H00102--00A, Front mat Genesis Read online

  selection, concentration, and assembly of life’s membranes, proteins,

  and genetic molecules, built in part on a scaffolding of rocks and min-

  erals. Eventually, these biomolecular structures formed self-replicating

  cycles—chemical systems that copied themselves and competed for a

  finite and dwindling supply of resources. Ultimately, competition be-

  tween different self-replicating cycles triggered evolution by natural

  selection, and life was on its way.

  Part I introduces the theory of emergence, which provides a con-

  ceptual framework for understanding the immensely complex path

  from nonlife to the first living cell. Parts II, III, and IV explore experi-

  mental and theoretical attempts to understand, step by step, the spe-

  cific emergent processes: the emergence of diverse biomolecules, the

  emergence of larger structures composed of many molecules, and fi-

  nally the emergence of self-replicating collections of molecules. Be

  warned. Not all of these attempts are success stories: Scientific progress

  demands time-consuming, often tedious effort; most experiments end

  in failure; we spend much of our scientific lives making mistakes as fast

  as we can and desperately trying not to make the same mistake twice.

  The narrative focuses on the chemical transition from a prebiotic

  Earth rich in organic molecules to the so-called RNA World of self-

  replicating genetic molecules. I do not examine the important subse-

  quent transition from the RNA World to the present world in which

  life is governed and sustained by DNA and proteins, nor do I examine

  the rich subject of life’s evolution following the appearance of the first

  cell—both are complex topics requiring their own books.

  As in any scientific field, much of the richness of origin-of-life

  research lies in the details and not the overview. Thus I provide an

  xvi

  PREFACE

  extensive section of notes and a bibliography that directs readers to

  primary literature and more detailed discussions of many issues. The

  notes include comments and corrections from numerous experts who

  reviewed drafts of this book; in many instances these remarks high-

  light current uncertainties and controversies among scientists, includ-

  ing disagreements about my interpretations. Nonetheless, this book is

  not encyclopedic and only begins to address a vast and rapidly ex-

  panding literature. I apologize to those scientists whose important

  studies are not detailed.

  Why should an earth scientist, trained in the fields of mineralogy and

  crystallography, write such a book? By the mid-1990s, my research ca-

  reer at the Carnegie Institution of Washington’s Geophysical Labora-

  tory had reached a respectable plateau, achieved through two decades

  of solid, serviceable work in the specialized field of high-pressure crys-

  tal chemistry. With secure federal funding and a steady stream of pub-

  lications, scientific life was good, at least on an immediate level; but

  something, I felt, was missing. The essence of science is the unmatched

  joy in seeking and finding answers to questions about the natural

  world, yet by the mid-1980s we had grasped the central principles of

  how crystals compress. The crystallographic questions I asked seemed

  increasingly narrow, while the answers provided few surprises. I was

  ready to try something new.

  There is another reason why origins research resonated. Since the

  late 1980s, I’ve worked with a colleague at George Mason University,

  the physicist James Trefil, to effect reform in undergraduate science

  education. Most undergraduate science requirements are discipline-

  bound—tied to physics, chemistry, or biology. Jim and I believe that

  this approach is flawed, providing precious little useful knowledge on

  which students can build and grow as lifelong science enthusiasts.

  Rather, we find that non-science majors benefit immeasurably from

  broadly integrated science courses that deal with overarching scientific

  principles and their applications to daily life.

  Origin-of-life research epitomizes why integrated science educa-

  tion is vital. The pursuit of life’s origins, like many other fascinating

  facets of the natural world, draws deeply on several branches of sci-

  PREFACE

  xvii

  ence—geology, biology, chemistry, physics, astronomy. It is thrilling to

  integrate the full spectrum of scientific ideas into the pursuit of one

  fundamental question.

  So, when the unanticipated opportunity arose, I jumped at the

  chance to change research directions and began investigating the an-

  cient question of life’s chemical origins. My efforts have gained en-

  couragement and focus through years of contact with theoretical

  biologist Harold Morowitz, another colleague at George Mason Uni-

  versity. Harold enticed me into the origins field, and his innovative

  ideas and persuasive approach have informed every step I’ve taken. His

  edict that “the unfolding of life involves many, many emergences” pro-

  vides an underlying theme of this book.

  Our first experiments, commenced with giddy optimism in the

  spring of 1996, proved laughably naïve. Our little group at Carnegie’s

  Geophysical Laboratory was unlikely to leapfrog a half-century of dedi-

  cated and creative origin-of-life research, yet we did enter the arena

  with fresh eyes and a lack of crippling preconceptions. Slowly, a frame-

  work for tackling the problem, and this book, emerged.

  The outline for Genesis: The Scientific Quest for Life’s Origin crys-

  tallized at a September 2000 conference in Modena, Italy, that focused

  on a single daunting question: “What is life?” Opinions among the

  hundred assembled scientists, philosophers, and theologians differed

  dramatically, but the most contentious debates occurred within the

  ranks of scientists. One aging expert on lipid molecules argued that

  life began with the first semipermeable lipid membrane, the structure

  that encloses cells. An authority on metabolism countered that life

  began with the first metabolic cycle, the process by which all cells con-

  vert energy and atoms to useful molecules. On the contrary, claimed

  several molecular biologists who specialized in genetics, the first living

  entity must have been an RNA-like genetic system that carried

  and duplicated biological information. One mineralogist even pro-

  posed that life began not as an organic entity, but as a self-replicating

  mineral.

  To a relative newcomer in the field, the unresolved debate was

  reminiscent of the old tale of the blind men and the elephant. Asked to

  describe the massive beast, each one’s perspective varied, based on

  which anatomical feature was close at hand—the rough ropelike tail,

  the mighty treelike legs, the twisting snakelike trunk, and so forth. Each

  blind man’s version was wrong, but each possessed an element of the

  xviii

  PREFACE

  more complex elephantine truth. Perhaps, I thought, the disparate

  claims of what constitutes life are likewise mere parts of the more com-

&nb
sp; plex truth of life’s identity and origin.

  Throughout the conception and writing of this book I have benefited

  from the aid, advice, and encouragement of numerous colleagues. In

  addition to Harold Morowitz, they include most prominently

  geochemist George Cody and petrologist Hatten S. Yoder Jr. of the Geo-

  physical Laboratory. I have continued to rely on them in countless ways.

  Hat Yoder’s death in August 2003 was a great personal and professional

  loss.

  I am indebted to many distinguished origin-of-life researchers who

  read the manuscript and provided invaluable comments and correc-

  tions, including Louis Allamandola, Gustaf Arrhenius, Graham Cairns-

  Smith, David Deamer, James Ferris, Friedemann Freund, Thomas

  Gold, Rosalyn Grymes, Bruce Jakosky, Gerald Joyce, Noam Lahav, An-

  tonio Lazcano, James Miller, Harold Morowitz, David Oldroyd,

  Norman Pace, Nick Platts, David Ross, Bruce Runnegar, Sara Seager,

  Jack Szostak, Günter Wächtershäuser, Malcolm Walter, and Nick

  Woolfe.

  Among the numerous other scientists who contributed directly or

  indirectly to the book through conversations, collaborations, and com-

  ments on portions of the manuscript are Bruce Alberts, Jeffrey Bada,

  Connie Bertka, Robert Downs, Glenn Goodfriend, Stephen Jay Gould,

  Nora Noffke, David Sholl, Henry Teng, and James Trefil.

  The establishment of the NASA Astrobiology Institute (NAI), of

  which our Carnegie Institution research team was a founding member

  in 1998, provided a wealth of new colleagues and collaborative ven-

  tures. I have benefited immeasurably from interactions with Aravind

  Asthagiri, Nabil Boctor, Alan Boss, Kevin Boyce, Jay Brandes, Hugh

  Churchill, Rachel Dunham, Janae Eason, Gözen Ertem, Mary Ewell,

  Tim Filley, Marilyn Fogel, Patrick Griffen, Chris Hadidiacos, Wes Hunt-

  ress, Andy Knoll, Jake Maule, Ken Nealson, David Olesh, Doug Rumble,

  James Scott, Sean Solomon, Andrew Steele, and Jan Toporski.

  The staff at the Joseph Henry Press tackled this project with dedi-

  cation, professionalism, and constant good humor. First and foremost,

  PREFACE

  xix

  my deep gratitude to executive editor Stephen Mautner, who embraced

  the concept of this book and has contributed to every stage of its de-

  velopment. Steve played many roles—enthusiastic reader, attentive

  confidant, generous friend, patient counselor. His influence appears

  on every page. Jim Gormley supervised the stylish graphics and design.

  Production editor Heather Schofield managed the project with grace

  and efficiency. Freelance editor Sara Lippincott provided an invaluable

  detailed review of the penultimate draft. Marketing director Ann Mer-

  chant developed the effective publicity campaign. I am grateful to all

  of the JHP staff for their help and guidance.

  Margaret Hazen contributed to every facet of this book, through

  countless hours of intense conversation, discerning critiques of innu-

  merable drafts, and unflagging support and encouragement.

  Prologue

  “Look at this!”

  Harold Morowitz beamed as he hustled into my office. “The di-

  electric constant of water drops to about 20 at a kilobar and 350

  degrees. That’s like an organic solvent!” He pulled up a chair to show

  me the data.

  It took me a moment to change gears and realize what he was so

  excited about. Harold and I both teach undergraduate science courses

  at George Mason University, where we often discuss biology’s “Big

  Questions,” including the chemical processes that might have led to

  life’s origin. For more than two decades Harold had puzzled over the

  role of water, which poses a persistent problem in origin-of-life sce-

  narios. Water is the medium of life, while carbon, by far the most ver-

  satile of all the chemical elements, forms the essential backbone of all

  biomolecules. Yet researchers find that several key chemical steps in

  assembling life’s carbon-based molecules do not work very well in wa-

  ter. So how could life have started on a wet planet?

  One intriguing, though untested, possibility is that life’s initial

  chemical reactions proceed more easily at high pressure and tempera-

  ture. This idea had received a boost in the late 1970s, when Oregon

  State University oceanographer Jack Corliss descended to the deep,

  dark ocean floor in the research submersible Alvin and observed as-

  tonishing ecosystems at undersea volcanic vents. In these hellish zones,

  without benefit of sunlight, life has found a way to survive crushing

  1

  2

  GENESIS

  pressures of 1,000 atmospheres—a kilobar—and scalding tempera-

  tures greater than 100°C. Perhaps, Corliss and co-workers suggested,

  life first arose at such hostile extremes, and in total darkness.

  Morowitz’s dense tabulation held a possible clue. Photocopied

  from a 1970s text, it recorded variations of water’s physical and chemi-

  cal properties with temperature and pressure. Sure enough, at extreme

  pressure-cooker conditions water appeared to be a remarkably

  different liquid from the stuff that comes out of the tap. Harold was

  onto something. “So maybe Jack Corliss is right—maybe it is the vents.”

  The mainstream origin-of-life community, wedded as they were

  to the tradition of a globe-spanning ocean of “primordial soup”

  bathed in sunlight, had rejected this speculation out of hand. In the

  intervening two decades, no one had bothered to try the relevant high-

  temperature and high-pressure experiments. Yet the so-called hydro-

  thermal-origins hypothesis was too intriguing and too testable to dis-

  appear. In the late 1980s, the German chemist and patent attorney

  Günter Wächtershäuser put flesh on the bones of this idea by propos-

  ing a detailed chemical scenario for origin events in a deep hydrother-

  mal zone rich in sulfide minerals.

  Now Harold had uncovered data showing how the physical and

  chemical properties of water might be very different at extreme condi-

  tions from those of everyday experience. Perhaps chemical reactions

  that fail at Earth’s ordinary surface conditions could take place at those

  extremes. There was only one way to find out.

  “So, can you do the experiments?” Harold knew that my nearby

  research base, the Carnegie Institution’s Geophysical Laboratory in

  Washington, D.C., maintained an arsenal of high-temperature and

  high-pressure apparatus. The laboratory specialized in chemical reac-

  tions at extreme conditions, so he hoped that my colleagues and I

  might be able to tackle the complex carbon chemistry that underpins

  the origin of life.

  I hardly gave it a second thought. “Sure, why not? It’s an easy ex-

  periment.” I had no idea where that hasty promise would lead.

  Harold Morowitz is one of the kindest scientists I know. More than a

  few scientists wear a veneer of kindness—a pro forma geniality that

  masks an intense and often competitive personality. Those of us whose

  PROLOGUE

  3

  lives are devoted to indulging our curiosity
tend to be a distracted, self-

  absorbed lot. Harold is different. He smiles winningly at the slightest

  encouragement and speaks with the calmness and quiet passion of a

  rabbi or father confessor. He loves a good idea, and shares his richly

  inventive vision of life’s origin without hesitation, without the conven-

  tional expectation of clever reciprocation. He came to George Mason

  University in 1988, after a full and productive career on the biology

  faculty at Yale, where he made his mark studying energy flow in cells.

  Harold argues persuasively that modern cells carry hints of life’s earli-

  est biochemical processes. Such molecular “fossils” persist in all of us

  in all our cells he claims, and these molecules point to the chemistry of

  life’s origins. [Plate 1]

  Although at the time a novice at origins research, I was delighted

  for an excuse to collaborate with Harold Morowitz. His name pro-

  vided instant credibility in a field at times tarnished by questionable

  data, contentious debates, or even outright quackery. Nevertheless, a

  successful experiment must be planned with care, and Harold had al-

  ready spent a lot of time thinking about strategy.

  “Let’s start with pyruvate,” he urged. Pyruvate, an energy-rich, 3-

  carbon molecule, was a natural choice for Harold, who had spent a

  lifetime studying metabolism, the processes by which cells gather at-

  oms and energy to sustain themselves, grow, and reproduce. Pyruvate

  is a key ingredient in every cell’s metabolism. Most cells gain energy by

  splitting the 6-carbon sugar, glucose, into two pyruvate molecules, af-

  ter which the pyruvate is broken into smaller molecules to release fur-

  ther energy. Morowitz explained that some cells also use pyruvate as a

  building block to construct larger molecules. So, for example, a pyru-

  vate molecule and a molecule of carbon dioxide (one carbon atom

  bonded to two oxygen atoms) can react to form a 4-carbon molecule

  called oxaloacetate, which undergoes further reactions in metabolism.

  It’s simple math: 3 + 1 = 4. But that reaction never works in water

  at room pressure, at least not without a complex biological catalyst—a

  molecule that greatly boosts the reaction rate. In the absence of a cell’s

  sophisticated catalytic chemical machinery, pyruvate tends to break