[Paleopsych] Scientific American: Did Life Come from Another World?

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Did Life Come from Another World?
http://www.sciam.com/print_version.cfm?articleID=00073A97-5745-1359-94FF83414B7F0000

    October 24, 2005

    New research indicates that microorganisms could have survived a
    journey from Mars to Earth
    By David Warmflash and Benjamin Weiss

    Most scientists have long assumed that life on Earth is a homegrown
    phenomenon. According to the conventional hypothesis, the earliest
    living cells emerged as a result of chemical evolution on our planet
    billions of years ago in a process called abiogenesis. The alternative
    possibility--that living cells or their precursors arrived from
    space--strikes many people as science fiction. Developments over the
    past decade, however, have given new credibility to the idea that
    Earth's biosphere could have arisen from an extraterrestrial seed.

    Planetary scientists have learned that early in its history our solar
    system could have included many worlds with liquid water, the
    essential ingredient for life as we know it. Recent data from NASA's
    Mars Exploration Rovers corroborate previous suspicions that water has
    at least intermittently flowed on the Red Planet in the past. It is
    not unreasonable to hypothesize that life existed on Mars long ago and
    perhaps continues there. Life may have also evolved on Europa,
    Jupiter's fourth-largest moon, which appears to possess liquid water
    under its icy surface. Saturn's biggest satellite, Titan, is rich in
    organic compounds; given the moon's frigid temperatures, it would be
    highly surprising to find living forms there, but they cannot be ruled
    out. Life may have even gained a toehold on torrid Venus. The Venusian
    surface is probably too hot and under too much atmospheric pressure to
    be habitable, but the planet could conceivably support microbial life
    high in its atmosphere. And, most likely, the surface conditions on
    Venus were not always so harsh. Venus may have once been similar to
    early Earth.

    Moreover, the expanses of interplanetary space are not the forbidding
    barrier they once seemed. Over the past 20 years scientists have
    determined that more than 30 meteorites found on Earth originally came
    from the Martian crust, based on the composition of gases trapped
    within some of the rocks. Meanwhile biologists have discovered
    organisms durable enough to survive at least a short journey inside
    such meteorites. Although no one is suggesting that these particular
    organisms actually made the trip, they serve as a proof of principle.
    It is not implausible that life could have arisen on Mars and then
    come to Earth, or the reverse. Researchers are now intently studying
    the transport of biological materials between planets to get a better
    sense of whether it ever occurred. This effort may shed light on some
    of modern science's most compelling questions: Where and how did life
    originate? Are radically different forms of life possible? And how
    common is life in the universe?

    From Philosophy to the Laboratory
    To the ancient philosophers, the creation of life from nonliving
    matter seemed so magical, so much the realm of the gods, that some
    actually preferred the idea that ready-made living forms had come to
    Earth from elsewhere. Anaxagoras, a Greek philosopher who lived 2,500
    years ago, proposed a hypothesis called "panspermia" (Greek for "all
    seeds"), which posited that all life, and indeed all things,
    originated from the combination of tiny seeds pervading the cosmos. In
    modern times, several leading scientists--including British physicist
    Lord Kelvin, Swedish chemist Svante Arrhenius and Francis Crick,
    co-discoverer of the structure of DNA--have advocated various
    conceptions of panspermia. To be sure, the idea has also had less
    reputable proponents, but they should not detract from the fact that
    panspermia is a serious hypothesis, a potential phenomenon that we
    should not ignore when considering the distribution and evolution of
    life in the universe and how life came to exist specifically on Earth.
      _________________________________________________________________

    Earth's biosphere could have arisen from an extraterrestrial seed.
      _________________________________________________________________

    In its modern form, the panspermia hypothesis addresses how biological
    material might have arrived on our planet but not how life originated
    in the first place. No matter where it started, life had to arise from
    nonliving matter. Abiogenesis moved from the realm of philosophy to
    that of experimentation in the 1950s, when chemists Stanley L. Miller
    and Harold C. Urey of the University of Chicago demonstrated that
    amino acids and other molecules important to life could be generated
    from simple compounds believed to exist on early Earth. It is now
    thought that molecules of ribonucleic acid (RNA) could have also
    assembled from smaller compounds and played a vital role in the
    development of life.

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    In present-day cells, specialized RNA molecules help to build
    proteins. Some RNAs act as messengers between the genes, which are
    made of deoxyribonucleic acid (DNA), and the ribosomes, the protein
    factories of the cell. Other RNAs bring amino acids--the building
    blocks of proteins--to the ribosomes, which in turn contain yet
    another type of RNA. The RNAs work in concert with protein enzymes
    that aid in linking the amino acids together, but researchers have
    found that the RNAs in the ribosome can perform the crucial step of
    protein synthesis alone. In the early stages of life's evolution, all
    the enzymes may have been RNAs, not proteins. Because RNA enzymes
    could have manufactured the first proteins without the need for
    preexisting protein enzymes to initiate the process, abiogenesis is
    not the chicken-and-egg problem that it was once thought to be. A
    prebiotic system of RNAs and proteins could have gradually developed
    the ability to replicate its molecular parts, crudely at first but
    then ever more efficiently.

    This new understanding of life's origins has transformed the
    scientific debate over panspermia. It is no longer an either-or
    question of whether the first microbes arose on Earth or arrived from
    space. In the chaotic early history of the solar system, our planet
    was subject to intense bombardment by meteorites containing simple
    organic compounds. The young Earth could have also received more
    complex molecules with enzymatic functions, molecules that were
    prebiotic but part of a system that was already well on its way to
    biology. After landing in a suitable habitat on our planet, these
    molecules could have continued their evolution to living cells. In
    other words, an intermediate scenario is possible: life could have
    roots both on Earth and in space. But which steps in the development
    of life occurred where? And once life took hold, how far did it
    spread?

    Scientists who study panspermia used to concentrate only on assessing
    the basic plausibility of the idea, but they have recently sought to
    estimate the probability that biological materials made the journey to
    Earth from other planets or moons. To begin their interplanetary trip,
    the materials would have to be ejected from their planet of origin
    into space by the impact of a comet or asteroid. While traveling
    through space, the ejected rocks or dust particles would need to be
    captured by the gravity of another planet or moon, then decelerated
    enough to fall to the surface, passing through the atmosphere if one
    were present. Such transfers happen frequently throughout the solar
    system, although it is easier for ejected material to travel from
    bodies more distant from the sun to those closer in and easier for
    materials to end up on a more massive body. Indeed, dynamic
    simulations by University of British Columbia astrophysicist Brett
    Gladman suggest that the mass transferred from Earth to Mars is only a
    few percent of that delivered from Mars to Earth. For this reason, the
    most commonly discussed panspermia scenario involves the transport of
    microbes or their precursors from Mars to Earth.

    Simulations of asteroid or comet impacts on Mars indicate that
    materials can be launched into a wide variety of orbits. Gladman and
    his colleagues have estimated that every few million years Mars
    undergoes an impact powerful enough to eject rocks that could
    eventually reach Earth. The interplanetary journey is usually a long
    one: most of the approximately one ton of Martian ejecta that lands on
    Earth every year has spent several million years in space. But a tiny
    percentage of the Martian rocks arriving on Earth's surface--about one
    out of every 10 million--will have spent less than a year in space.
    Within three years of the impact event, about 10 fist-size rocks
    weighing more than 100 grams complete the voyage from Mars to Earth.
    Smaller debris, such as pebble-size rocks and dust particles, are even
    more likely to make a quick trip between planets; very large rocks do
    so much less frequently.

    Could biological entities survive this journey? First, let us consider
    whether microorganisms could live through the ejection process from
    the meteorite's parent body. Recent laboratory impact experiments have
    found that certain strains of bacteria can survive the accelerations
    and jerks (rates of changes of acceleration) that would be encountered
    during a typical high-pressure ejection from Mars. It is crucial,
    however, that the impact and ejection do not heat the meteorites
    enough to destroy the biological materials within them.

    Planetary geologists formerly believed that any impact ejecta with
    speeds exceeding the Martian escape velocity would almost certainly be
    vaporized or at least completely melted. This idea was later
    discounted, though, following the discovery of unmelted, largely
    intact meteorites from the moon and Mars. These findings led H. Jay
    Melosh of the University of Arizona to calculate that a small
    percentage of ejected rocks could indeed be catapulted from Mars via
    impact without any heating at all. In short, Melosh proposed that when
    the upward-propagating pressure wave resulting from an impact reaches
    the planetary surface, it undergoes a 180-degree phase change that
    nearly cancels the pressure within a thin layer of rock just below the
    surface. Because this "spall zone" experiences very little compression
    while the layers below are put under enormous pressure, rocks near the
    surface can be ejected relatively undeformed at high speeds.

    Next, let us consider survivability during the entry into Earth's
    atmosphere. Edward Anders, formerly of the Enrico Fermi Institute at
    the the University of Chicago, has shown that interplanetary dust
    particles decelerate gently in Earth's upper atmosphere, thus avoiding
    heating. Meteorites, in contrast, experience significant friction, so
    their surfaces typically melt during atmospheric passage. The heat
    pulse, however, has time to travel a few millimeters at most into the
    meteorite's interior, so organisms buried deep in the rock would
    certainly survive.

    Over the past five years a series of papers by one of us (Weiss) and
    his colleagues analyzed two types of Martian meteorites: the
    nakhlites, a set of rocks blasted off Mars by an asteroid or comet
    impact 11 million years ago, and ALH84001, which left the Red Planet
    four million years earlier. (ALH84001 became famous in 1996 when a
    group of scientists led by David McKay of the NASA Johnson Space
    Center claimed that the rock showed traces of fossilized
    microorganisms akin to Earth's bacteria; a decade later researchers
    are still debating whether the meteorite contains evidence of Martian
    life.) By studying the magnetic properties of the meteorites and the
    composition of the gases trapped within them, Weiss and his
    collaborators found that ALH84001 and at least two of the seven
    nakhlites discovered so far were not heated more than a few hundred
    degrees Celsius since they were part of the Martian surface.
    Furthermore, the fact that the nakhlites are nearly pristine rocks,
    untouched by high-pressure shock waves, implies that the Martian
    impact did not heat them above 100 degrees C.

    Many, though not all, terrestrial prokaryotes (simple one-celled
    organisms such as bacteria that lack a membrane-bound nucleus) and
    eukaryotes (organisms with well-defined nuclei) could survive this
    temperature range. This result was the first direct experimental
    evidence that material could travel from planet to planet without
    being thermally sterilized at any point from ejection to landing.

    The Problem of Radiation
    For panspermia to occur, however, microorganisms need to survive not
    only ejection from the first planet and atmospheric entry to the
    second but the interplanetary voyage itself. Life-bearing meteoroids
    and dust particles would be exposed to the vacuum of space, extremes
    in temperature and several different kinds of radiation. Of particular
    concern is the sun's high-energy ultraviolet (UV) light, which breaks
    the bonds that hold together the carbon atoms of organic molecules. It
    is very easy to shield against UV, though; just a few millionths of a
    meter of opaque material is enough to protect bacteria.

    Indeed, a European study using NASA's Long Duration Exposure Facility
    (LDEF), a satellite deployed by the space shuttle in 1984 and
    retrieved from orbit by the shuttle six years later, showed that a
    thin aluminum cover afforded adequate UV shielding to spores of the
    bacterial species Bacillus subtilis. Of the spores protected by the
    aluminum but exposed to the vacuum and temperature extremes of space,
    80 percent remained viable--researchers reanimated them into active
    bacterial cells at the end of the mission. As for the spores not
    covered by aluminum and therefore directly exposed to solar UV
    radiation, most were destroyed, but not all. About one in 10,000
    unshielded spores stayed viable, and the presence of substances such
    as glucose and salts increased their survival rates. Even within an
    object as small as a dust particle, solar UV would not necessarily
    render an entire microbial colony sterile. And if the colony were
    inside something as large as a pebble, UV protection would be sharply
    increased.

    Informative as it was, the LDEF study was conducted in low Earth
    orbit, well within our planet's protective magnetic field. Thus, this
    research could not say much about the effects of interplanetary
    charged particles, which cannot penetrate Earth's magnetosphere. From
    time to time, the sun produces bursts of energetic ions and electrons;
    furthermore, charged particles are a major component of the galactic
    cosmic radiation that constantly bombards our solar system. Protecting
    living things from charged particles, as well as from high-energy
    radiation such as gamma rays, is trickier than shielding against UV. A
    layer of rock just a few microns thick blocks UV, but adding more
    shielding actually increases the dose of other types of radiation. The
    reason is that charged particles and high-energy photons interact with
    the rocky shielding material, producing showers of secondary radiation
    within the meteorite.

    These showers could reach any microbes inside the rock unless it was
    very big, about two meters or more in diameter. As we have noted
    above, though, large rocks make fast interplanetary voyages very
    infrequently. Consequently, in addition to UV protection, what really
    matters is how resistant a microbe is to all components of space
    radiation and how quickly the life-bearing meteorite moves from planet
    to planet. The shorter the journey, the lower the total radiation dose
    and hence the greater the chance of survival.

    In fact, B. subtilis is fairly robust in terms of its radiation
    resistance. Even more hardy is Deinococcus radiodurans, a bacterial
    species that was discovered during the 1950s by agricultural scientist
    Arthur W. Anderson. This organism survives radiation doses given to
    sterilize food products and even thrives inside nuclear reactors. The
    same cellular mechanisms that help D. radiodurans repair its DNA,
    build extra-thick cell walls and otherwise protect itself from
    radiation also mitigate damage from dehydration. Theoretically, if
    organisms with such capabilities were embedded within material
    catapulted from Mars the way that the nakhlites and ALH84001
    apparently were (that is, without excessive heating), some fraction of
    the organisms would still be viable after many years, perhaps several
    decades, in interplanetary space.

    Yet the actual long-term survival of active organisms, spores or
    complex organic molecules beyond Earth's magne-tosphere has never been
    tested. Such experiments, which would put the biological materials
    within simulated meteoritic materials and expose them to the
    environment of interplanetary space, could be conducted on the surface
    of the moon. In fact, biological samples were carried onboard the
    Apollo lunar missions as part of an early incarnation of the European
    radiation study. The longest Apollo mission, though, lasted no more
    than 12 days, and samples were kept within the Apollo spacecraft and
    thus not exposed to the full space-radiation environment. In the
    future, scientists could place experimental packages on the lunar
    surface or on interplanetary trajectories for several years before
    returning them to Earth for laboratory analysis. Researchers are
    currently considering these approaches.

    Meanwhile a long-term study known as the Martian Radiation Environment
    Experiment (MARIE) is under way. Launched by NASA in 2001 as part of
    the Mars Odyssey Orbiter, MARIE's instruments are measuring doses of
    galactic cosmic rays and energetic solar particles as the spacecraft
    circles the Red Planet. Although MARIE includes no biological
    material, its sensors are designed to focus on the range of space
    radiation that is most harmful to DNA.

    Future Studies
    As we have shown, panspermia is plausible theoretically. But in
    addition, important aspects of the hypothesis have made the transition
    from plausibility to quantitative science. Meteorite evidence shows
    that material has been transferred between planets throughout the
    history of the solar system and that this process still occurs at a
    well-established rate. Furthermore, laboratory studies have
    demonstrated that a sizable fraction of microorganisms within a piece
    of planetary material ejected from a Mars-size planet could survive
    ejection into space and entry through Earth's atmosphere. But other
    parts of the panspermia hypothesis are harder to pin down.
    Investigators need more data to determine whether radiation-resistant
    organisms such as B. subtilis or D. radiodurans could live through an
    interplanetary journey. And even this research would not reveal the
    likelihood that it actually happened in the case of Earth's biosphere,
    because the studies involve present-day terrestrial life-forms; the
    organisms living billions of years ago could have fared much worse or
    much better.

    Moreover, scientists cannot quantify the likelihood that life exists
    or once existed on planets other than Earth. Researchers simply do not
    know enough about the origin of any system of life, including that of
    Earth, to draw solid conclusions about the probability of abiogenesis
    occurring on any particular world. Given suitable ingredients and
    conditions, perhaps life needs hundreds of millions of years to get
    started. Or perhaps five minutes is enough. All we can say with any
    certainty is that by 2.7 billion years ago, or perhaps several hundred
    million years earlier, life-forms were thriving on Earth.

    Because it is not possible at this time to quantify all the steps of
    the panspermia scenario, investigators cannot estimate how much
    biological material or how many living cells most likely arrived at
    Earth's surface in a given period. Moreover, the transfer of viable
    organisms does not automatically imply the successful seeding of the
    planet that receives them, particularly if the planet already has
    life. If, for example, Martian microbes arrived on Earth after life
    independently arose on our planet, the extraterrestrial organisms may
    not have been able to replace or coexist with the homegrown species.
    It is also conceivable that Martian life did find a suitable niche on
    Earth but that scientists have simply not identified it yet.
    Researchers have inventoried no more than a few percent of the total
    number of bacterial species on this planet. Groups of organisms that
    are genetically unrelated to the known life on Earth might exist
    unrecognized right under our noses.

    Ultimately, scientists may not be able to know whether and to what
    extent panspermia has occurred until they discover life on another
    planet or moon. For example, if future space missions find life on the
    Red Planet and report that Martian biochemistry is very different from
    our own, researchers would know immediately that life on Earth did not
    come from Mars. If the biochemistries were similar, however,
    scientists might begin to wonder if perhaps the two biospheres had a
    common origin. Assuming that Martian life-forms used DNA to store
    genetic information, investigators could study the nucleotide
    sequences to settle the question. If the Martian DNA sequences did not
    follow the same genetic code used by living cells on Earth to make
    proteins, researchers would conclude that Mars-Earth panspermia is
    doubtful. But many other scenarios are possible. Investigators might
    find that Martian life uses RNA or something else entirely to guide
    its replication. Indeed, yet-to-be-discovered organisms on Earth may
    fall into this category as well, and the exotic terrestrial creatures
    might turn out to be related to the Martian life-forms.

    Whether terrestrial life emerged on Earth or through biological
    seeding from space or as the result of some intermediate scenario, the
    answer would be meaningful. The confirmation of Mars-Earth panspermia
    would suggest that life, once started, could readily spread within a
    star system. If, on the other hand, researchers find evidence of
    Martian organisms that emerged independently of terrestrial life, it
    would suggest that abiogenesis can occur with ease throughout the
    cosmos. What is more, biologists would be able to compare Earth
    organisms with alien forms and develop a more general definition of
    life. We would finally begin to understand the laws of biology the way
    we understand the laws of chemistry and physics--as fundamental
    properties of nature.