A recent origin-of-life text features an appendix with scientific definitions of life written by 48 different authorities. The entry contributed by the distinguished evolutionary biologist John Maynard Smith describes life as “any population of entities which has the properties of multiplication, heredity and variation.” Alternatively, information theorist Stuart Kauffman claims that “life is an expected, collectively self-organized property of catalytic polymers.” Other equally renowned experts propose that “Life is the ability to communicate,” “Life is a flow of energy, matter and information,” “Life is a self-sustained chemical system capable of undergoing Darwinian evolution.” The definitions go on and on. Remarkably, no two definitions are the same.

This lack of agreement represents an obvious problem for those who search for signs of living organisms on other worlds, as well as for origin-of-life researchers. It is difficult to be sure that you’ve discovered life—or deduced the process of life’s origin, for that matter—when you can’t define what it is. In spite of generations of work by hundreds of thousands of biologists, in spite of countless studies of living organisms at every scale from molecules to continents, we still have no widely accepted definition.

This frustrating lack is not particularly surprising. For one thing, the question “What is life?” is asked in different contexts by different professions. Theologians hotly debate it in relation to the beginning of human life. Does life start at the moment of conception, when the fetal

brain first responds, or when the unborn heart first beats? In some theologies, life commences not with a physical process, but at the unknowable supposed instant of ensoulment. At the other end of the human journey, doctors and lawyers require a definition of life in order to deal ethically with patients who are brain dead or otherwise terminally unresponsive.

In contrast to these ethically complex and emotionally charged issues are the more abstract scientific efforts to define life. Biologists rely on straightforward genetic analysis—tests for DNA or diagnostic proteins—to identify the presence of life-forms on Earth today. But a more general definition that distinguishes all imaginable living objects from the myriad nonliving ones remains elusive. We know relatively little about the diversity of cellular life on Earth, not to mention the vast range of plausible noncellular life-forms that might await discovery elsewhere in the universe. Endorsing a sweeping definition of life based on such scanty knowledge is akin to defining “music” after listening to a single recording of Bach’s solo cello suites over and over again. The suites are a sublime example of music, but hardly sufficient to characterize the entire genre.

Scientists crave an unambiguous definition of life, and they adopt two complementary approaches in their efforts to distinguish that which is alive from that which is not. Many scientists adopt the “top-down” approach. They scrutinize all manner of unambiguous living and fossil organisms to identify the most primitive entities that are, or were, alive. For origin-of-life researchers, primitive microbes and ancient microfossils have the potential to provide relevant clues about life’s early chemistry. This strategy is limited, however, because all known life-forms, whether living or fossil, are based on biochemically sophisticated cells containing DNA and proteins. Any definition of life based on top-down research is correspondingly limited.

By contrast, a small army of investigators pursues the so-called “bottom-up” approach. They devise laboratory experiments to mimic the emergent chemistry of ancient Earth environments. Eventually, the bottom-up goal is to create a living chemical system in the laboratory from scratch—an effort that might clarify the transition from nonlife to life. Such research leads to an amusing range of passionate opinions

regarding what is alive, because each scientist tends to define life in terms of his or her own chosen specialty. One group will focus on the origin of cell membranes; to them, life began when the first encapsulating membrane appeared. Another team studies the emergence of metabolic cycles, so naturally for them the origin of life coincided with the origin of metabolism. Still other groups investigate primordial RNA (DNA’s presumed precursor genetic material), viruses, or even artificial intelligence, and each group hawks its own definition of life’s first appearance.

Into this mix, philosophers and theologians inject a more abstract view and speculate on the full range of phenomena that might be said to be alive—robotic life, computer life, even a self-aware Internet. Such debates can at times sound like a science fiction convention, but defining life is no idle exercise. The scientific community, with the full support of NASA and other governmental agencies, holds regular meetings to debate the question. After all, one of NASA’s prime missions is to look for life on other worlds, so a clear definition is essential for planning future missions.

It’s amazing how the “What is life?” question sparks arguments and fosters hard-line positions. Scientists excel at many things, but compromise is not always one of them. Nevertheless, Gerald Joyce of The Scripps Research Institute, serving on a NASA Exobiology panel, proposed a widely cited “working definition” for life in the context of space exploration. “Life is a self-sustained chemical system capable of undergoing Darwinian evolution,” he suggested.

According to this opinion, life combines three distinctive characteristics. First, any form of life must be a chemical system. Computer programs, robots activated with microchips, or other electronic entities are not alive according to this definition. Life also grows and sustains itself by gathering energy and atoms from its surroundings—the essence of metabolism. Finally, living entities must display variation. Natural selection of the more fit individuals will inevitably lead to evolution and the emergence of more complex entities. This NASA-inspired definition is probably as general, useful, and concise as any we are likely to come up with—at least until we discover more about what is actually out there.

Even armed with this functional definition, it’s difficult to know what Earth’s very first life-form was like. Our planet’s earliest life may have been vastly different from anything we know today. Many experts

suspect that the first living entity was not a single isolated cell as we know it, for even the simplest cell incorporates astonishing chemical complexity. That first life-form probably did not use DNA, given the exceedingly intricate genetic mechanism of life on Earth today. It may not even have used proteins, the chemical workhorses of cellular life.

Experts in different fields propose different ideas regarding Earth’s first life-form. As a geologist, trained in the ways of rocks, my favorite theory is that the very first entity to fit NASA’s trial definition may have been an extremely thin molecular coating on rock surfaces. Such “flat life” would have spread across mineral grains in a layer only a few billionths of a meter thick, exploiting energy-rich mineral surfaces while slowly spreading like a lichen from rock to rock.

Whatever the first life-form looked like, it must have arisen from chemical reactions of ocean, atmosphere, and rocks. Yet the overarching problem with studying life’s origin is that even the simplest known life-form is vastly more complex than any nonliving components that might have contributed to it. How does such astonishing, intricate complexity arise from lifeless raw materials? Emergence can help.

French anthropologist Claude Lévi-Strauss, who investigated the mythologies of many cultures, identified a deep-seated human tendency to reduce complex situations to oversimplified dichotomies: friend and enemy, heaven and hell, good and evil. The history of science reveals that scientists are in no way immune to this mindset. In the eighteenth century, the neptunists, who favored a watery origin for rocks, fought with the plutonists, who favored heat as the causative agent. Both, it turns out, were right. A similar contentious and ultimately misleading dichotomy raged between the eighteenth-century catastrophists and uniformitarians, the former espousing a brief and cataclysmic geological history for Earth and the latter holding that geological processes are gradual and ongoing. Once-doctrinal distinctions between plants and animals or between single-celled and multicellular organisms have become similarly blurred.

Attempts to formulate an absolute definition that distinguishes between life and nonlife represents a similar false dichotomy. Here’s why. The first cell did not just appear, fully formed with all its chemical sophistication and genetic machinery. Rather, life must have arisen

through a sequence of emergent events—diverse processes of organic synthesis, followed by molecular selection, concentration, encapsulation, and organization into diverse molecular structures. The emergence of self-replicating molecules of increasing complexity and mutability led to molecular evolution through the process of natural selection, driven by competition for limited raw materials. That sequential process is an organizing theme of this book.

What appears to us as a yawning divide between nonlife and life obscures the fact that the chemical evolution of life occurred in this stepwise sequence of successively more complex stages of emergence. When modern cells emerged, they quickly consumed virtually all traces of the earlier stages of chemical evolution. “Protolife” became a rich source of food, wiped clean by the consuming cellular life, like a clever murderer leaving the scene of the crime.

Our challenge, then, is to play detective—to establish a progressive hierarchy of emergent steps leading from a prebiotic Earth enriched in organic molecules, to functional clusters of molecules perhaps arranged on a mineral surface, to self-replicating molecular systems that copy themselves using resources in their immediate environment, to encapsulation in membranes—that is, to cellular life. (Recall the words of Harold Morowitz: “The unfolding of life involves many, many emergences.”) The nature and sequence of these steps may vary in different environments, and we may never know the exact sequence (or sequences) that occurred on the early Earth. Yet many of us suspect that the inexorable direction of the chemical path is similar on any habitable planet or moon.

Such a stepwise scenario informs attempts to define life. To define the exact point at which such a system of gradually increasing complexity becomes “alive” is intrinsically arbitrary. Where you, or I, or anyone else chooses to draw such a line is more a question of perceived value than of science. Do you value the intrinsic isolation of each living thing? Then for you, life’s origin may correspond to the entrapment of chemicals by a flexible cell-like membrane. Or is reproduction—the extraordinary ability of one creature to become two and more—your thing. Then self-replication becomes the demarcation point. Many scientists value information as the key and argue that life began with a genetic mechanism that passed information from one generation to the next.

“What is life?” is fundamentally a semantic question, a subjective matter of taxonomy. Nature holds a rich variety of complex, emergent chemical systems, and scientists increasingly are learning to craft such systems in the laboratory. No matter how curious or novel their behavior, none of these systems comes with an unambiguous label: “life” or “nonlife.”

To be sure, labels are important and scientists convene earnest conferences and appoint august committees to decide on taxonomic issues. Valid taxonomy is vital for effective communication and provides a foundation for any scientific pursuit. The problem facing us today, however, is that valid taxonomies rely on a minimum level of understanding. Early attempts at classifying animals purely by color, shape, or other superficial features ultimately failed. Similarly, the classification of chemical elements by their physical state—solid, liquid, or gas—was unhelpful in developing a predictive chemical theory.

Recently, the philosopher Carol Cleland of the University of Colorado and the planetary scientist Christopher Chyba of the SETI (Search for Extraterrestrial Intelligence) Institute compared current attempts to define life with similar eighteenth-century efforts to characterize water. Before the discovery of molecules and atomic theory, water could be characterized only by a series of non-unique traits. Water is clear and wet, but so are many oils (and muddy water isn’t all that clear). Water sustains life, but so do many foods (and water with a few invisible pathogens can kill you). Water freezes when it gets cold, water soaks into wood, water flows downhill, on and on the list grows; but none of these traits, nor any combination of these traits, is both necessary and sufficient. No definition devised in the eighteenth century could have captured the true essence of water—the molecule with two hydrogen atoms and one oxygen atom.

By the same token, they argue, scientists in the early twenty-first century are in no position to define life. We have yet to articulate the theoretical underpinnings of biology; we have nothing analogous to the periodic table for living entities. And with only one unambiguous example, cellular life on Earth, we are in no position to lock ourselves into any precise definition. Better, therefore, to keep an open mind and simply describe the characteristics of whatever we find.

I suspect that any universal theory of life will rest, at least in part, on the ideas of emergence. If life arose as a sequence of emergent steps, then each of those steps represents a taxonomically distinct, funda-

mentally important stage in life’s molecular synthesis and organization. Each step deserves its own label.

Ultimately, the key to defining the progressive stages between nonlife and life lies in experimental studies of relevant chemical systems under plausible geochemical environments. The concept of emergence simplifies this experimental endeavor by reducing an immensely complex historical process to a more comprehensible succession of measurable steps. Each emergent step provides a tempting focus for laboratory experimentation and theoretical modeling.

This nontraditional view of life’s definition as a stepwise transition from chemistry to biology is of special relevance to the search for life elsewhere in the universe. It’s plausible, for example, that Mars, Europa, and other bodies in our solar system progressed only part way along the path to cellular life. If so, that’s crucial to know, at least from NASA’s point of view. If each step in life’s origin produced distinctive and measurable isotopic, molecular, and structural signatures in its environment, and if such markers can be identified, then these chemical features become observational targets for planned space missions. It’s possible, for example, that primitive prebiotic isotopic, molecular, and structural forms are inevitably eaten by more advanced cells and survive as “fossils” only if cellular life never developed in their environs. Thus prebiotic features may serve as extraterrestrial “abiomarkers”—clear evidence that molecular organization and evolution never progressed beyond a certain precellular stage. As scientists search for life elsewhere in the universe, they may be able to characterize extraterrestrial environments according to their degree of emergence along this multistep path.

Consider Saturn’s recently visited moon Titan as a choice example. Cloud-enshrouded Titan possesses an atmosphere one-and-a-half times thicker than Earth’s and is rich in methane and ammonia. Organic molecules, which color the atmosphere a hazy orange, rain onto the surface to form thick accumulations of organic gunk. Lakes of methane and ethane occur side-by-side with frozen expanses of rockhard water ice, though conditions are generally much too cold for liquid water or significant chemical progress toward life.

From time to time, however, the impact of a large comet or aster-

oid may have melted regions of ice on Titan. For periods of hundreds or even thousands of years, gradually cooling ice-covered lakes might have supported the first chemical steps in the path toward life, only to become frozen again. Such primitive biochemistry, though lost forever on Earth’s scavenged surface, might conceivably survive in the deep-freeze of Titan.

But so much for speculation and conjecture. Observations of the living world, coupled with relevant experiments, will illuminate the emergence of life both here on Earth and even elsewhere in our solar system.