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.
ORIGINS AND EMERGENCE
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, fundamentally important stage in life's molecular synthesis and organization. Each step deserves its own label.
AN EXPERIMENTAL STRATEGY
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 asteroid 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.
3
Looking for Life
Scientists turn reckless and mutter like gamblers who cannot stop betting.
Alan Lightman, Einstein's Dreams, 1993
The profound difficulty in crafting an unambiguous definition of what is (or was) alive came into dramatic focus in 1996 with the discovery of supposed cellular fossils in a meteorite from Mars. Of the countless thousands of meteorites that have been collected on Earth's surface, only a precious two dozen or so came to us from Mars. In the 1980s, chemists deduced the distant origins of these rocks from the diagnostic composition of gas trapped inside them—gas that matches perfectly the known idiosyncrasies of the Martian atmosphere. Theorists maintained that giant asteroid impacts on Mars could easily have hurled rocky debris into orbit around the Sun. And while the Sun and Jupiter, the two most massive objects in our solar system, eventually (often after millions of years) sweep up most of that Martian detritus, a tiny fraction of the rubble inevitably finds its way to Earth. With the discovery of Martian meteorites, scientists could, for the first time, investigate actual pieces of another planet.
Naturally, these nondescript chunks of dark-colored rock are highly prized and receive the closest examination by earthbound scientists. Most of them are hunks of ancient igneous formations—material formed from once-molten rock near the Martian surface. We expect such meteorites to be devoid of life. But one Mars meteorite proved strikingly different from the others, and it naturally attracted extra close scrutiny. Collected in 1984 from the Allan Hills region of Antarctica (hence its now famous designation, ALH84001), this meteorite held a suite of minerals that suggested to some scientists the possibility of ancient interactions with liquid water.
A team of biologists, planetary scientists, and meteorite experts led by NASA's David McKay subjected pieces of the two-pound rock to a battery of analytical tests. They probed the meteorite with X-rays, lasers, gamma rays, and beams of electrons, recording characteristics as small as a billionth of an inch across. No one had ever expected to find hard evidence for Martian life, but even a hint of freely flowing water on Mars would constitute a major discovery. Yet gradually, as the data piled up, McKay and his colleagues began to believe that they had found the smoking gun for Martian life.
LIFE ON MARS: THE ALLAN HILLS STORY
On August 7, 1996, the Allan Hills team publicly claimed the discovery of tiny elongated objects that were once alive. “LIFE ON MARS!” screamed the headlines, while the prestigious periodical Science published an article with the equally giddy title (at least for a scientific journal), “Search for Past Life on Mars: Possible relic biogenic activity in Martian meteorite ALH84001.” President Clinton got into the act by holding a national press conference, during which he basked in the reflected glory of NASA's triumph.
McKay and his eight co-workers pointed to five separate types of data, which they presented point by point like a zealous prosecutor at a jury trial. Point number one: The meteorite was found to contain a suite of organic molecules, including carbon-based compounds called PAHs (polycyclic aromatic hydrocarbons). These sturdy, long-lasting molecules, which feature interlocking rings of six carbon atoms, often arise when once-living cells are subjected to high temperature. Since carbon is the key element of life as we know it, its presence in ALH84001, which distinguished that specimen from the other Martian meteorites, was of extraordinary significance.
Point two: The meteorite held microscopic globules of carbonate minerals, similar to those that make the graceful formations on th
e walls of caves on Earth. Such carbonates are often deposited through the action of liquid water passing through a system of cracks and fissures. Liquid water is the presumptive medium of all cells and thus a necessary condition for life. What's more, their tiny structures, about a ten-thousandth of an inch in diameter, reminded some observers of minerals precipitated by microbes on Earth.
The third and fourth points relied on sophisticated analytical tools. The NASA team used an electron microscope to discover and characterize two iron-bearing minerals, an iron sulfide called pyrrhotite and an iron oxide called magnetite. Of particular interest were the curious chainlike arrays of minuscule magnetite crystals. Magnetite is a magnetic mineral found in abundance in rocks of all types, but the perfect shape of these alien crystals and their unusual chemical purity, coupled with their distinctive linear arrangements, seemed unlike anything ever seen except in a few remarkable types of bacteria. These “magnetotactic” microbes tend to live in thin layers of sediment where chemical conditions change rapidly with depth, and they use their internal magnets to distinguish “up” from “down,” by sensing the inclination of Earth's magnetic field. So sensitive are these organisms to their vertical position that magnetotactic bacteria from the Northern Hemisphere move in the wrong direction and die when placed in Southern Hemisphere soils, where magnetic “up” and “down” are reversed. The NASA scientists claimed that no known inorganic process could have produced such an ordered crystalline array.
Genesis: The Scientific Quest for Life's Origin Page 5