Genesis: The Scientific Quest for Life's Origin

by Robert M. Hazen

  All these approaches and more inform the search for a law of emergence; all provide a glimpse of the answer. Yet each seems too abstract to apply to benchtop chemical experiments on the origin of life. An experimentalist needs to decide on the nitty-gritty details: What should be the starting chemicals at what concentrations; how acidic or basic the solution; what run temperatures, pressures, and times? Is there any way that the ideas of emergence can help?

  A classic scientific approach to discovering general principles and laws is to examine the behavior of specific systems. The study of simple systems that display emergent behavior may well point to physical factors that lead to patterning in much more complex systems, including life. We can hope that observations of specific systems will eventually point to more general rules.

  PATTERNS IN THE SAND

  You don't need a laboratory to observe emergent phenomena. In fact, you can't go on a hike without seeing dozens of examples of emergence in action. Among my favorite emergent phenomena are interactions of water and sand, which provide a convenient and comprehensible example of structures arising from the energetic interactions of lots of agents (not to mention a great excuse to spend the day at the shore). When moving water (or wind, for that matter) flows across a flat layer of sand, new patterns arise. Periodic sand ripples appear, as sand grains are sorted by size, shape, and density. The system thus becomes more orderly and patterned as energy—the flow of wind or water—dissipates.

  My favorite emergent sandy system lies at the base of the fossil-rich hundred-foot-tall cliffs that border the Chesapeake Bay's western shore in Calvert County, Maryland. Fifteen-million-year-old whale bones, razor-sharp sharks' teeth, branching bleached corals, and robust fist-sized clamshells abound in the wash zone, where waves constantly wear away the soft sediments. Walks along those majestic formations often lead to thoughts about the factors that contribute to complexity.

  At times of unusually low tide, especially near a new moon in the cold clear winter months, receding waters expose a gently sloping pavement of ancient sediments below the base of the cliff—a formation called blue marl. Treacherously slippery when wet, this firm flat surface commonly accumulates a thin layer of sand—particles that display emergent patterns when subjected to the wash of shallow water. Over the years, I've noticed four distinct factors that contribute to the emergence of complex sand patterning.

  Factor 1:

  The Concentration of Agents

  The first obvious factor in achieving a patterned, complex system is simply the density of sand grains—that is, the number of interacting particles per square centimeter of the blue marl's surface. It's easy to estimate this number by collecting almost every grain of sand from an area 10 centimeters square, about the size of a small paper napkin. I collect the sand in a plastic bag or bottle, take it back to the lab, dry it, and weigh the sample. Using a microscope, I count out 100 grains from the sample and then weigh that batch. As it turns out, the total number of grains per square centimeter is approximately equal to the total weight of sand from the 100 square-centimeter (10 × 10) area divided by the weight of 100 sand grains.

  I find that with fewer than about 100 sand grains per square centimeter, the dusting of particles is too sparse for any noticeable patterns to emerge. Given the minute size of the average sand grain, typically less than half a millimeter in diameter, 100 grains per square centimeter provides a sparse coverage over less than 10 percent of the smooth blue marl surface. Increase the sand concentration to about 1,000 grains per square centimeter, however, and an intriguing pattern of regularly spaced sand piles, each a centimeter or two across, appears on the hard blue surface. What's more, a small circle of darker sand grains typically crowns each little tan pile. Evidently a minimum concentration of several hundred grains per square centimeter is required to initiate patterning in sand.

  Patterns in sand grains emerge as the concentration of grains increases. At about a thousand grains per square centimeter (A) small, black-topped piles are observed; at a few thousand grains per square centimeter (B) discontinuous bands arise; and above 10,000 grains per square centimeter (C) continuous ripples cover the surface.

  Increase the sand concentration slightly to a few thousand grains per square centimeter and you get discontinuous short bands of sand at right angles to the gentle back-and-forth wave motion of the shallow water. As with the mini-sandpiles, each tan band is topped by a line of darker grains. And as sand concentration exceeds 10,000 grains per square centimeter, continuous, evenly spaced, black-capped ripples form across the hard pavement. I've seen this classic rippled surface cover hundreds of square meters of shallow water in patterns so hypnotically regular that I hesitated to disturb the symmetry by walking on it.

  And that's it. Higher concentrations of sand simply provide a deeper base for the regular ripples. Buried sand grains don't participate in the process so no new structures arise beyond the elegant, wavelike, periodic forms on the surface.

  This systematic behavior suggests that the concentration of interacting agents plays a fundamental role in the emergent complexity of a system. Below a critical threshold, no patterns are seen. As particle concentrations increase, so too does complexity, but only to a point. Above a critical saturation of agents, we find no new behaviors.

  Similar observations have been made about other emergent systems. One ant species—Eciton burchelli, the army ant—stays close to home as long as the colony consists of fewer than about 80,000 individuals. Exceed that number of army ants, however, and the colony exhibits new emergent behavior; like a bursting dam, the ants pour out in a massive “swarm raid” to attack adjacent colonies. At higher populations, half of the ants may spontaneously leave to form a new colony. Studies of termite colonies also reveal that the construction of pillar-type mounds requires a critical density of individuals.

  At a much greater scale, spiral galaxies require a minimum number of about 100 million stars to trigger development of the familiar spiral arm structure. According to theoretical models of astrophysicists, the majestic arms form as a result of gravitational instabilities caused in part by a large central mass of stars.

  Human consciousness and self-awareness also emerge from the interactions of trillions of neurons. Sadly, as those of us who watch friends and relatives afflicted with Alzheimer's disease must observe, when a critical number of cells and their connections are destroyed, self-awareness fades away.

  These findings suggest that the emergence of life might have depended on achieving some minimal concentration of biomolecules, the essential agents of cellular life. Too few molecules, no matter how friendly the environment, and life could not arise. That's a useful idea to bear in mind when designing origin-of-life experiments.

  Factor 2:

  The Interconnectivity of Agents

  Sand grains influence each other by direct contact, the simplest local way to interact. A rounded grain at the surface of a sandpile typically touches about a half-dozen adjacent grains. The balance between these stabilizing contacts and gravity on the one hand, and the restless, disruptive flow of water on the other, leads to a controlled shuffling of grains and ultimately to the rippled patterning of sand. By contrast, ants in an ant colony interact over much greater distances, by marking the ground with a variety of pheromones, which are chemical signals that point other ants to food, alert them to danger, and provide other vital information. In this way, any given ant has the potential to interact with thousands of colony mates in varied ways. These differences in interconnectedness provide part of the reason why ant colonies are more complex than water-shaped sandpiles.

  The conscious brain, the most complex system we know, is also the most complexly interconnected. Each of the trillions of neurons in your brain interacts with hundreds of other nearby cells through a branching network of dendrites. Electrical signals between any two neurons, furthermore, may be stronger or weaker, like the current controlled by the dimmer switch on your lamp. Int
erconnections of the brain are vastly more intricate than those of sand or ants.

  These observations of emergent systems suggest that life's origin must have relied on a wide repertoire of chemical interactions. Experiments that optimize the number and type of molecular contacts might thus be more likely to display emergent behaviors of interest.

  Factor 3:

  Energy Flow Through the System

  Regardless of how many sand grains or ants or neurons are present, no pattern can emerge without a flow of energy through the system. Sand grains will not start hopping without a certain minimum water-wave speed (typically about 1/2 to 1 meter per second along the shores of the Chesapeake Bay). More energetic waves with greater speed and amplitude move grains more easily and generate sand patterns more quickly, though these patterns do not appear to differ fundamentally in their shapes.

  But every complex patterned system has a limit to the magnitude of energy flow it can tolerate. During energetic storms, crashing waves obliterate sand ripples and other local sedimentary features. Black and tan sand grains become jumbled and all signs of emergent patterning disappear.

  The human brain exhibits strikingly similar behavior in terms of energy flow. During normal waking hours, the brain maintains a moderate level of electrical impulses—the normal healthy flow of energy through the neural system. Deepest sleep corresponds to a sharp drop in electrical activity as we slip from consciousness, whereas the excessive electrical intensity of an epileptic seizure thwarts conscious action by scrambling the usual patterned electrical flow.

  The emergence of complex patterns evidently requires energy flow within rather restrictive limits: Too little flow and nothing happens; too much flow and the system is randomized—entropy triumphs. This conclusion is important for the experimental study of life's chemical origins. A reliable source of energy is essential, to be sure, but lightning, ultraviolet radiation, and other intense forms of ionizing energy can blast molecules apart and may be too extreme to jump-start life. We must look for gentler chemical energy sources, like the steady, reliable chemical potential energy stored in a flashlight battery, to sustain the metabolism of primitive life.

  Factor 4:

  Cycling of Energy Flow

  Many natural systems are subject to cycles of energy: day and night, summer and winter, high tide and low tide. Such cycles may play a fundamental role in the evolution of emergent systems, though it's often difficult to document the effects of these subtle cycles in nature.

  Laboratory wave tanks, though considerably less scenic than the Chesapeake Bay in January, facilitate the study of sand-ripple formation under controlled conditions. Recent research on natural patterned systems reveals that cycling of energy flow through a system is a fascinating and previously unrecognized fourth factor in generating complex sand patterns. In 2001, physicist Jonas Lundbek Hansen at the Niels Bohr Institute and his Danish colleagues announced this surprising wrinkle in the mechanics of ripple formation. Most previous experiments had involved fixed wave amplitudes (that is, wave height) and frequencies (how many waves pass a given point in a second). Such studies typically generate perfectly spaced, straight ripples. Instead, Hansen and his colleagues wondered what might happen if they cycled these variables. Over periods of several minutes, they increased and then decreased the amplitude or the frequency of their water waves. The results were breathtaking. Rather than simple parallel sand ripples, they produced elegant intertwined and branching sand structures. These new patterns appear remarkably similar to sand features that commonly arise along the Chesapeake Bay when the water is only a few inches deep—conditions that apparently favor periodic fluctuations in wave amplitude.

  Alert to the potential power of energy cycling, PhD student Mark Kessler and Professor Brad Werner of the University of California, San Diego, recently analyzed amazing stone circles and other so-called “patterned grounds” in Alaskan Arctic terrain that is subject to cyclical freezing and thawing. With each thaw, rounded boulders shift slightly, interacting with one another over many years to produce remarkable fields covered by natural circles of stone. [Plate 2]

  The role of cycling in the emergence of patterns represents a frontier area of study that is keenly watched by some origin-of-life investigators. After all, the primitive Earth was subject to many cycles—day/night, high tide/low tide, wet/dry, and more. Perhaps such cycles, which can be duplicated in a controlled experimental environment, contributed to the emergence of life itself.

  FORMULATING EMERGENCE

  So what might a mathematical law of emergence look like? My guess is that the expression will take the form of a mathematical inequality, something like this:

  That's a short-hand way of saying that the emergent complexity of a system, denoted by the letter C (for “complexity”), is a number less than or equal to some value that is a mathematical function (f) of the concentration of interacting particles (n), the degree of those particles' interconnectivity (i), the time-varying energy flow through the system [E(t)], and perhaps other variables as well.

  At least two daunting impediments thwart the completion of this potentially simple formulation. First, as previously noted, we lack a precise definition of complexity. It's impossible to quantify something when you don't really know what that something is. And second, we are woefully ignorant of the exact mathematical relationships between complexity and the three possible key factors: the concentration of interacting agents, the interconnectivity of those agents, and the cyclical energy flow. Simple systems yield tantalizing clues, but we are still a long way from any definitive formula.

  This quest to characterize emergent phenomena, though initially couched in mathematical abstraction, is not ultimately an abstract exercise. Emergent systems frame every aspect of our experience. Our environment, our bodies, our minds, the patterns of our lives and our culture—all display emergent complexity. A comprehensive theory of emergence will foster applications to myriad problems in everyday technology: long-range weather prediction, computer network design, traffic control, the stabilization of ecosystems, the control of epidemics, perhaps even the prevention of war. Armed with such a law, we will acquire a deeper understanding of any system of many interacting agents—indeed, even of the origin of life itself.

  2

  What Is Life?

  I know it when I see it.

  Justice Potter Stewart, 1964

  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 proces
s, 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.

  “TOP-DOWN” VERSUS “BOTTOM-UP”

  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.