H00102--00A, Front mat Genesis

by Charles Baum

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

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  GENESIS

  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 bil-

  lionths 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 overarch-

  ing 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 my-

  thologies 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 geologi-

  cal 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 be-

  come 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

  WHAT IS LIFE?

  29

  through a sequence of emergent events—diverse processes of organic

  synthesis, followed by molecular selection, concentration, encapsula-

  tion, and organization into diverse molecular structures. The emer-

  gence 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 se-

  quential 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 ar-

  ranged 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 dif-

  ferent 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 com-

  plexity 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 entrap-

  ment of chemicals by a flexible cell-like membrane. Or is reproduc-

  tion—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.

  30

  GENESIS

  “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 behav-

  ior, none of these systems comes with an unambiguous label: “life” or

  “nonlife.”

  To be sure, labels are important and scientists convene earnest con-

  ferences and appoint august committees to decide on taxonomic is-

  sues. 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 under-

  standing. Early attempts at classifying animals purely by color, shape,

  or other superficial features ultimately failed. Similarly, the classifica-

  tion 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 Colo-

  rado 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 s
ustains life, but so do many foods (and water with a few invis-

  ible 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-

  WHAT IS LIFE?

  31

  mentally important stage in life’s molecular synthesis and organiza-

  tion. 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 sim-

  plifies 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 ex-

  perimentation and theoretical modeling.

  This nontraditional view of life’s definition as a stepwise transi-

  tion 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 chemi-

  cal 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 “abio-

  markers”—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 ex-

  traterrestrial 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. Or-

  ganic 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 rock-

  hard water ice, though conditions are generally much too cold for liq-

  uid water or significant chemical progress toward life.

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

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  GENESIS

  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.

  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, eventu-

  ally (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 sci-

  entists. Most of them are hunks of ancient igneous formations—mate-

  rial 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 mete-

  33

  34

  GENESIS

  orite held a suite of minerals that suggested to some scientists the pos-

  sibility 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 prestig
ious periodical Science pub-

  lished 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 Mar-

  tian meteorites, was of extraordinary significance.

  Point two: The meteorite held microscopic globules of carbonate

  minerals, similar to those that make the graceful formations on the

  walls of caves on Earth. Such carbonates are often deposited through

  the action of liquid water passing through a system of cracks and fis-

  sures. Liquid water is the presumptive medium of all cells and thus a

  necessary condition for life. What’s more, their tiny structures, about a

  LOOKING FOR LIFE

  35

  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 pyrrho-

  tite 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 any-

  thing 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 in-