubtless, they have traveled in the other direction too. Deep inside the meteorite, microbes would be shielded from harmful radiation, and the near-absolute-zero temperatures would keep the frozen organisms in pristine condition. "You could take E. coli [bacteria] and cool it rapidly to 10°K [Kelvin] and leave it for 10 billion years and then put it back in [water and] glucose, and I suspect you would have 99 percent survival," says Orgel. Takeoff and reentry are problematic, off course. But we know that the interiors of meteorites can remain cold during reentry. As for the initial impact that kicks the rock into space, Orgel draws an analogy to the circus performers who allow themselves to be shot from guns. "They use slow-burning gunpowder," he said, "so they survive the acceleration." Similarly, an impact that generated large amounts of expanding gas could accelerate a rock into space gently enough to spare the lives of the microbes within it.

    Bada and Orgel are members of a NASA committee that concerns itself with the possible risks of bringing samples of extraterrestrial rocks back to Earth. Could microbes from another world set off a lethal pandemic here? The maverick British cosmologist Fred Hoyle has been saying so for years; in fact, he believes that some of the great epidemics of human history were caused by bacteria and viruses that came from space.

    According to Orgel, much depends on whether the Life to which those microbes belong is related to our own Life—whether one is the parent or sibling of the other, so to speak. If so, the microbes would likely be similar enough to us in their basic biochemistry that they might be able to subvert our metabolic pathways, just as pathogenic terrestrial microbes do. But if those microbes belonged to a completely independent Life, he tells us reassuringly that "they could only eat us—nothing more subtle."

    Orgel wraps up our discussion on a characteristic note. "So, have we finished the origins of life? I suspect we have, haven't we? Nothing's known about it—what more is there to say? All theories are bad."

Our tour ends in a more upbeat vein, with a visit to Gerald Joyce, a molecular biologist at the Scripps Research Institute, almost next door to the Salk Institute on the bluffs overlooking the Pacific. Joyce must be about half the age of most of the other members of the NSCORT group, and he certainly plays the role of the young Turk. Where Orgel is cautious to a fault, Joyce is brazenly optimistic. A couple of years ago, he asserted that life would be created in a test tube by the end of the twentieth century.

    Of course, that brings up the question: What is `life'? Philosophers, theologians, and scientists have been torturing themselves over this for centuries. According to Joyce, there is a folk definition and a scientific definition. The folk definition of life is "that which is squishy." While open to criticism, this definition does seem to capture something about life: its plasticity, its vulnerability, and the notion that we will intuitively recognize it when we see it.

    For a scientific definition, Joyce offers the product of a NASA workshop on the subject: life is a "self-sustained chemical system capable of undergoing Darwinian evolution." That means a system that can undergo reproduction, mutation, and natural selection. Of course, that definition wouldn't sit well with the people at the Museum of Creation, who don't believe that evolution happens. And later, we will have the opportunity to question other aspects of it: whether, for example, a nonchemical system such as a computer program could ever be considered "alive." But it seems like a reasonable starting point.

    A "self-sustained" system, by the way, doesn't mean a closed system. To maintain itself in the face of the second law of thermodynamics—the inevitable increase in disorder or entropy in a closed system—life must use energy from outside the system. This energy could be sunlight, it could be chemical energy locked in minerals, organic compounds, or other living creatures, it could even be electricity or gravitational energy. But whatever the energy source, this requirement ensures that life can only be understood in terms of its relationship with the environment.

    Joyce thinks that between life and non-life there's a sharp boundary. The origin of life wasn't the sequential appearance of gradually more `lifelike' systems, from ones that were 1 percent living to ones that were 100 percent living. Systems gradually became more complex, certainly, but the appearance of life was at the point where such systems were able to record their phylogenetic history—which means informational macromolecules that can be replicated.

    Like Orgel, Joyce works with RNA. In fact, he trained with Orgel at the Salk Institute before organizing his own lab at Scripps. But rather than focus on how RNA came to be, Joyce is engaged in the attempt to create a self-sustaining, evolving RNA system in a test tube. He wants to bring the RNA world back to life.

    To do this, Joyce uses a special class of RNA molecules called ribozymes. These molecules have the property not only of encoding information, but also of catalyzing enzymatic reactions—a function generally performed by proteins. To some extent, the existence of such molecules was implicit in the theory of the RNA world when it was first proposed, but it became a reality in 1983 when Thomas Cech of the University of Colorado and Sidney Altman of Yale discovered naturally occurring RNA molecules that are able to enzymatically alter other nucleic acid molecules. These "ribozymal" RNAS are not a structurally distinct group in the sense of having, say, a chemically different backbone from other RNAS. Rather, it's simply that the particular sequence of nucleotide bases in a ribozyme causes the RNA molecule to fold up into a three-dimensional shape that is capable of catalyzing a particular reaction, just as the particular sequence of amino acids in a protein gives the molecule its particular enzymatic talent.

    Joyce is after a ribozyme that can catalyze the replication of RNA—an "RNA replicase." Such a molecule, of course, could be the central player in an RNA world. If it could be found, many of the problems faced by Orgel would be circumvented. But no such ribozyme exists in the world today, as far as anyone knows. If it existed in the RNA world, as Joyce believes, it has long since gone extinct. So how to re-create it? Just trying out a bunch of randomly chosen RNA sequences doesn't seem like a workable strategy, given the numbers involved. The molecule would probably have to be at least fifty bases long to do a reasonable job, but there are about [10.sup.30] possible RNA sequences of this length—that's one with thirty zeroes after it. If you laid out the sequences from here to the most distant observable object in the universe, you'd have to pack a hundred billion different sequences into every millimeter of the way.

    Joyce's solution is to make evolution run backward. He starts with an existing ribozyme that can, to some limited degree, carry out a reaction that is a part of the desired "replicase" function. For example, it might be able to join two short stretches of RNA. He puts this in a test tube along with a set of molecules, including DNA, protein enzymes, and nucleotides, which collectively allow the ribozyme to replicate. Within an hour, a single ribozyme molecule has multiplied a trillion-fold. The trick is that Joyce arranges things so that the ribozyme's enzymatic activity is made part of the replication process. Thus, any ribozyme molecule that by chance mutates slightly, so as to perform a better enzymatic job than its peers, replicates faster. It becomes the dominant type of molecule in the tube. Thus, the mixture self-evolves in a direction set by the experimenter.

    The aim is to have the ribozyme evolve to a point where it can do without the DNA and proteins. In other words, the ribozyme has to evolve backward from our present world to the RNA world. Joyce hasn't achieved that yet. But he, along with other researchers who have used similar techniques, has made significant progress. Continuous in vitro evolution, as it is called, seems to be a powerful technique for moving in a directed fashion through "sequence space"—the multidimensional realm of possible RNA sequences.

    So will life be created in a test tube by the end of the century, as Joyce prophesied a while back? "Sure," he says blithely, "but remember the century isn't really over till January 2001."

Figuring out how a Life gets started—whether on Earth or elsewhere—has proved to be an enormously difficult task. One wonders what Darwin would think if he could learn of all the head-spinning theories, tortuous experiments, and confusing blind alleys that have bedeviled efforts to breathe life into his "warm little pond." Would he applaud the progress, or would he throw up his hands and retreat to the notion of intentional creation? It's hard to say. But Leslie Orgel, who in many ways is the most pessimistic of the La Jolla scientists, nevertheless maintains a basic loyalty to the idea of a natural origin. "The best I can say," he says, "is that there doesn't seem to be any requirement for magic to happen."

    The underlying question that remains unanswered has to do with likelihoods. The polymers that are the chemical basis of life are complicated. To assemble them by chance rolls of the dice seems to ask too much, even with an astronomical number of planets on which to roll them and 15 billion years to roll them in, for astronomical numbers are no match for hyperastronomical improbabilities. Fred Hoyle put it succinctly, when he commented that the chances of forming the simplest life by random processes were about the same as the chances that a whirlwind sweeping through a junkyard might put together a Boeing 747 airliner.

    The question is: Are there forces that guide the assembly process through the near-infinite maze of possibilities? Joyce's in vitro evolution hints at such a force. But what is needed, it seems, is a theory that explains the self-organization of complex systems at a more fundamental, abstract level. In Chapter 6, we will visit one place where the search for such a theory is underway.

    In spite of the partial and incomplete picture that the La Jolla researchers have been able to paint, there is the thread of a story. In particular, the development of the DNA/RNA/protein world, with which we are familiar today, probably involved several successive stages of increasing complexity, with at least two major transitions: from a pre-RNA world to the RNA world, and from the RNA world to our own world.

    There is something ominous about those transitions. Did RNA "invent" DNA for example, in order to have a safer repository for its genetic information, only to have DNA "mutiny" and relegate RNA to a subservient role? These human analogies are foolish, of course, when talking about evolving chemical systems, but they're hard to avoid.

    We ask Orgel whether he thinks that our own biological world will in time be taken over by the next level of complexity. We're thinking of intelligent computers or some such thing. But Orgel, ever the chemist, has his own ideas. "I claim it would happen with some weird alkaloid," he says. Alkaloids are elaborate, nitrogen-containing compounds that include, for example, heroin and caffeine. "Alkaloids look to us the way nucleotides looked to the pre-RNA world—complicated. And then one of them turns out to be a Frankenstein."

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Table of Contents

Introduction 1
1 Origins: How Life on Earth Began 11
2 Going to Extremes: The Habitats and Requirements for Life 39
3 The Incredible Shrinking Martians: Searching for Life in the Solar System 63
4 The Death and Life of Stars: Organic Molecules and the Evolution of Solar Systems 91
5 The Planet Finders: Searching for Life Beyond the Sun 109
6 What Happens in Evolution? Chance and Necessity in the Origin of Biological Complexity 131
7 SETI: The Search for Extraterrestrial Intelligence 159
8 Dreamland: The Science and Religion of UFOs 183
9 Exotica: Life as We Don't Know It 195
10 Many Worlds: Cosmology and the Anthropic Principle 219
Conclusions 237
Notes 247
Index 257
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