by David Toomey
WEIRD LIFE
THE SEARCH FOR LIFE THAT IS VERY,
VERY DIFFERENT FROM OUR OWN
David Toomey
Contents
Cover
Title Page
Prologue
CHAPTER ONE: Extremophiles
CHAPTER TWO: A Shadow Biosphere
CHAPTER THREE: Defining Life
CHAPTER FOUR: Starting from Scratch
CHAPTER FIVE: A Bestiary of Weird Life
CHAPTER SIX: Life from Comets, Life on Stars, and Life in the Very Far Future
CHAPTER SEVEN: Intelligent Weird Life
CHAPTER EIGHT: Weird Life in Science Fiction
CHAPTER NINE: Weird Life in the Multiverse
Epilogue
Glossary
Notes
Works Cited
Index
Picture Section
Also by David Toomey
Additional Praise for Weird Life
Copyright
Prologue
One of my favorite books as a child (and, truth be told, one of my favorites today) is Dr. Seuss’s If I Ran the Zoo. The “I” of the title is Gerald McGrew, a boy of perhaps age ten or twelve. The book opens as young McGrew visits his local zoo and finds its animals—a few sleepy-eyed bears and lions—uninspiring. He wishes for more exotic creatures, and so begins an extended daydream. Our youthful protagonist imagines himself as the zoo’s manager, releasing the bears and lions, which shamble off, one presumes, to find more stimulating environs. Then he fantasizes himself the zoo’s procurer, outfitted with pith helmet and butterfly net, in the heroic mold of Mallory and Burton, scaling mountains and crossing oceans in search of ever more exotic animals. And he finds them. In the “Desert of Zind” he captures a ferocious sort of camel called a “Mulligatawny,” and on the “Island of Gwark” he catches a gigantic bird called a “Fizza-ma-Wizza-ma-Dill.” And then he assures us that he’s just warming up.
Young Gerald McGrew may be a fictional character in a children’s book, but his urges resonate. Humans, it seems, have always been less than satisfied with actual fauna, and so moved to invent alternatives. We all remember a few: the sphinx, the griffin, the basilisk, the phoenix. But ancient cultures created many more. Margaret Robinson’s Fictitious Beasts (one of the most thorough and authoritative catalogues) lists several hundred, each description replete with details of behavior and in many cases an instructive encounter with a god or heroic mortal.1 To a biologist, what is striking about this imaginary bestiary, especially in comparison with the bestiary that nature actually has produced, is its paucity. The fact is that no one knows exactly how many species reside on our planet at present, but a conservative guess is 3.6 million, and some estimates are as high as 100 million.2 To those who prefer the real to the imaginary, it should come as good news that Robinson’s work has a real-world cognate. The Encyclopedia of Life, an effort to build an online compendium of every extant species, is now at 500,000 web pages and growing.
Peruse Robinson’s work at a rate of a second per page, and you’ll close the back cover after about four minutes. To get through the Encyclopedia of Life at the same rate you’ll need six weeks. Even then, you’ll have only a hint of the astonishing diversity of life over time. There have been at least 30 billion distinct species in the history of Earth.3 Suppose there were a book devoting a page to each species. To read it at a rate of a second per page, you’d need nearly ten centuries.
It’s not just the numbers. By comparison with reality, the creatures of myth suffer in another way. Most are little more than portmanteaus—with, for instance, the head of one animal sewn onto the body of another. Any imaginatively sadistic schoolchild equipped with scissors and glue, you might think, could do as well. Even the parts list is severely scaled back, derived, as it is, almost entirely from the single branch of the tree of life that bears mammals, lizards, and birds. There are exceptions like “Grandmother Spider,” who figures in many Native American creation myths; and the kraken, the giant squid that inhabits the imaginations of coastal dwellers on several continents. And there are some rather wondrous hybrids. The “vegetable lamb of Tartary” (Agnus scythicus or Planta Tartarica Barometz), for instance, was a legendary plant of central Asia believed to grow sheep as its fruit. The sheep were connected to the plant by an umbilical cord, and when they had eaten all they could reach, the whole plant withered and died.4 But that is about as bizarre as the mythical beasts get. The overwhelming majority, if we were to classify them within standard taxonomies, would be vertebrates in the phylum Chordata—essentially, animals with backbones.
For most of human history, anyone seeking a little strangeness in the proportion had to be satisfied with these. Then, in the mid-seventeenth century, natural philosophers discovered another bestiary, one that had two advantages over the imagined one. First, its animals were real. Second, they did not inhabit an exotic and distant country. In fact, they lived among us. And on us. A great many of them lived inside us.
By the late 1620s, the first of the type of instrument we call the microscope had been crafted and named. Half a century later, a twenty-eight-year-old British naturalist named Robert Hooke began to make his own, to use them to examine a great many things, and to sketch what he saw. Hooke’s work was not always easy. He had two sorts of microscopes. One, rather like the instrument we know, was a set of lenses fixed and aligned inside a small tube; the other, far more difficult to use, was a glass bead the size of a pinhead, held in a brass mounting. Of course, the subjects themselves could be uncooperative. The only way Hooke could immobilize ants without half crushing them was to get them drunk on brandy.
Despite such challenges, by 1664 Hooke had produced a work called Micrographia. Even now, in an age of high-definition television and IMAX 3D, it can be a startling experience to open the book to a folded page and gently pull its leaves apart to reveal, for instance, a copperplate engraving of a flea measuring a monstrous half meter across. One of Hooke’s contemporaries, Samuel Pepys, called the book the “most ingenious” he had ever read. Micrographia became a best seller, and many of its readers would have agreed with Hooke’s observation of the flea: “the strength and beauty of this small creature, had it no other relation at all to man, would deserve a description.”5 Still, a few criticized Hooke’s pursuit of knowledge with no obvious practical application, and many more were simply uninterested. The reason may have been that the animals discovered by Hooke and his successors were utterly unlike anything known at the time; they were perhaps too strange. Of course, there was another reason for the dis-interest: Hooke’s creatures were very, very small. Then, as now, humans equate small with unimportant—a prejudice that, as we shall see, is as misguided as it is dangerous.
Some two centuries after Micrographia, natural philosophers discovered another bestiary—this one populated by animals that were as large by comparison with us as we are compared to house cats. They are long vanished from the Earth, but their appeal, perhaps especially to a certain demographic of seven-year-olds, has proved enduring. Some of the reasons are obvious. Dinosaurs are strange enough to inspire wonder, but not so strange as to be wholly unfamiliar. Tyrannosaurus rex (a favorite of the aforementioned demographic) saw through eyes not unlike our own, breathed through nostrils, and walked over ground.*
As the flea and Tyrannosaurus rex demonstrate, nature itself will outperform the uninformed imagination every time. If these creatures showed the limits of human imagination, they also enlarged it. With the discovery of microscopic and sub-microscopic life came que
stions about nourishment, reproduction, and mobility. Was there cooperation? How small was it possible for a living thing to be? The dinosaurs, likewise, produced new questions. How could such enormous masses be supported? How large was it possible for a living thing to be? And of course, why did they vanish?
Even as naturalists pondered these questions, there came a new understanding of life at fundamental levels. In 1859, Charles Darwin, prodded by the independent discoveries of British naturalist Alfred Russel Wallace, published On the Origin of Species by Means of Natural Selection. Despite attacks by religious conservatives, the book was widely read (it would see six editions in Darwin’s lifetime), and within a decade several works were published in its support. In the early years of the twentieth century, biologists rediscovered Gregor Mendel’s laws of heredity, and American geneticist Walter Sutton found evidence that chromosomes carry units of inheritance. In the 1940s, Julian Huxley and George Gaylord Simpson consolidated natural selection and genetics, and DNA was found responsible for hereditary changes in bacteria. In 1953, James Watson and Francis Crick published the structure of DNA in the journal Nature.
Meanwhile, as to the variety of forms that might be made with that DNA, there were—to the delight of most and the consternation of some—more surprises. Again and again, scientists discovered animals and plants that broke all the rules, surpassing what many had assumed to be limits in size, shape, and behavior. Nonetheless, by the mid-twentieth century most biologists had reason to believe that life would survive within only a narrow range of pressures and temperatures. There seemed to be some limits that could not be surpassed.
By this time there were at least nine specialties in biology, the study of living organisms and life processes. Probably it shouldn’t be surprising that practitioners in each specialty tended to define life in the terms of that specialty, and that they had no shared definition of the core subject at all. But what is surprising is that no one thought this lack of consensus much of a problem. Taxonomists, molecular biologists, and embryologists went about their business identifying species, studying chemical reactions that maintained a metabolism, and culturing microbes. If asked to define life by, say, an upstart philosophy major at an interdepartmental faculty reception, they would say they knew it when they saw it and that, thank you, was quite enough.
In the early 1970s, however, it became obvious that the biologists’ confidence in their powers of recognition, whether justified or unjustified, was not quite enough. It was about that time that NASA asked scientists to submit designs for life-detecting experiments to be carried to the planet Mars aboard the two unmanned Viking spacecraft. These would be the first in situ attempts to discover life on another world. To detect something with a miniature laboratory that would be operated remotely by a radio signal sent from a transmitter more than a million miles away was no small challenge. It seemed reasonable that the task would be made easier if the “something” to be detected was properly defined first.
The three experiments chosen by NASA were ingenious but, at least in the view of some, lacking in imagination. Two were designed with the assumption that Martian life would need water, and all three were designed under the assumption that life would survive in only a narrow (in fact, a rather Earthlike) range of temperatures. Depending on whom you ask, the results of that reconnaissance meant that either there was no life in the spacecrafts’ vicinity or (this from one experiment) there might be some very unusual life indeed. In any case, the results did little to change larger ideas of life’s boundaries.
Then, in a series of discoveries in the 1980s and 1990s back on Earth, scientists found that they had underestimated nature’s ingenuity; the realm of life was (again) greater than they had dared imagine. In places where no one thought life possible, organisms were not merely surviving; they were thriving. Once biologists began to look, they found them everywhere. And there were enough to satisfy an army of Gerald McGrews.
No one expected life in water much above its boiling point. But scientists found bacteria living in volcanic hydrothermal vents on the ocean floor, one species merrily reproducing at a scalding 235°F. No one thought life could survive in water at temperatures much below its freezing point. But in Antarctic ice floes, scientists found channels of slushy brine in which single-celled algae were harvesting energy from the sunlight filtered through ice and assimilating nutrients from the water below.
Biologists had assumed other limits as well. They had thought that organisms would tolerate only a narrow range of pH levels. Then they discovered life flourishing in hot sulfur springs and growing vigorously in soda lakes. They had assumed that aquatic life would tolerate only so much salt. But they found bacteria that had adapted perfectly to saturated salt lakes. They had believed that high levels of radiation would kill any organism. But they discovered a bacterium that, by efficiently repairing broken DNA strands, could withstand radiation energies at a thousand times the level that would kill a human. They had assumed that life required a “substrate”—a surface on which its molecules could interact easily and often. But they found microbes that may be living through their entire life cycles—growing, metabolizing, reproducing—in clouds.6
Biologists knew that many creatures living in the dark on the seafloor ate organic material that fell slowly from the surface, and that some survived by drawing energy from chemical reactions. Still, they assumed that all life depended—perhaps indirectly but nonetheless ultimately—on the Sun. But in 1996, a group of scientists reported the discovery, more than a mile beneath Earth’s surface, of assemblages of bacteria and fungi that gained all their energy from inorganic chemicals in the rock around them.
All these organisms—the bacteria in the hydrothermal vent, the algae in the Antarctic brine, the rock-eating fungi, and the rest—came to be known collectively as extremophiles—lovers of extremes.
The physical boundaries within which life is possible are unknown and undefined, but most biologists believe that they must exist, for the simple reason that there are temperatures and pressures under which the structures of organisms—cells, DNA, and proteins—will break down, no matter how well protected. In short, life must have ultimate limits. If life exists outside them, it must be something other than what has been called, in that venerable phrase that can hardly be improved upon, “life as we know it.” It must be fundamentally different. It must be, in a word, weird.
Exactly what can we say about such life? At the very least, we can say what it is not. All life we know has DNA, the same twenty or so amino acids and proteins, and a biochemistry that employs the same thousands of chemical pathways (the complex chemical reactions by which a metabolism is maintained) and that uses liquid water as a solvent. It is for these reasons that biologists believe that all life we know—you, me, the flea, the megalosaurus, Charles Darwin, your neighborhood creationist, and all the extremophiles—is descended from a single common ancestor. Weird life, if it exists, would not be descended from that ancestor, and it could be weird in any number of ways. It might have as its basis a molecule other than DNA, it might use other amino acids, or it might use a solvent like ammonia or liquid methane.
Whether such substitutions are possible, or whether the fundamental features of life we know are necessary to all life, is far from clear. But it may be that some, or many, or most of those features are the products of happenstance, and that life on Earth might as easily have taken a different path, the result being organisms whose fundamental features would be utterly different from those we know. What is clear is that the discovery of even one example of such life would profoundly change our understanding of biology. The familiar illustration of all life we know is a great tree, its trunk splitting and splitting again into branches representing phylogenetic categories, each less fundamental and more populous than that from which it sprouted, finally ending in millions of twigs representing individual species. There have been many discoveries of new species, and recently, even a new phylum.7 But an example of another sort of l
ife would mean that the tree itself is not unique, and that it may be only one of many, perhaps one in an entire forest. Such a discovery would be cause for humility on our part—another demonstration that we occupy a smaller part of the universe than we now believe. It would also be cause for renewed wonder at a cosmos that is stranger and vastly richer than we now imagine.
Papers on what we now call weird life appeared sixty years ago, but they were few and scattered across disciplines. There was no overview and nothing like an ongoing discussion. The first gesture in those directions came in 2002. NASA and the European Space Agency (ESA) were thinking about (if not exactly planning) unmanned missions to the outer Solar System—to Saturn’s moon Titan, to Neptune’s moon Triton, to the comets. If life existed in these places, it would be radically different from anything we knew, and radically different from anything that Viking looked for on Mars.
The idea is not new, and we’ve seen it again and again in science fiction: An astronaut on a desolate but otherwise unremarkable planet sees what he assumes is an odd rock formation. He looks away, and it moves. He looks back and realizes his mistake, and (as they say) dramatic complications ensue. But this was not science fiction. Now NASA and ESA were taking the possibility of weird life quite seriously, and making it clear that much was at stake. The discovery of extraterrestrial life—whether it developed independently or had migrated from planet to planet via meteor, solar wind, or some other means—would have profound and lasting consequences not merely for the life sciences or even for science in general, but for our understanding of our very place in the universe. But would we recognize life if we saw it? And if we did not recognize it, might we, by inattention or carelessness, destroy it?
In 2002 the National Research Council (NRC) assembled a group of twenty-five scientists from research laboratories and institutes across the United States. The group called itself the Committee on the Limits of Organic Life in Planetary Systems, and its task was nothing if not ambitious. Its members were to define life, to identify the traits necessary to life as we know it, and to determine the outer limits of living systems. As if this were not enough, they were handed a second, more provocative challenge: to imagine possibilities for weird life in some detail. For five years they read and discussed papers, collected data, and talked. By summer 2007 they had published a report summarizing their work. It was titled The Limits of Organic Life in Planetary Systems.8 Publication of the NRC report represented a watershed moment in the history of thinking about the boundaries of life as we know it, and what sort of life might lie beyond those boundaries. It also provides much of the foundation for this book.
