Faint Echoes, Distant Stars

by Ben Bova

  The next year, Nagy and George Claus, a microbiologist at the New York University Medical Center, made a much more startling claim: They reported finding “organized elements” in the Orgueil meteorite and another chondrite, Ivuna. They claimed that these “organized elements” were the remains of fossilized algae and other life-forms. Pasteur had examined the Orgueil meteorite about a century earlier and found no discernable organisms, but Nagy and his colleagues had much better microscopes and analytical instruments.

  The first extraterrestrial life had been found, Nagy and Claus believed.

  Or had it? Critical scientists were quick to point out that Orgueil and Ivuna had lain on museum shelves for decades. Chondrites are porous, wide open to contamination by terrestrial microbes. The “organized elements” were more likely to be contaminants from Earth than alien fossils, the critics insisted.

  Sagan, already a leader in the search for extraterrestrial life, helped to demolish Claus and Nagy’s position. Because he dearly wanted to believe what they were claiming, Sagan looked for any possible argument that could contradict them (while hoping he would not find any). “Extraordinary claims require extraordinary evidence,” he maintained. He wanted to be sure. Unfortunately, the evidence to support the extraterrestrial-life claim soon fell apart.

  The polarized light tests showed that the organic molecules in Orgueil all spiraled to the right, whereas organics in living terrestrial creatures all spiral to the left. In laboratory experiments where organic molecules are generated artificially, they show no preference for one rotational direction or the other: Lab-produced organics come out in equal amounts of right-handed and left-handed twists. Sagan postulated that the same was originally true of the Orgueil meteorite: It contained both right- and left-handed organic molecules. But over the century in which it had laid on a dusty museum shelf, terrestrial microorganisms had infiltrated the porous stone and eaten the left-handed molecules, which they could metabolize, while leaving the “indigestible” righties alone.

  The hardest blow came from the University of Chicago, where geochemist Edward Anders and pathologist Frank Fitch showed that most of the “organized elements” were fragments of terrestrial contaminants such as ragweed pollen. While a few of the “elements” remained unidentified, the scientific community rejected the idea that they were fossils of extraterrestrial organisms. As Sagan put it, “Somewhat reluctantly we must choose the . . . alternative . . . that the Orgueil meteorite was contaminated by ragweed pollen.”

  Although the brief flurry of excitement over Claus and Nagy’s “organized elements” died out, it became incontrovertibly clear that meteorites do bring organic molecules to Earth, including PAHs and amino acids. In 2001, an international research team led by George Cooper of the Ames Research Center discovered the sugar dihydroxyacetone and glycerol in two carbonaceous meteorites, Murchison and Murray. Dihydroxyacetone is a precursor for more complex sugars, including the ribose backbone of ribonucleic acid (RNA) and deoxyribonucleic acid (DNA), which are found in every living creature. Glycerol is a constituent of the membranes that enclose living cells.

  Astrobiologists now believe that prebiotic chemistry takes place in the carbon- and water-bearing chondritic meteorites, as well as in the icy bodies of comets. The latest discovery (as of this writing) comes from the Tagish Lake meteorite, which was recovered from a frozen lake in Canada shortly after it fell in 2001. Examined before any significant terrestrial contamination could infect it, the Tagish Lake meteorite proved to contain relatively complex organic compounds, including PAHs.

  This growingly detailed picture of the Earth’s origin and early days has helped astrobiologists understand how life got started on our world.

  THE MOLTEN EARTH

  The Sun began to shine more than 4 billion years ago. The raging fury of thermonuclear hydrogen fusion at the core of our particular star turned the Sun’s surface into a blindingly brilliant sphere of light. But there was darkness on the face of the Earth, and its surface looked more like the pits of hell than the green lovely world we call our home.

  The Earth was hot. Hot enough to melt solid rock. In the accretion process that built the planets, the Earth (together with the other planets) was constantly bombarded by infalling meteoroids and comets, the same bombardment we can see on the pockmarked face of the Moon. This eons-long pounding heated the Earth’s surface to the melting point. When a sizable chunk of rock hits the ground at the hypersonic speeds that meteoroids attain, the high temperature created by the explosion literally boils rock.

  As we have seen, one titanic collision with a Mars-sized planetesimal blasted enough of the young Earth’s crust into orbit to lead to the creation of the Moon. The long bombardment kept the Earth’s surface seething and red-hot.

  The young Earth was also heated from within by the radioactive elements in the planet’s crust and interior, elements such as uranium, thorium, and radium. Radioactivity generates heat. Our world has been nuclear heated from its beginning. During the 4-plus billion years of Earth’s existence, much of these radioactive elements have decayed into lead and helium and other inert elements. But uranium and the other long-term radioactives still exist, and they still help to heat our planet’s interior.

  Four billion years ago the Earth was molten. The rocks were liquid and flowed just as lava flows from an erupting volcano. The heavier elements sank to the core of our planet, giving our world a dense metallic core surrounded by lighter layers of silicate rock. Gradually, the outermost layers of rock radiated much of their heat away into space, cooled, and the surface of the Earth became solid, trapping the heat remaining in the planet’s interior beneath the insulating crust of rock.

  That primeval heat still seethes beneath our feet today. We get glimpses of its fury when a volcano spews molten rock or blasts a mountaintop into ashes. Even the “solid ground” of the continents we live on are actually islands of rock floating on the hotter, denser, more plastic rocks beneath them. These huge plates of granite and basalt inch along this way and that like titanic stone barges creeping slowly across deep, molten seas, throwing up mountain chains where they grind inexorably into one another, shuddering the ground with earthquakes where they meet.

  And deep beneath it all, nearly 3,000 kilometers below the surface, the Earth’s core of nickel and iron is still hot enough to be fluid, except at its very heart where the titanic pressures of a whole world pressing down on it makes the innermost core solid metal.

  Up on the surface of the newly formed Earth, there was nothing but darkness and barren, seething hot rock. Not a likely place to expect life to take root.

  Yet the precursors of living organisms already existed in the comets and asteroids that were raining down on Earth’s surface.

  10

  Opportunistic,Tenacious Life

  We . . . can’t rule out the possibility that life arose a number of times and on each occasion (except the last) was wiped out. The rock record of Earth’s first billion years is so fragmentary that several giant, sterilizing impacts could have occurred, and we’d have no way of knowing about them.

  —David Darling

  Life Everywhere: The Maverick Science of Astrobiology

  EVEN WHEN THE SURFACE of the Earth cooled enough to solidify, the planet was still dark and totally unlike the world we know. There was no life. Not a blade of grass or a microscopic bacterium. There were no oceans, no seas. The world was bare rock, covered with thick clouds that must have blanketed the Earth for eons.

  We know that life eventually arose on our world, whether it was carried to Earth by the asteroids and comets that bombarded the Earth for the first billion years or so of its existence or came into being on our planet without extraterrestrial seeding.

  Life may have arisen more than once, only to be wiped out by shattering explosions as the heavy bombardment pounded the newly formed planet. The titanic collision that led to the Moon’s formation would have undoubtedly scrubbed our world clean of any life that might ha
ve gotten started by that time. Over the long course of eons, solitary asteroid hits have wiped out huge numbers of living species. As noted earlier, the most recent of these extinction events occurred about 65 million years ago, killing off at least one-third of all the living creatures on Earth, including the dinosaurs.

  Yet life persists, even flourishes, despite these setbacks. But how did it begin?

  THE VITAL ROLE OF WATER

  More than 4 billion years ago, not even the Earth’s air was the same as it is today. The Earth’s original atmosphere was composed largely of the gases from the cloud that had been the progenitor of the Solar System, mainly hydrogen and helium, with smaller amounts of nitrogen and hydrogen compounds such as methane and ammonia. At the Earth’s distance from the Sun, however, once the Sun began to shine, the free hydrogen and helium of the primeval atmosphere soon boiled away into space, leaving the heavier gases.

  Meteors flashed through that dark, cloud-covered sky, chunks of rock and metal continued to bombard the Earth. And icy comets carried organic molecules to Earth, plus a lot of water. As they exploded in Earth’s atmosphere, the comets rained megatonnages of water onto the surface, perhaps enough to fill the basins that eventually became our seas and oceans.

  Water is the most common triatomic molecule in the universe; undoubtedly there was considerable water already contained within the Earth’s interior, boiling up to the surface just as steam erupts from volcanoes today. Still, the contribution from the comets must have been significant.

  Water vapor may well have been the smallest ingredient in the Earth’s early atmosphere, but it turned out to be the most important. Our planet is situated squarely in the Sun’s habitable zone, where water can exist in all three of its physical forms: vapor, ice, and—crucially important for the existence of life—liquid.

  The thick black clouds covering the Earth contained a good deal of water vapor, and for untold eons they rained down on the bare rocks below. At first the rocks were still so hot that most of the rain flashed immediately into steam and rose back up through the atmosphere to form clouds again. Gradually, however, as the rocks cooled, liquid water began to collect on the surface of our world. Rivers began to flow across the land. Ponds, lakes, seas, and mighty oceans began to grow.

  The clouds eventually broke apart and sunlight smiled down on the surface of a barren, rocky world. But it was a world that was partially covered now with liquid water. It was a world ready to bear life.

  WHAT THE ROCKS TELL US

  The primitive Earth had all the necessary ingredients for life to flourish. The temperatures and pressures on the surface (and for kilometers belowground) were suitable for long-chain carbon molecules to form. Very likely such organic molecules had already been deposited on our planet by infalling comets and asteroids, assuming that the long-chain molecules could survive the heat of their fiery entry into the atmosphere and their explosive crash-landing.

  There was plentiful liquid water: Stephen J. Mojzsis of the University of Colorado at Boulder and colleagues from the University of California at Los Angeles and Curtin University in Australia have found evidence that liquid water was present on Earth’s surface 4.3 billion years ago. Whole seas and oceans were forming, and water permeated the rocks for many kilometers underground. There was also energy welling up from the hot interior, as well as falling from the sky in the form of sunlight. There was energy from radioactivity and lightning, as well.

  Yet the Earth was lifeless. The rocks were still hot and bare, although they had cooled enough to solidify in most places. The atmosphere was now mainly nitrogen, carbon dioxide, methane, ammonia, and inert gases such as argon, neon, etc. And water vapor.

  The oldest known rocks are the Acasta gneiss, from Canada’s Northwest Territories, which have been dated at slightly less than 4 billion years old. Gneiss is a metamorphic type of rock, formed under conditions of high temperature and pressure. The Acasta gneiss probably formed deep underground and, over the course of geologic ages, was lifted to the surface by tectonic forces.

  The fact that geologists have found rocks that can be reliably dated at nearly 4 billion years old means that the Earth’s crust must have cooled from its molten condition by then, some half a billion years after its original formation. Thanks to the rock samples returned from the Moon by the Apollo astronauts, planetary astronomers have concluded that the thundering heavy bombardment of the solar system’s accretion phase ended about 3.5 billion years ago.

  Life had already taken root on Earth by then.

  Indirect evidence shows that living organisms may have existed 3.87 billion years ago—370 million years before the heavy bombardment ended.

  The evidence comes from samples of graphite found on Akilia Island, off the southwest coast of Greenland, and in the rocks of the Isua formation inland. In 1996, Mojzsis and Gustaff Arrhenius, oceanographer at the Scripps Institution in California (and grandson of the panspermia theorist), reported that they found elevated levels of carbon-12 in the graphite, which they attributed to the existence of living organisms in the rock. Carbon comes in three forms (which chemists call isotopes): carbon-12, -13, and -14. When cells take up carbon dioxide and use it in their metabolism, the lighter carbon-12 is slightly enriched in the cell over the heavier carbon-13. When the Scripps oceanographers found higher levels of carbon-12 than carbon-13 in the Greenland rocks, they drew the conclusion that biological processes had been at work in them 3.87 billion years ago. (Radioactive carbon-14, which decays in a matter of thousands of years, was long gone from their samples.)

  Other scientists have argued that nonbiological processes could also elevate the carbon-12 levels. There is some controversy, as well, over the exact age of the Akilia rocks.

  Remember the story from the prologue of the scout at the edge of the canyon? These scientists are at the edge of the known, gazing across a chasm into uncharted territory. Even with the best tools available to them, no one can yet say for certain what they have seen. We will run across this situation time and again, especially when we consider the possibility that microbial life existed on Mars more than 3 billion years ago.

  WHAT THE EXTREMOPHILES TELL US

  What kind of life could have existed on Earth nearly 4 billion years ago? Could life actually have arisen so early in our planet’s history? If there were cells metabolizing carbon dioxide out of the atmosphere that long ago, then the origins of life must date to an even earlier time.

  Remember, the Earth was still hot. The rocks may have cooled enough to solidify, but we would find the temperature distinctly uncomfortable, if not lethal. Clouds still covered the sky; sunshine was rare, and when it did reach the ground it was unfiltered by an ozone layer. Killing levels of ultraviolet light reached the ground and even penetrated some depth into the seas. There was no free oxygen in the air; Earth’s atmosphere was a choking, deadly mix of gases that are unbreathable.

  Unbreathable for us, that is.

  There are organisms that thrive at high temperature and pressure, as we saw in Chapter 4. Most of them are anaerobic; they do not need oxygen. Indeed, oxygen kills them.

  The extremophiles, especially those anaerobic microbes that now live deep underground or in brutal conditions of high temperature and salinity, may well represent the oldest lines of living organisms on Earth. They very likely arose when conditions that we consider “extreme” were the perfectly ordinary, everyday environment of our homeworld. If conditions on the primitive Earth seem hellish to us, the extremophiles were content, in John Milton’s words from Paradise Lost, to “make a heaven of hell.”

  In other words, life takes advantage of whatever conditions exist, as long as those conditions fulfill the three requirements of carbon, water, and energy. Even at the far edges of temperature, pressure, salinity, etc., as long as carbon atoms can form chains, water remains liquid, and energy is available, life can flourish. Life is opportunistic. And tenaciously persistent.

  It was only later, billions of years later, when li
ving organisms began to greatly alter the Earth’s environment, that the extremophiles were driven to the odd corners of the world.

  ORGANIC SOUP OR A BIT OF CLAY?

  But how did it all begin? Somewhere, somehow life got started on Earth and transformed this planet into the world we live on. How?

  As we have seen, Svante Arrhenius suggested that life began elsewhere and came to Earth as spores that drifted through interstellar space. We know that organic molecules were carried to Earth by comets and asteroids. Yet these are only the precursors of life, not living organisms. We are still left with the question of how life began—wherever it started. How can living creatures arise out of materials that are not alive?

  Charles Darwin, founder of the concept of evolution through natural selection, confined his thinking to considerations of how life might have begun on Earth without any extraterrestrial input. In 1871, he wrote:

  [W]e could conceive in some warm little pond, with all sorts of ammonia and phosphoric salts, light, heat, electricity, etc. present that a protein compound was chemically formed, ready to undergo still more complex changes . . .

  In other words, life might have sprung spontaneously from a mixture of chemicals in “some warm little pond.” For almost a century, most biologists accepted Darwin’s basic idea that life began in the water, somewhere, and the chemical reactions that led to life’s beginning were probably powered by sunlight.

  Their reasoning went this way: When the oceans first formed, they were made of fresh water, as fresh as rain or any sparkling mountain lake. As centuries turned to millennia, and millennia to eons, rain and other forms of weathering eroded the rocks and washed much of their minerals into the streams that emptied into the oceans and seas. This continuous input of minerals turned the oceans saltier and saltier. The seas were turning into what the biologists called an “organic soup,” water that is laced with organic compounds rich in carbon and the other elements necessary for life. The soup might also have been seeded with organic molecules deposited by comets or meteorites.